AMT, NIIT & the Additional Medicare Tax

If you've spent any time reading about taxes, you've probably run into three names that arrive with a little jolt of fear and almost no explanation. "Watch out for the AMT — it can blindside you." "There's a 3.8% surtax on investment income now." "Why do I owe more Medicare tax at filing than came out of my paychecks?" They get mentioned the way you'd mention a pothole you can't quite see — something that might swallow you if your income gets high enough, lurking just off the edge of the ordinary tax return. This lesson exists to turn on the lights. Because here's the honest truth about all three, and it's the opposite of how they're usually described: each one has a clear, published threshold, and you can tell in about a minute whether it touches you at all.

Let's name them plainly. The Alternative Minimum Tax (AMT) is a *parallel* tax system: you (or your software) compute your tax a second way, with some deductions removed, and pay whichever result is higher. The Net Investment Income Tax (NIIT) is a flat 3.8% surtax on investment income — interest, dividends, capital gains — once your income crosses a line. The Additional Medicare Tax is a 0.9% surtax on wages and self-employment income above that same line. Two of the three are simple surtaxes with a fixed rate and a fixed threshold; the third, the AMT, is the only genuinely complicated one, and even it comes down to a single comparison. None of them is a trap for ordinary filers. All three are aimed at higher incomes, and all three, once you can see the threshold, stop being scary.

We'll learn this through two households at the income levels where these taxes actually bite. Priya and Raj Malhotra — Priya a software engineer earning $185,000 with stock options, Raj a consultant with about $90,000 of self-employment income, filing jointly in Seattle with a real brokerage account — are the rare case that carries all three at once: Priya exercises incentive stock options and triggers the AMT, their investments draw the NIIT, and their two incomes together cross the Additional Medicare line. Nina Kowalski, a Boston physician earning $310,000, carries two of them — her high wages bring the Additional Medicare Tax, and her investment income brings the NIIT — and she shows what happens when the numbers are simpler. By the end you'll be able to look at your own return and say, for each of the three, "this one reaches me" or "this one doesn't" — and know what to do either way.

Before we open any one of them, it helps to see all three on a single map, because they have more in common than their scary reputations suggest. Two facts tie them together. First, they all live on Schedule 2 — the "other taxes" schedule that adds amounts on top of your regular income tax after the normal bracket calculation is done. You met Schedule 2 in the foundation lessons as the home of self-employment tax and a handful of add-ons; these three are its higher-income residents. Second, two of the three share the exact same income line — $250,000 for a married couple filing jointly, $200,000 for a single filer, $125,000 if married filing separately. The NIIT and the Additional Medicare Tax use identical thresholds; learn the line once and you've learned it for both.

Read the map for the one structural distinction that will save you confusion later: the two 3.8%/0.9% surtaxes carve up your income between them and never overlap. The Additional Medicare Tax is a surtax on *earned* income — wages and self-employment. The NIIT is a surtax on *investment* income — interest, dividends, capital gains, rents. A single dollar is either earned or invested, so it can be hit by at most one of the two. That's why you can owe both (Priya & Raj do) without any dollar being taxed twice: the 0.9% rides on their paychecks and Raj's consulting, the 3.8% rides on their brokerage account. The AMT is the odd one out — it's not a surtax on a slice of income but a whole parallel calculation of your tax, and it lands in a different part of Schedule 2 (Part I, which feeds Form 1040 line 17) than the two surtaxes (Part II, which feeds line 23). Keep that geography in your pocket; we'll return to it at the end when we watch all three land on one return.

Start with the strangest of the three, because once the AMT stops being mysterious the other two are easy. The name is unhelpful, so let's translate it. Alternative Minimum Tax means: an *alternative* way of computing your tax, designed to make sure high-income filers pay at least a *minimum* amount even after piling up deductions. The mechanism is genuinely a second, parallel tax system. You compute your tax the normal way you've learned — income, deductions, brackets. Then you compute it again under the AMT's rules, which disallow certain deductions and add back certain items. You compare the two results, and — this is the whole rule — you pay the higher of the two. If your normal tax is higher, the AMT is invisible; you never think about it. If the AMT calculation comes out higher, you pay that instead, and the difference is what people call "owing the AMT."

Why does a second system exist at all? Decades ago, Congress noticed that a small number of very high earners were using stacks of perfectly legal deductions to drive their tax bills to nearly zero. The AMT was the response: a floor. It strips out the deductions and preferences it considers too generous, recomputes your income on that stripped-down basis, and applies its own rates. The politics of who it *should* hit have always been contested, and — as we'll see — the 2017 tax law and the 2025 One Big Beautiful Bill Act between them dramatically shrank who actually pays it. But the machine itself is unchanged: compute twice, pay the higher.

The diagram shows the shape with Priya and Raj's real numbers, which we'll build up piece by piece over the next few sections — for now just absorb the structure. On the left, their tax computed normally: a regular income tax of $46,442 (that's the tax on their income after the standard deduction and brackets, which you learned to compute in the tax-calculation lesson). On the right, the same couple run through the AMT machine: their income is recomputed as $418,642 of "alternative minimum taxable income," and after the AMT's own exemption and rates, the AMT system says their tax should be $69,174. Two numbers, same couple, same year. The rule says pay the higher — $69,174 — so the AMT costs them the difference: $69,174 − $46,442 = $22,732. That $22,732 is the number we're going to earn, line by line, and then — reassuringly — largely get back. But first, the two ideas that make the right-hand column work: what gets added back, and the exemption that shields almost everyone from ever seeing this column at all.

