How Much AMT Will an ISO Exercise Cost You? (w/Examples) + FAQs

This article reflects federal rules and California state rules as of June 2026 and covers tax years 2025 and 2026. Tax law changes — confirm current figures before you file.

Quick Answer

Your AMT cost on an ISO exercise is roughly 26% to 28% of your bargain element (fair market value minus strike price) that exceeds your AMT exemption — $90,100 single or $140,200 married filing jointly for 2026. Exercise 10,000 shares with a $50 spread, and your AMT bill can top $100,000.

When you exercise an incentive stock option (ISO) and hold the shares past December 31, the gap between the stock’s value and what you paid becomes “phantom income” for the alternative minimum tax — a real tax bill on a paper gain you can’t spend. You owe it at filing even though you sold nothing and received no cash.

The stakes are high and the timing is unforgiving. Once the calendar year closes, the AMT is locked in no matter what happens to the stock price afterward — a lesson thousands learned in the 2000 dot-com crash when they owed huge AMT on shares that later went worthless.

💵 How to calculate your exact AMT bill before you exercise a single share.
📉 How to find your “crossover point” — the most shares you can exercise tax-free.
🔁 How to get your AMT money back later through the minimum tax credit.
🌴 How California piles its own 7% AMT on top of the federal hit.
⚠️ The 7 mistakes that turn a planned exercise into a six-figure surprise.

What an ISO Exercise Actually Triggers

An incentive stock option (ISO) is a right your employer grants you to buy company stock at a fixed “strike price.” Exercising means you pay that strike price and become a shareholder. For regular federal income tax, exercising an ISO and holding the shares is a non-event — you report nothing and pay nothing that year.

The catch lives in a parallel tax system called the alternative minimum tax (AMT). Under IRC Section 56(b)(3), the “bargain element” of your exercise — the fair market value (FMV) at exercise minus your strike price — is an AMT “preference item.” It gets added to your income for AMT purposes in the year you exercise, even though no cash changed hands.

The consequence is direct: you can owe tens or hundreds of thousands of dollars in tax on a gain you haven’t touched. The bargain element formula is simple — (FMV − Strike Price) × Shares Exercised — but the resulting AMT can be brutal for pre-IPO employees who can’t sell shares to raise the cash.

A common misconception is that exercising ISOs is “tax-free” because nothing appears on your W-2. It isn’t. The IRS receives a copy of Form 3921 from your employer reporting the exercise, and its matching system flags returns that leave the preference off Form 6251.

What you should do about it: before you exercise, pull your most recent 409A valuation (or the public share price), subtract your strike price, multiply by the shares you plan to exercise, and run the AMT math in the next section. Do this before clicking “exercise,” not after the year closes.

How the AMT Calculation Works, Step by Step

The AMT is a separate tax system that runs alongside your regular tax. You compute your tax two ways and pay whichever is higher. The difference, when AMT is higher, is the extra tax you owe.

Here is the sequence the ESO Fund AMT guide and Form 6251 both follow:

Compute your regular federal tax on your normal income, ignoring the ISO exercise entirely.
Compute your alternative minimum taxable income (AMTI) — your taxable income plus the ISO bargain element plus other preference items.
Subtract the AMT exemption ($90,100 single / $140,200 joint for 2026).
Apply the AMT rate — 26% on the first $244,500 of remaining AMTI (2026), then 28% above that — to get your tentative minimum tax (TMT).
Compare. If TMT is higher than regular tax, the gap is your AMT.

The reason this matters is that long-term capital gains and qualified dividends keep their favorable 0/15/20% rates inside the AMT system, but the ISO bargain element does not — it is taxed at the full 26% or 28%. That single rule is why ISO exercises are, per Charles Schwab, the single most common AMT trigger for employees.

The consequence of getting step 2 wrong is an IRS CP2000 notice proposing additional tax plus interest, because the agency already has your Form 3921. What you should do: enter your exercise into tax software twice — once saying “no” to the ISO question to capture your baseline, then “yes” to see the AMT delta. The difference is your AMT cost.