The AMT's second calculation starts from your regular taxable income and then adds back the deductions and preferences it doesn't allow. You don't need to memorize the full list — for an individual it's a handful of items, and most people have none of them. Here are the ones that matter:

• The standard deduction. The AMT does not allow it. If you took the standard deduction (for 2026, $32,200 for a married couple), the AMT adds it right back to your income. This mostly matters when some *other* preference has already pulled you toward the AMT — as it will for Priya and Raj.
• State and local taxes (SALT) you deducted on Schedule A. The AMT gives zero benefit for state and local income and property taxes, so if you itemized and deducted them, the full amount is added back. This used to be *the* classic AMT trigger — high earners in high-tax states got pulled in constantly. Two law changes gutted it: the regular deduction for SALT is now capped ($40,000 under the 2025 law, and only $10,000 before that), so there's far less SALT sitting on Schedule A to add back in the first place.
• The incentive-stock-option (ISO) bargain element. The big one, and the whole reason Priya lands in the AMT. When you exercise an ISO and hold the shares, the "bargain" you got — the gap between the stock's value and what you paid — is invisible to the regular tax but fully counted by the AMT. We'll give this its own section; it's the star of the show.
• A few specialized items most people never touch: interest on certain "private activity" municipal bonds (tax-free for regular tax, taxable for AMT), and differences in how depreciation is figured for a business. Personal and dependent exemptions used to be a preference too — they're what pushed large families into the AMT before 2018 — but they were set to zero by the 2017 law and kept there, so they no longer create AMT exposure.

Put those changes together and you get the single most important fact about who owes the AMT today: almost nobody does, and that's deliberate. Before 2018, more than 5 million households paid the AMT every year — largely because of the SALT add-back and personal exemptions catching upper-middle-class families in high-tax states. The 2017 law raised the exemption sharply, capped SALT, and zeroed out personal exemptions, and the number of AMT payers collapsed to roughly 200,000. The 2025 One Big Beautiful Bill Act then made that high exemption *permanent* — which matters enormously, because the high exemption had been scheduled to expire, and if it had, the AMT would have snapped back to catch millions of ordinary families again. It didn't. So for the vast majority of filers, the AMT is a non-event, and the middle-class "AMT trap" of the 2000s is genuinely gone.

The reason the AMT column comes out lower than the regular tax for almost everyone is a single, generous number: the AMT exemption. Before the AMT applies its rate, it subtracts a large fixed amount from your alternative minimum taxable income. For tax year 2026, verified against the IRS's official inflation figures (Revenue Procedure 2025-32), the exemption is $140,200 for a married couple filing jointly and $90,100 for a single filer ($70,100 married filing separately). That's an enormous shield: it means the AMT only starts to matter once your recomputed income, after the add-backs, is well into six figures. For a typical filer, the exemption swallows the whole AMT calculation and the tentative minimum tax comes out far below the regular tax — so they never owe a cent of AMT and never even know the calculation ran.

But the exemption doesn't last forever as your income climbs. Past a threshold, it phases out — it shrinks as your income rises, so very high earners lose the shield. This is where the 2025 law made its high-end change, and the 2026 numbers are worth stating exactly:

Read the mechanic underneath the table, because it's what makes the AMT sharper at the top. Once your AMTI passes the phase-out threshold, the exemption shrinks by 50 cents for every dollar over the line — and that 50% rate is itself a 2026 change, doubled up from 25 cents on the dollar in prior years. So a married couple keeps the full $140,200 exemption up to $1,000,000 of AMTI, then loses it at 50 cents per dollar, until it's completely gone at $1,280,400. The practical effect: the exemption disappears twice as fast as it used to, and it starts disappearing at a much lower income than in 2025 (when the married phase-out didn't even begin until $1,252,700). Priya and Raj, happily, are well under the $1,000,000 line, so they keep their full $140,200 — one thing that stays simple in their calculation.

One last piece and the AMT machine is fully built: the rates. After subtracting the exemption, the AMT taxes what's left — the "taxable excess" — at just two rates: 26% on the first slice and 28% on the amount above a breakpoint, which for 2026 is $244,500 of taxable excess. (Long-term capital gains and qualified dividends keep their lower capital-gains rates inside the AMT too — the AMT doesn't punish those.) That's the entire engine: take AMTI, subtract the exemption, apply 26% then 28%, and you have the tentative minimum tax. Compare it to your regular tax, pay the higher. Now let's feed Priya's ISO exercise into it and watch a real AMT bill appear.

Priya, like many engineers at growing companies, holds incentive stock options (ISOs) — a right to buy company stock at a fixed "strike" price set when the options were granted, usually far below what the stock is worth later. This year she exercises a block of them: she pays the strike price and receives the shares, and — believing in the company — she *holds* the shares rather than selling. We're not here to teach how options are granted and vest (that's the equity-compensation lesson); we care about one number this creates, because it's the spark that lights the AMT.

That number is the bargain element — the gap between what the shares are worth when she exercises and what she paid for them. Priya's options have a strike price of $5 a share, the stock is worth $25 a share when she exercises, and she exercises 6,000 shares. So her bargain element is the $20-per-share "bargain" times 6,000 shares:

Here is the quiet cruelty that makes ISOs the AMT's signature trigger, and it turns entirely on the difference between the two tax systems. For the regular tax, exercising an ISO and holding the shares is a *non-event* — you bought some stock, you sold nothing, you have no income, you owe nothing. That's the whole appeal of an ISO. But the AMT doesn't see it that way: it counts the full $120,000 bargain element as income the moment you exercise, on Form 6251, line 2i. So Priya has $120,000 of income for AMT purposes that doesn't exist for regular-tax purposes — income she can't spend, because it's locked up in shares she hasn't sold. Tax professionals call this phantom income: a tax bill on value you truly received but hold no cash against.

Sit with why this is the trigger and not, say, her salary. Priya's $185,000 salary is taxed the same under both systems — wages get no special break the AMT claws back, so they don't create an AMT problem. The ISO bargain element is different precisely *because* the regular tax gives it a break (letting you defer all tax until you sell the shares) that the AMT refuses to honor. The AMT exists to catch exactly this kind of gap. So the mental model to carry: a large exercise-and-hold of ISOs is the one ordinary event most likely to drop a normal high earner into the AMT — and the bigger the bargain element, the harder the landing. Now let's run Priya's full parallel calculation and see the $22,732 appear.

We now have every part we need, so let's build both columns of Priya and Raj's 2026 return and compare them. Start with the regular tax, the way you learned it. Priya's wages are $185,000; Raj's consulting nets $90,000 (his self-employment tax on that is a separate matter we'll handle later — it doesn't enter the AMT comparison); and they have $30,000 of investment income (long-term gains and qualified dividends). After the deduction for half of Raj's self-employment tax, their adjusted gross income is $298,642, and after the $32,200 standard deduction their taxable income is $266,442. Running that through the 2026 married brackets (and taxing the $30,000 of investment income at the 15% capital-gains rate) gives a regular income tax of $46,442. That's the left column, and the number the AMT has to beat.

Now the AMT column. Build AMTI by starting from that $266,442 of taxable income and adding back what the AMT disallows: the $32,200 standard deduction (the AMT doesn't allow it) and the $120,000 ISO bargain element (Priya's phantom income). Nothing else — they took the standard deduction, so there's no SALT to add back:

Walk the punchline. Their AMTI is $418,642 — comfortably under the $1,000,000 where the exemption would start shrinking, so they keep the full $140,200 exemption. Subtracting it leaves $278,442 of taxable excess. The AMT taxes that at 26% up to $244,500 and 28% on the $33,942 above it (their $30,000 of long-term gains stays at 15% inside the AMT, just as it does for the regular tax), producing a tentative minimum tax of $69,174. That beats their regular tax of $46,442, so they pay the $69,174 — and the AMT, reported on Form 6251, is the difference: $22,732. Every dollar of that $22,732 traces to the $120,000 ISO bargain element; without the exercise, their tentative minimum tax would have been *below* their regular tax and the AMT would have been zero. This is the phantom-income bill in the flesh: $22,732 of real tax owed on stock she hasn't sold and can't spend.

All of that computation lands on one form — Form 6251, Alternative Minimum Tax — Individuals. Your tax software fills it in and tells you whether you owe the AMT, so you'll rarely touch it by hand. But you should be able to *read* it, because reading it is how you understand why you owe, confirm the ISO adjustment is right, and later track the credit coming back. Let's read Priya and Raj's, the way you'd check your own — every section shown whole, with the lines that carry their story highlighted.

Read it top to bottom. Part I — Alternative Minimum Taxable Income is the add-back section. It begins on line 1 with their regular taxable income ($266,442), then line 2a adds back the standard deduction ($32,200) and line 2i — the line to know — adds the ISO bargain element ($120,000), labeled on the form as "Exercise of incentive stock options (excess of AMT income over regular tax income)." (A common mix-up: the ISO line is 2i; line 2m, right below it, is for passive activities — a different item entirely.) Those add-backs bring line 4, AMTI, to $418,642. Part II — Alternative Minimum Tax does the arithmetic we just did: it subtracts the $140,200 exemption (line 5), leaves $278,442 of taxable excess (line 6), applies the 26%/28% rates to reach the $69,174 tentative minimum tax (line 7), and then line 9 brings in their $46,442 regular tax for the comparison. Line 11 reports the excess — $22,732 — as the AMT, and a pointer sends it to Schedule 2, Part I, line 2. Every number we computed by hand is sitting right there on the form, which is exactly how you'd verify a software result or a preparer's work.

Here is the part of the AMT story almost nobody tells you, and it changes how the whole thing feels. The $22,732 Priya paid is not gone — most or all of it comes back to her in future years, as the AMT credit. The AMT she owes because of the ISO isn't a permanent extra tax; it's a *timing* difference. The regular tax will eventually tax the same gain (when she sells the shares), so making her pay it early through the AMT would be double-taxing her — and the tax code knows it. The fix is the minimum tax credit (claimed on Form 8801): the AMT you paid because of a timing item becomes a credit that offsets your *regular* tax in later years. It sits and waits, carrying forward indefinitely, until a year when your regular tax exceeds your tentative minimum tax — and then it comes back to you.

The key distinction — and it's the one thing to understand about the credit — is deferral items versus exclusion items. Some AMT add-backs are *timing* differences that reverse later: the ISO bargain element is the prime example, because the regular tax will pick up that gain when the shares are sold. These are deferral items, and the AMT you pay because of them generates the credit — it comes back. Other add-backs are *permanent* differences that never reverse: the standard deduction and SALT are gone for good under the AMT, and you never get regular-tax credit for them later. These are exclusion items, and the AMT you pay because of them does *not* generate a credit — that portion is a true, permanent extra tax.

What makes the credit actually come back is a second consequence of the ISO exercise: a dual basis in the shares. For the *regular* tax, Priya's cost basis in the 6,000 shares is what she paid — the $5 strike, so $30,000. For the *AMT*, her basis is the value the AMT already taxed her on — the $25 exercise price, so $150,000. She has two different bases in the same shares. When she eventually sells, the AMT gain (measured from the higher $150,000 basis) is $120,000 *smaller* than the regular-tax gain (measured from the $30,000 basis). That gap is what flips the comparison in the sale year: her regular tax exceeds her tentative minimum tax, and the minimum tax credit releases to offset the regular tax on that larger regular gain. The prepayment and the recovery are two ends of the same rope.