2025 vs. 2026: The Numbers You Must Anchor To

The figures changed between tax years, and the One Big Beautiful Bill Act (OBBBA) made the higher exemptions permanent while resetting the phase-out zone. Anchoring to the wrong year produces a wrong bill.

For tax year 2025, the AMT exemption was $88,100 for single filers and $137,000 for married filing jointly, per the 2025 inflation adjustments. For tax year 2026, the exemption rose to $90,100 single and $140,200 joint, with the 26%/28% rate breakpoint at $244,500.

The bigger 2026 story is the phase-out reset. Beginning in 2026, the exemption starts phasing out at $500,000 of AMTI for single filers and $1,000,000 for joint filers — much lower than the prior near-million single threshold — which pulls more high earners into AMT. There is also industry debate over whether OBBBA doubled the phase-out rate from 25% to 50% of each dollar above the floor; the underlying IRC Section 55(d) phase-out has historically been 25%, so confirm the finalized 2026 rate with a professional before relying on it.

2025 vs. 2026 Federal AMT Figures	Amount
2025 exemption — single (IRS)	$88,100
2025 exemption — married filing jointly	$137,000
2026 exemption — single (Reed Corp)	$90,100
2026 exemption — married filing jointly	$140,200
2026 exemption — married filing separately	$70,100
2026 rate breakpoint (26% below / 28% above)	$244,500
2026 phase-out floor — single / joint	$500,000 / $1,000,000

Which Situation Applies to You?

The right move depends on where you are in the ISO journey. Find your row below and read the matching section.

You haven’t exercised yet and want to plan. Read “Finding Your AMT Crossover Point” — you can likely exercise some shares tax-free every year.
You’re at a private, pre-IPO company with no way to sell. You face the highest risk, because a same-year sale isn’t an option. Focus on partial exercises and the December/January split.
You’re at a public company or post-IPO. A same-year sale (“disqualifying disposition”) can erase the AMT entirely by converting the spread to ordinary income.
You already exercised and now face a surprise bill. The AMT is owed, but read “Getting Your Money Back” — it becomes a minimum tax credit you can recover.
You’re a high earner above $500,000 (single) or $1,000,000 (joint). The 2026 phase-out shrinks your exemption, raising your effective marginal rate toward 35%.

The consequence of misreading your situation is large. A pre-IPO employee who assumes a same-year sale will save them — when no market exists — can be stuck with an AMT bill and no cash to pay it. What to do: confirm whether your shares are sellable this calendar year before you exercise, because that single fact changes the entire strategy.

Worked Example 1: A Single Filer at a Startup (2026)

Meet Priya, a single software engineer at a Series B startup. She earns $200,000 in W-2 wages and exercises 50,000 ISOs at a $0.50 strike when the 409A valuation is $4.50 per share. She plans to hold the shares.

Here is her math, following the ESO Fund method:

Bargain element: ($4.50 − $0.50) × 50,000 = $200,000.
Regular taxable income: $200,000 wages − $15,750 standard deduction ≈ $184,250; regular federal tax ≈ $36,000.
AMTI: $185,000 + $200,000 spread = $385,000.
Subtract 2026 exemption: $385,000 − $90,100 = $294,900 (no phase-out — she’s under $500,000).
Apply AMT rates: 26% × $244,500 = $63,570, plus 28% × $50,400 = $14,112. TMT = $77,682.
AMT due: $77,682 − $36,000 = $41,682.

Priya owes about $41,682 in federal AMT at filing — on stock she cannot sell. The consequence is a real cash demand for a paper gain, which is why she should set aside that cash before exercising or consider exercising fewer shares.

Worked Example 2: A Married Couple With a Big Spread (2026)

Meet David and Maya, married filing jointly. David earns $250,000, and he exercises 10,000 ISOs at a $1 strike when his company’s value hits $51 per share, holding the shares through year-end.