Two honest caveats keep this from sounding too magical. First, the credit is nonrefundable and comes back only gradually — you use it in a given year only up to the amount your regular tax exceeds your tentative minimum tax, so it can trickle back over several years, and if you keep exercising ISOs and staying near the AMT line, it can take a long time to fully recover. Second, the recovery is real but not instant cash — it's a smaller future tax bill, not a refund check today. Still, the framing matters enormously for the fear this lesson is dismantling: the AMT on an ISO is far closer to an interest-free (or low-interest) *prepayment* of tax you'd owe eventually than to a genuine penalty. Knowing that is the difference between panic and a plan.

Because the AMT on ISOs is predictable, it's also manageable — the whole danger is in exercising blind. Three levers, none of them exotic, turn a potential ambush into a decision:

1. Model the AMT before you exercise — always. The bargain element, and therefore the AMT, is knowable before you click the button: it's (current value − strike) × shares. Run the two-column calculation (or have software or a professional run it) *before* exercising, so the $22,732 is a number you chose to accept, not a surprise in April. This is the single most important habit, and it's the one rule of the Scam Watch below.
2. Exercise up to the "crossover" if you want to owe zero AMT. There's an amount of bargain element you can add before your tentative minimum tax overtakes your regular tax — the crossover point. Exercise up to (but not past) it and you build AMT basis and start the long-term holding clock at *no* AMT cost, then repeat next year. People with large option grants often exercise in annual slices sized to the crossover, spreading the exercise across years to keep each year's AMT at or near zero.
3. Exercise early in the year, so you keep an escape hatch. If you exercise-and-hold early in the year and the stock later falls, you can do a same-year sale to cancel the AMT preference entirely (see the next item). Exercise in late December and you've given up that option — you're locked into the AMT on a value that might evaporate. Early-year exercises leave you room to react.
4. Consider a disqualifying disposition if the exercise goes wrong. If you exercise ISOs and sell the shares in the *same* year, the AMT preference simply doesn't arise — the bargain element becomes ordinary income taxed the normal way instead (reported on your W-2), and there's no line 2i AMT adjustment. It's called a disqualifying disposition (you gave up the ISO's special long-term treatment), and it's a common escape hatch: if the stock tanks after you exercise, selling by year-end can wipe out an AMT bill on value that no longer exists.

Leave the AMT behind — the hard one is done — and meet the first of the two simple surtaxes. The Net Investment Income Tax (NIIT) is a flat 3.8% added on top of your regular tax when two things are both true: your income is over the threshold ($250,000 married filing jointly / $200,000 single), *and* you have investment income. It was created to help fund Medicare, it's been in place since 2013, and — a detail that matters more than it sounds — its thresholds have never been adjusted for inflation. They were $250,000/$200,000 in 2013 and they're still $250,000/$200,000 for 2026. Every year that incomes drift up, a few more people cross a line that never moves; the NIIT quietly reaches further into the upper-middle class over time even though nothing about it "changes."

The rate is simple, but the *base* — what the 3.8% actually applies to — is the clever part, and it's designed so the tax never hits more than it should. You don't pay 3.8% on all your investment income, and you don't pay it on all your income over the threshold. You pay it on the lesser of the two:

Why the lesser of the two? Think about what each piece protects. If you have a lot of investment income but your total income only barely tops the threshold, the tax should reach only the sliver above the line — so it's capped at (MAGI − threshold). If your income is far above the threshold but you have only a little investment income, the tax should reach only that little bit of investment income — so it's capped at your net investment income. Taking the smaller of the two caps means you're only ever taxed on investment income that genuinely sits above the threshold. It's a fair design once you see it, and it's exactly how Form 8960 computes it.

The NIIT lives or dies on one question: what is "net investment income"? Get the category right and the tax is trivial; get it wrong and you'll either panic over income that doesn't count or miss income that does. The rule tracks intuition surprisingly well — it's income your *money* earned, not income *you* earned by working.

A few edges are worth naming because they trip people up. Wages and self-employment income never count — they're earned income, and they're the domain of the *other* surtax, the Additional Medicare Tax (this is that non-overlap again). Retirement-account distributions don't count either — your IRA or 401(k) withdrawal isn't net investment income, even though it's technically investment growth — but watch the sneaky second-order effect: a big withdrawal *raises your MAGI*, which can push you over the threshold and drag your *other* investment income into the NIIT. Municipal bond interest is the quiet winner — it's excluded from net investment income *and* it doesn't raise your MAGI, making it a genuine double shelter from this tax. And home-sale gain is mostly protected: the portion excluded under the home-sale exclusion ($250,000 single / $500,000 married) isn't investment income, but any taxable gain above that exclusion is.

The widget shows the shape; the takeaway is that the NIIT is almost always simpler than its reputation. For most people who owe it, the answer is: their income is over the line, they have some interest, dividends, and capital gains, and 3.8% of that (or of the amount over the line, whichever is smaller) is the tax. It's a real cost — but it's a knowable, computable one, not a lurking mystery. Let's compute Priya and Raj's exactly.

Priya and Raj have $30,000 of net investment income for 2026 — say $2,000 of interest, $10,000 of dividends, and $18,000 of net long-term capital gains from their brokerage account. Their MAGI is their AGI, $298,642. Now apply the lesser-of rule. Candidate (a) is their net investment income: $30,000. Candidate (b) is MAGI minus the married threshold: $298,642 − $250,000 = $48,642. The NIIT applies to the *smaller* of the two, which is the $30,000 of investment income:

So their $30,000 of investment income, already taxed at the 15% long-term capital-gains and qualified-dividend rates, faces an *additional* 3.8% — meaning the true federal rate on those investment dollars is 18.8%, not 15%. That's the practical meaning of the NIIT for an investor: it quietly lifts the headline 15% capital-gains rate to 18.8% (or the 20% top rate to 23.8%) once you're over the threshold. The $1,140 itself isn't huge, but the reframe is worth holding — when you're an over-the-threshold investor, every capital-gains and dividend decision should be priced at 18.8%, not 15%.

And because the NIIT is mechanical, it's also *reducible* — legally, and often substantially. The levers all work by shrinking one of the two inputs (net investment income, or MAGI over the threshold):

• Harvest investment losses. Because net investment income is *net* — gains minus losses — realizing a capital loss directly shrinks the NIIT base alongside the regular capital-gains tax. This is the two-birds bonus we noted in the harvesting lesson: a harvested loss works against the 15%/20% capital-gains tax *and* the 3.8% NIIT at once, so a higher earner's harvested loss is worth about 18.8% a dollar, not 15%.
• Time income to stay under the threshold. If your MAGI is near the line, spreading a big capital gain across two years — or realizing it in a lower-income year — can keep more of your income below $250,000/$200,000, where the NIIT can't reach it at all.
• Favor municipal bonds where they fit. Muni interest is the double shelter — excluded from investment income *and* from MAGI — so shifting some taxable bond holdings to munis can lower both inputs at once (weigh the lower yield against the tax saved).
• Use tax-advantaged accounts. Investment income earned inside an IRA, 401(k), or HSA isn't reported until distribution (and retirement distributions aren't NIIT income), so the more of your investing that happens inside those accounts, the less investment income is exposed to the 3.8% at all.

The NIIT gets its own one-page form, Form 8960, and it's the friendliest of the three forms in this lesson — it's essentially the lesser-of calculation written out as lines you can follow. Let's read Priya and Raj's so the $1,140 is fully transparent, top to bottom.

Follow it through. Part I — Investment Income simply gathers the pieces: $2,000 of taxable interest, $10,000 of dividends, and $18,000 of net gain from Schedule D, summing to $30,000 of total investment income on line 8. Part II would subtract any investment expenses allocable to that income (they have none material, so nothing changes). Part III — Tax Computation is the lesser-of, spelled out: line 12 carries the $30,000 of net investment income; line 13 is their MAGI, $298,642; line 14 is the $250,000 married threshold; line 15 subtracts to get $48,642 over the threshold; line 16 takes the smaller of line 12 and line 15 — the $30,000; and line 17 multiplies by 3.8% to reach $1,140, with a pointer to Schedule 2, Part II, line 12. That's the entire tax. If you ever want to sanity-check a NIIT number, this form is short enough to read in a minute — and now you can.

The third shadow tax is the twin of the NIIT — same thresholds, similar era, similar simplicity — but it rides on the *other* kind of income. The Additional Medicare Tax is a 0.9% surtax on *earned* income — wages and self-employment income — above $250,000 married filing jointly / $200,000 single / $125,000 married filing separately. It's an extra layer on top of the regular Medicare tax (the 1.45% that's always come out of your paycheck), it was created alongside the NIIT to help fund Medicare, and like the NIIT its thresholds have never been indexed for inflation — they've sat at $250,000/$200,000 since 2013.

The math is the simplest of the three: 0.9% of your earned income above the threshold. For Nina, our Boston physician, it's almost trivial. She's single, earns $310,000 in wages, and the single threshold is $200,000, so she owes 0.9% on the $110,000 above it: 0.9% × $110,000 = $990. That's it — no lesser-of, no parallel system, just a flat 0.9% on the earned income over the line. If the tax were only ever this straightforward, it wouldn't have a scary reputation. But it does, and the reason isn't the math — it's the *withholding*.

Here's the question this tax is famous for: "Why do I owe more Medicare tax at filing than came out of my paychecks all year?" The answer is a genuine design gap — the per-job withholding mismatch — and understanding it is what turns a surprise balance into an expected one. Your employer is *required* to withhold the extra 0.9%, but only on the wages it pays you over $200,000 in the year — and it does this without regard to your filing status, your other jobs, or your spouse. The employer uses a flat $200,000 trigger per job, because that's all it can see. Your *actual* liability, though, depends on your whole household's earned income against your filing-status threshold. Those two numbers rarely match.

Watch how the mismatch plays out three ways, because you'll recognize your own situation in one of them:

• Two earners, each under $200,000 — the classic surprise. A married couple where each spouse earns $150,000 has $300,000 of combined wages, $50,000 over their $250,000 threshold — so they owe 0.9% × $50,000 = $450. But *neither* employer withheld a cent, because each spouse individually stayed under the $200,000 per-job trigger. The whole $450 shows up as a balance due at filing, withheld by nobody.
• One high-wage job, single filer — usually about right. Nina's single $310,000 job: her employer withholds 0.9% on the $110,000 over $200,000 = $990, and because the single threshold is *also* $200,000, that's almost exactly her real liability. A single filer with one big job is the case the system gets right.
• Over-withheld, and you get it back. If you have one job just over $200,000 plus a modest second job, or your single job crosses $200,000 but your total lands you below your threshold in some configuration, the employer may withhold *more* 0.9% than you actually owe — and Form 8959 credits the excess back to you as if it were extra income-tax withholding.