Bargain element: ($51 − $1) × 10,000 = $500,000.
Regular taxable income: roughly $215,000 after deductions; regular federal tax ≈ $40,000.
AMTI: $215,000 + $500,000 = $715,000 — above the $560,800 point where the joint exemption fully phases out, so little to no exemption survives.
TMT: roughly 28% × $715,000 ≈ $200,200.
AMT due: $200,200 − $40,000 = about $160,200.

This scenario from Reed Corp shows how fast AMT scales with the spread. A $500,000 bargain element produces a six-figure AMT bill. The lesson: a large one-year exercise is the most expensive way to do it, and spreading the exercise over several years would have cut the annual hit sharply.

Worked Example 3: The Phantom-Income Cautionary Tale

Meet Carlos, a single filer who exercised 30,000 ISOs in early 2025 at a $2 strike when the FMV was $42, creating a $1.2 million bargain element. He held the shares hoping for an IPO and owed roughly $330,000 in AMT for tax year 2025.

Then the startup’s funding fell through, and by the time he could sell, the shares were worth $3 each. Carlos paid hundreds of thousands in tax on value that evaporated — the exact trap the dot-com crash created for thousands in 2000 and 2001.

The consequence of holding illiquid, volatile shares after exercise is permanent: the AMT is owed for the year of exercise regardless of what the stock does next. The recovery (a minimum tax credit) helps, but it can take a decade and only works if he has future regular tax to absorb it. What Carlos should have done: sell enough shares in a same-year disqualifying disposition to fund the tax, or exercise a smaller block.

Finding Your AMT Crossover Point

The “crossover point” is the largest bargain element you can add to your income before AMT kicks in — the sweet spot where your tentative minimum tax just equals your regular tax. Exercise up to that line and you owe zero extra AMT, per SmartAsset.

To find it, use tax software the way the ESO Fund recommends. First, enter your return with no ISO exercise and note your regular tax. Then add an ISO exercise and slowly increase the number of shares until your total tax just begins to rise above that baseline. The share count right before the increase is your crossover point.

The reason this works is the exemption buffer. The first $90,100 (single, 2026) of AMTI above your regular taxable income is shielded, plus there’s headroom while your TMT stays under your regular tax. The consequence of ignoring the crossover is paying AMT you could have legally avoided by exercising fewer shares.

What you should do: calculate your crossover every year and exercise just up to it, then repeat next year. Over several years you can convert a large grant to long-term-capital-gain-eligible shares while paying little or no AMT along the way.

Getting Your AMT Money Back: The Minimum Tax Credit

AMT on an ISO exercise is not a permanent loss — it’s a prepayment. Under IRC Section 53, the AMT you pay generates a “minimum tax credit” you can use against regular tax in future years when your regular tax exceeds your tentative minimum tax.

This works because you carry two cost bases after exercise. Your regular tax basis is the strike price, and your AMT basis is the FMV at exercise. When you eventually sell, the difference between the two creates a negative AMT adjustment that lets the credit flow back to you, claimed on Form 8801.

The catch is timing and refundability. The credit is not refundable — it can only offset regular tax, not generate a check. The consequence: if you take a low-income year (sabbatical, residency, early retirement), there may be no regular tax to absorb it, and the credit sits unused, though it carries forward indefinitely. What to do: track your AMT basis and credit carryforward every year, and time future income so you have regular tax available to soak up the credit.

California’s Own AMT Makes It Worse

Most states have no AMT, but California does — and it hits ISO holders hard. California imposes a 7% AMT rate on top of the federal hit, computed on California Schedule P (540).

This matters because federal rules start the analysis, but California runs its own parallel calculation with its own (lower) exemptions and its own treatment of the ISO bargain element. A California engineer with a large spread can owe federal AMT and California AMT in the same year, stacking the two bills. Per ESO Fund, Iowa, Minnesota (5.8%), and Colorado also impose state-level AMT, while no-income-tax states like Texas, Florida, Washington, and Nevada impose none.

The consequence of forgetting the state layer is a serious cash shortfall at filing. Priya from Example 1, if she lives in California, could owe several thousand dollars more in state AMT on top of her $41,682 federal bill. What to do: if you live in California or another AMT state, run your state’s AMT form alongside Form 6251 before exercising, and budget for both.