Now Priya and Raj's own number, which shows the self-employment wrinkle. Priya's $185,000 of wages is under the $200,000 per-job trigger, so her employer withheld no extra Medicare tax; Raj is self-employed, so nothing was withheld from him either. But their combined earned base crosses the $250,000 married threshold, so they owe the tax on the excess. The precise figure is small — $163 — because of how the form counts self-employment income (it's reduced to 92.35% before the tax applies, and their wages fill up most of the threshold first). The exact mechanics land on Form 8959, which we'll read next. The point isn't the size — $163 is minor — it's that nobody withheld it, so it arrives as a balance due, and if it's large enough (as it easily can be for two high earners), it can even trigger an underpayment penalty for not having paid it during the year.

Form 8959 is where the Additional Medicare Tax is computed *and* reconciled against what your paychecks withheld — and it's the reconciliation that makes it worth reading, because it's how the surprise-or-refund gets settled. It has five parts; the first four figure the tax, and the fifth does the true-up. Let's read Priya and Raj's.

Walk the five parts. Part I — Wages takes Priya's Medicare wages ($185,000, from W-2 box 5) and compares to the $250,000 married threshold; her wages are under it, so the Additional Medicare Tax on wages is $0. Part II — Self-Employment handles Raj: his self-employment income for this purpose is $83,115 (his $90,000 net, reduced to 92.35% the way self-employment tax always is), and here's the key coordination — the threshold for his self-employment income is reduced by the wages already counted, so it drops from $250,000 to $250,000 − $185,000 = $65,000. That leaves $83,115 − $65,000 = $18,115 of his self-employment income above the reduced threshold, and 0.9% of it is $163. (Wages fill the threshold first; this is what stops the couple from getting *two* full thresholds.) Part III (railroad retirement) is zero for them, and Part IV totals the tax to $163 on line 18, pointing to Schedule 2, Part II, line 11.

Part V — Withholding Reconciliation is the part that answers "why do I owe this?" It looks at the Medicare tax actually withheld on their W-2s (box 6), subtracts the ordinary 1.45% that everyone pays, and whatever's left is the *extra 0.9% that was withheld.* For Priya, box 6 shows only the regular 1.45% of her wages — because she never crossed the $200,000 per-job trigger, her employer withheld $0 of Additional Medicare Tax. So Part V confirms nothing extra was withheld, the entire $163 is due at filing, and there's no offsetting credit to carry to Form 1040 line 25c. Had an employer over-withheld (Nina's near-match, or the over-withheld case), Part V is exactly where that excess would be credited back. This form is the true-up machine — it's why the tax and its withholding, which almost never match during the year, come out even on the return.

Now step back and watch the three taxes land together, because Priya and Raj's return is the rare one that carries all of them — and seeing them side by side is what makes the whole system click. Each was computed on its own form; each flows to Schedule 2; each adds to their tax. Here's the full picture:

Read the geography one more time, because it's the payoff of the map we drew at the start. The AMT sits in Part I of Schedule 2 and feeds Form 1040 line 17, right next to the regular tax — because it *is* a recomputation of the income tax. The two surtaxes sit in Part II ("other taxes," alongside Raj's self-employment tax) and feed Form 1040 line 23 — because they're add-ons, not a re-figuring of the income tax. And the two surtaxes never double-count: the 0.9% rode on the earned income (Priya's wages, Raj's consulting), the 3.8% rode on the investment income, and no dollar was touched by both. Their three shadow taxes total $24,035 on top of their regular tax — real money, but every dollar of it is now explained, and the biggest chunk (the $22,732 AMT) is a prepayment they'll largely recover.

And Nina, our second household, shows how much simpler it usually is: she carries just *two* of the three. Her $310,000 salary owes $990 of Additional Medicare Tax (correctly withheld by her single employer, so no surprise), and her investment income owes the 3.8% NIIT — but with no ISOs and no unusual preferences, she has no AMT at all; her wages are taxed identically under both systems. That's the far more common shape: a high earner brushes one or two of these taxes, not all three, and the ones they brush are the two simple surtaxes. Priya and Raj's all-three return is the exception that lets us see the whole machine; most people who owe anything here owe a clean 3.8% or 0.9% and nothing more.

These three taxes create three characteristic dangers — one is an honest surprise, one is a self-inflicted cash crunch, and one is an outright scam. You don't need to memorize them; you need the tells and the one rule that covers all three.

Maybe you're reading this because one of these already caught you — an ISO exercise triggered an AMT bill you didn't see coming, or you filed and discovered you owed an Additional Medicare true-up nobody had withheld, or a surtax showed up and you're not sure it's even right. Take a breath. These three are genuinely among the least-explained corners of the tax code, and getting surprised by one doesn't mean you did anything wrong or missed something obvious — it means the system handed you a bill it never warned you about. And crucially, almost all of it is either recoverable or fixable.

The single most important reassurance is the one from the AMT credit section, and it's worth repeating because it's the exact thing people panic about: the AMT you paid because of an ISO exercise is largely a prepayment, not a penalty — it comes back to you as a credit in later years. You didn't lose that money; you paid it early, and the tax code hands it back through Form 8801 when your regular tax later exceeds your tentative minimum tax (which the higher AMT basis in your shares helps make happen when you sell). Here's what to do depending on what caught you:

• You owed a big AMT bill from an ISO exercise. First, make sure Form 8801 is generating the minimum tax credit so you actually recover it in future years — this is the step people miss, and the credit doesn't apply itself indefinitely without being tracked. Second, if the exercise went wrong because the stock fell *within the same year*, check whether a disqualifying disposition (selling by December 31) still makes sense to cancel the phantom income. Going forward, model every future exercise before you make it.
• You owed an Additional Medicare true-up you didn't expect. This is normal for two-earner and multi-job households — nobody withheld it because no single employer could see your whole picture. Pay the balance (it's usually modest), and set up extra withholding or estimated payments so next year it's covered in advance and can't trigger a penalty.
• A surtax showed up and you're not sure it's correct. Read the relevant form — Form 8960 for the NIIT, Form 8959 for the Additional Medicare Tax — using the walkthroughs in this lesson. They're short, and now you can trace every line. If the software or preparer got an input wrong (miscounted your investment income, or your MAGI), you fix it by amending with Form 1040-X.
• A notice arrived proposing one of these taxes. If the IRS's computers matched your income and proposed an AMT or surtax you didn't report, that's usually a CP2000 — a proposed correction, not an audit. Read it, check it against the form, and either agree and pay a modest difference or explain why it's already handled. Opening it is far less frightening than the imagined version.

And if what you feel is regret — that you exercised without checking the AMT, or that you could have timed a gain to duck the NIIT — set that down too. These are exactly the moves that even experienced people learn by getting surprised once. You now know the thresholds, the credit, and the levers, which is more than most filers ever learn. The worst outcome here was never the tax itself; it was not understanding it. You're past that.

Help for these three taxes has a natural ladder, and — unlike most of the tax code — this is one area where paying a professional *before* the fact genuinely earns its fee. A large ISO exercise or a complex investment year is precisely when good advice pays for itself, because the mistakes are expensive and the planning is worth real money. Here's the stack, ordered by when to reach for each rung.

1. A CPA or Enrolled Agent — before a large ISO exercise, not after. This is the top of the ladder for a reason, and for these taxes it belongs at the *front* of the process. Before you exercise a big block of options, a professional can model the AMT, find your crossover, plan a multi-year exercise, and tell you the cash you'll need — turning a potential ambush into a decision. The same is true for a complex investment year: someone who models the NIIT and the surtaxes prospectively saves far more than they cost. If you take one thing from this stack: for a big equity-comp event, get the advice *before* you act.
2. The IRS forms and their instructions — free, authoritative, and readable. The instructions for Form 6251 (AMT), Form 8960 (NIIT), and Form 8959 (Additional Medicare Tax) are the plain-language rulebooks, free at IRS.gov, and — as you've seen — the forms themselves are short enough to follow line by line. For the AMT credit, Form 8801 and its instructions are the source. These are what the professionals are reading too; for confirming a number or understanding a line, they beat any secondhand summary.
3. Tax software — for the computation itself. For all three taxes, quality tax software runs the calculations automatically: it generates Form 6251 and tells you if you owe the AMT, computes the NIIT on Form 8960, and does the Form 8959 true-up. If your situation is a straightforward NIIT or Additional Medicare Tax (income over the line, some investment income or two W-2s), software handles it cleanly — the professional is for the *planning*, especially around ISOs, not the arithmetic.
4. The Taxpayer Advocate Service (TAS) — if one of these becomes a dispute you can't resolve. If a notice about one of these taxes spirals, or you're facing a hardship (say, an AMT bill you genuinely can't pay because the ISO shares are illiquid), TAS is the free, independent office inside the IRS that steps in when normal channels stall. And remember that an AMT bill you can't pay in full can go on an installment agreement — you're not required to produce it all at once. TAS is the backstop, not the first stop, but it's there.

The same handful of shadow-tax questions come up again and again. Quick, plain answers, each pointing back to where the fuller story lives in this lesson.

• How do I know if I owe the AMT? Software tells you automatically — if it generated a Form 6251 with a positive line 11, you owe it. By hand, the check is: did you do something unusual (most often, exercise and hold incentive stock options), and is your income high? If neither, the $140,200 (married) / $90,100 (single) exemption almost certainly shields you and you owe nothing.
• I exercised ISOs — will the AMT hit me? Possibly, if you held the shares past year-end and the bargain element (value at exercise minus what you paid, times shares) is large. Exercising and holding is invisible to the regular tax but fully counted by the AMT. Model it before you exercise — the number is knowable in advance — and remember most of any AMT you pay comes back as a credit.
• What is this 3.8% on my investment income? The Net Investment Income Tax — a 3.8% surtax on investment income (interest, dividends, capital gains, rents) once your income tops $250,000 married / $200,000 single. It applies to the lesser of your investment income or the amount you're over the line, so it never reaches more than the investment income actually above the threshold.
• Why do I owe more Medicare tax at filing than was withheld? The per-job withholding mismatch. Your employer withholds the extra 0.9% only on wages *it* pays over $200,000, ignoring your spouse and other jobs — so two-earner couples crossing $250,000 combined often have nothing withheld and owe it at filing. Form 8959 trues it up. Cover it in advance with extra withholding or estimates.
• Do the AMT and the surtaxes ever come back to me? The AMT largely does — when it's caused by a timing item like an ISO, you get it back as a credit (Form 8801) in later years. The two 3.8%/0.9% surtaxes do not come back; they're permanent taxes on income over the threshold. So the AMT is closer to a prepayment; the surtaxes are a true cost.
• Can I avoid the NIIT? You can shrink it, legally: harvest investment losses (which cut the NIIT base and the regular capital-gains tax at once), time income to stay under the threshold, favor municipal bonds (excluded from both the tax and your MAGI), and invest inside tax-advantaged accounts. You can't make it vanish through a promoter's "structure" — those are scams.
• Do I owe both the NIIT and the Additional Medicare Tax? You can, but never on the same dollar. The 0.9% rides on earned income (wages, self-employment); the 3.8% rides on investment income. If you have a lot of both over the threshold — like Priya and Raj — you owe both, but each dollar is hit by at most one.
• Is the AMT credit guaranteed to come back in full? Not guaranteed, and not instant. It's nonrefundable and returns only in years when your regular tax exceeds your tentative minimum tax, so it can trickle back over several years — and if you keep exercising ISOs, it can take a while. Selling the shares (where the higher AMT basis shrinks the AMT gain) is what usually releases it.
• These thresholds — do they rise with inflation? The NIIT and Additional Medicare thresholds ($250,000/$200,000/$125,000) do NOT — they've been frozen since 2013, so the taxes quietly reach further every year. The AMT exemption and its phase-out DO adjust for inflation each year (the 2026 figures are in this lesson).
• Did the 2025 tax law change any of this? For the AMT, yes and in two directions: it made the high exemption permanent (protecting the middle class from a snapback) but lowered the phase-out threshold and doubled its speed (tightening the AMT on high earners and ISO exercisers). The NIIT and Additional Medicare Tax were left unchanged — same rates, same frozen thresholds.