Three Common Scenarios

These three patterns from the research cover most ISO situations.

Exercise Strategy	What It Costs You
Exercise and hold past year-end at a private company	Full AMT on the bargain element, due at filing; recoverable later as a minimum tax credit, but you need cash now (ESO Fund)
Exercise and sell in the same calendar year (disqualifying disposition)	No AMT; the spread is taxed as ordinary income on your return instead, so you give up long-term capital gain treatment (ESO Fund)
Exercise just up to your crossover point each year	Little or no AMT; you spread a large grant over several tax years and keep long-term gain potential (SmartAsset)

How to Report It: Form 3921, Form 6251, and Form 8801

Three IRS forms run the ISO-AMT lifecycle, and each has a deadline and a consequence for getting it wrong.

Form 3921 is the informational form your employer sends you by January 31 of the year after you exercise. It reports your strike price, the FMV at exercise, and the number of shares. The consequence of ignoring it: the IRS has the same copy and will issue a notice if your Form 6251 doesn’t match.

Form 6251 is where the AMT is actually computed and filed with your Form 1040 by the April 15 deadline. You enter the bargain element on the ISO line, the form applies your exemption and phase-out, computes TMT, and compares it to regular tax. Tax software generates it automatically once you enter the exercise.

Form 8801 is how you later claim back the minimum tax credit against regular tax. You file it in years after the AMT year, tracking the carryforward until it’s used up. The consequence of skipping it is leaving recoverable tax on the table — one of the most-missed items on self-prepared returns.

Mistakes to Avoid

Exercising your entire grant in one year. A huge single-year bargain element wipes out your exemption and can produce a six-figure AMT bill, as David and Maya’s $160,200 example shows.
Forgetting cash for the tax. You owe AMT at filing even though you sold nothing, so an illiquid exercise can leave you unable to pay.
Ignoring Form 3921. The IRS matches it; leaving the spread off Form 6251 reliably triggers a CP2000 notice with proposed tax and interest.
Assuming the regular cost basis is the AMT basis at sale. Using the wrong basis overstates your gain and overpays tax when you finally sell.
Holding volatile, illiquid shares after exercise. A price crash leaves you taxed on phantom value that disappeared, like Carlos.
Forgetting the minimum tax credit. Not filing Form 8801 in later years leaves recoverable AMT unclaimed, sometimes for a decade.
Overlooking your state’s AMT. California’s 7% AMT (plus Iowa, Minnesota, Colorado) stacks on the federal bill and surprises filers who budgeted federal-only.
Crossing the 2026 phase-out floor blindly. Above $500,000 single / $1,000,000 joint, your shrinking exemption pushes the effective marginal rate toward 35%.

Do’s and Don’ts

Do calculate your AMT before exercising — run the two-way math so the bill is never a surprise.
Do find your crossover point each year — you can often exercise a meaningful block tax-free.
Do keep two basis records — strike price for regular tax, FMV at exercise for AMT, so your eventual sale is correct.
Do set aside cash for the tax — because AMT is due at filing whether or not you sold shares.
Do file Form 8801 in later years — to recover the AMT credit you prepaid.
Don’t exercise everything at once — concentrated spreads trigger the largest bills.
Don’t assume your state has no AMT — confirm, because California and a few others do.
Don’t hold illiquid shares you can’t afford to lose — a crash leaves you taxed on vanished value.
Don’t ignore the December/January split — it can double your exemption headroom across two years.
Don’t go it alone above six figures — the cost of a planning mistake routinely exceeds the cost of a CPA.