You've seen all three taxes worked out on Priya & Raj and Nina. Now run your own situation through them. The checker below takes a few inputs — your filing status, income, investment income, and any ISO bargain element — and tells you, for each of the three shadow taxes, whether it reaches you and roughly how much: whether your AMT calculation beats your regular tax, your 3.8% NIIT on the lesser-of, and your 0.9% Additional Medicare true-up. It's pre-filled with Priya & Raj's exact figures so you can confirm the lesson's math, then clear it and enter your own.

As you experiment, watch two things. Set your investment income and ISO bargain element to zero and raise your wages: notice that below the threshold all three read "doesn't apply," and that the two surtaxes switch on the moment you cross the line — that's the one-minute self-check made visible. Then add an ISO bargain element and watch the AMT column overtake the regular tax: that's the phantom-income trigger, and it's why you model an exercise before you make it. This is a teaching estimate — it uses the 2026 figures but simplifies the full AMT preference list and skips state tax and the finer edges — but it makes the shape of your own exposure visible, which is the entire point of a lesson about taxes that are supposed to be invisible.

Every term this lesson introduced, in one place — the vocabulary of the three shadow taxes.

• Alternative Minimum Tax (AMT) — a parallel tax system (Form 6251) that recomputes your income without certain deductions, applies a 26%/28% rate, and makes you pay the higher of it or your regular tax.
• Alternative minimum taxable income (AMTI) — the AMT's version of taxable income: your regular taxable income with the standard deduction and AMT preference items added back.
• AMT preference item / adjustment — an item the AMT treats less favorably than the regular tax and adds back to income (the ISO bargain element, the standard deduction, full SALT, private-activity-bond interest, depreciation differences).
• Tentative minimum tax (TMT) — AMTI minus the AMT exemption, taxed at 26%/28%; you owe AMT only to the extent it exceeds your regular tax.
• AMT exemption — a large amount subtracted from AMTI before the rate applies (2026: $140,200 married / $90,100 single); the shield that keeps most people out of the AMT.
• AMT exemption phase-out — the shrinking of the exemption once AMTI passes a threshold (2026: $1,000,000 married / $500,000 single) at 50 cents per dollar over, gone entirely by $1,280,400 / $680,200.
• ISO bargain element — the spread between an incentive stock option's value at exercise and its strike price, times the shares; a regular-tax non-event on exercise-and-hold but a positive AMT preference (Form 6251 line 2i) — "phantom income" with no cash.
• Phantom income — income you're taxed on but received no cash against — the ISO bargain element under the AMT being the classic example.
• Dual basis (ISO shares) — the two cost bases an ISO exercise creates: a regular-tax basis (the strike price) and a higher AMT basis (the value at exercise), so the AMT gain on a later sale is smaller.
• AMT credit / minimum tax credit (Form 8801) — AMT paid because of a deferral item (like an ISO) that comes back as a credit against regular tax in later years when regular tax exceeds the tentative minimum tax; carries forward indefinitely.
• Deferral item vs. exclusion item — AMT from timing differences that reverse (ISO exercise, depreciation) generates the credit; AMT from permanently-lost deductions (standard deduction, SALT) does not.
• Disqualifying disposition — selling ISO shares in the same year you exercised; no AMT preference arises and the bargain element is taxed as ordinary income instead — a common way to sidestep the AMT hit.
• Net Investment Income Tax (NIIT) — a 3.8% surtax (Form 8960) on the lesser of your net investment income or the amount your MAGI exceeds $250,000 married / $200,000 single / $125,000 married filing separately (thresholds fixed since 2013).
• Net investment income — interest, dividends, capital gains, rents, royalties, non-qualified annuities, and passive-business income; NOT wages, self-employment income, active-business income, retirement distributions, Social Security, or muni interest.
• MAGI (for the NIIT) — modified adjusted gross income: your AGI plus the foreign earned income exclusion; for filers without foreign income, simply your AGI.
• Additional Medicare Tax — a 0.9% surtax (Form 8959) on wages and self-employment income above $250,000 married / $200,000 single / $125,000 married filing separately, on top of regular Medicare tax, with no employer match.
• The per-job withholding mismatch — an employer withholds the 0.9% only on the wages it pays over $200,000, ignoring filing status, other jobs, and a spouse — so two-earner couples are routinely under-withheld and get a surprise trued up on Form 8959.
• Form 8959 Part V (withholding reconciliation) — the part that compares the extra 0.9% actually withheld against what's owed, settling the surprise-or-refund on the return.