Pros and Cons of Exercising ISOs Early

Pro — smaller spread. Exercising when FMV is close to strike means little or no AMT, because the bargain element is tiny.
Pro — starts the clock. Early exercise begins your long-term capital gains and ISO holding periods sooner.
Pro — credit recovery. Any AMT you do pay becomes a future minimum tax credit, not a permanent cost.
Pro — lower effective rate. AMT’s 26–28% rate is below the 37% top ordinary rate, so the spread is taxed favorably versus a disqualifying sale.
Pro — control. Spreading exercises lets you manage AMT to near zero across several years.
Con — cash at risk. You spend real money on strike price and possibly AMT for shares you can’t sell.
Con — phantom-income danger. If the stock falls, you’ve paid tax on a gain that disappeared.
Con — credit may sit unused. A low-income year can leave the AMT credit stranded with no regular tax to offset.
Con — complexity. Dual basis tracking and Form 8801 carryforwards are easy to mishandle.
Con — state stacking. In California and similar states, you owe a second AMT on top of the federal one.

What to Do Next

Pull your numbers. Get your current 409A valuation or share price, your strike price, and your share count, and compute (FMV − Strike) × Shares.
Run the two-way tax math. Use tax software to find your baseline regular tax, then add the exercise to see your AMT delta.
Find your crossover point. Increase the share count until tax just rises above baseline — exercise up to that line tax-free.
Budget the cash. Set aside both the strike-price cost and the AMT (federal plus any state) before you exercise.
Gather records. Keep Form 3921, your exercise confirmation, and your 409A documentation for dual-basis tracking.
Call a professional when the spread tops six figures, the shares are illiquid, or you live in an AMT state like California. A CPA or tax attorney will model multi-year exercise, same-year-sale, and credit-recovery scenarios — work that routinely saves far more than it costs.

This article is educational and is not a substitute for advice from a licensed tax professional who knows your specific situation.

Frequently Asked Questions

What is the 2026 AMT exemption amount?

$90,100 for single filers and $140,200 for married filing jointly for tax year 2026. Married filing separately is $70,100. This amount is subtracted from your AMTI before the 26%/28% rate applies, per the 2026 figures.

How much AMT will I owe on my ISO exercise?

Roughly 26% to 28% of the bargain element that exceeds your exemption, minus your regular tax. Multiply (FMV − strike) by your shares, subtract the exemption, and apply the AMT rates to estimate your bill.

Do I owe AMT if I exercise and hold ISOs?

Yes, if the bargain element is large enough to push your tentative minimum tax above your regular tax. Holding past December 31 makes the spread an AMT preference item under IRC Section 56(b)(3).

Can I avoid AMT on my ISOs entirely?

Yes, by exercising only up to your crossover point each year, or by selling in the same calendar year. A same-year sale is a disqualifying disposition that converts the spread to ordinary income and erases the AMT, per ESO Fund.

Is the AMT I pay on ISOs gone forever?

No. It becomes a minimum tax credit under IRC Section 53 that you recover against future regular tax using Form 8801, though recovery can take years.

When do I pay the AMT on an ISO exercise?

At tax filing for the year you exercised — by April 15 of the following year. You owe it even though you sold no shares and received no cash, which is the core phantom-income problem.

Does California charge AMT on ISO exercises?

Yes. California imposes its own 7% AMT on Schedule P (540), stacking on top of the federal AMT. Iowa, Minnesota, and Colorado also have state AMTs; most other states do not.

What form reports my ISO exercise?

Form 3921, sent by your employer by January 31, reports the exercise details. You then compute the AMT on Form 6251 with your Form 1040. The IRS receives a matching copy of Form 3921.

What was the AMT exemption for 2025?

$88,100 single and $137,000 married filing jointly for tax year 2025, per the IRS inflation adjustments. The 2026 amounts rose to $90,100 and $140,200.

When does the AMT exemption phase out in 2026?

Above $500,000 of AMTI for single filers and $1,000,000 for joint filers in 2026. Your exemption shrinks for each dollar above the floor, raising your effective marginal rate, per Reed Corp.

What is a disqualifying disposition?

Selling ISO shares in the same year you exercised (or before meeting holding periods). It eliminates the AMT preference, taxing the spread as ordinary income instead, which removes double-taxation risk, per ESO Fund.

Should I hire a professional for ISO planning?

Yes, when your spread tops six figures, your shares are illiquid, or you live in an AMT state. A CPA or tax attorney models multi-year exercise and credit-recovery scenarios; the cost of a mistake routinely runs into six figures.

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