AMT on ISO Exercise: Calculation, Credit, and the $100K Limit

Exercising an incentive stock option and holding the shares is invisible to your regular tax return, but the alternative minimum tax treats the spread between your strike price and the stock’s fair market value as income in the year you exercise. If that spread is large enough to push your AMT calculation above your regular tax, you owe the difference in cash by April 15 of the following year, before you’ve sold a single share. The AMT on an ISO exercise is recoverable over time through a credit, but recovery is slow and the up-front liquidity hit is real.¹

What Triggers the AMT Bill

Section 56(b)(3) of the Internal Revenue Code strips out the favorable ISO treatment that Section 421 provides under the regular tax system. For AMT purposes, the entire spread between the stock’s fair market value on the exercise date and the strike price you paid counts as income in the year of exercise.² This spread is the bargain element. Strike at $10, stock worth $60, 5,000 shares exercised: you’ve added $250,000 to your AMT income for the year.

For public stock, fair market value is the closing price on the exercise date. For a private company, it’s typically the most recent 409A valuation. Your employer reports the exercise on Form 3921, showing grant date, exercise date, strike price, fair market value per share, and share count, and must send it to you by January 31 of the following year.³

How the AMT Calculation Works

The AMT runs alongside your regular tax return. You pay whichever is higher, and the excess of the tentative minimum tax over the regular tax is what shows up on your return as AMT.¹

You start with regular taxable income, add the ISO bargain element and other AMT adjustments to arrive at alternative minimum taxable income (AMTI), then subtract an exemption. For 2026, the exemption is $90,100 for single filers and $140,200 for joint filers. It phases out at 50 cents per dollar once AMTI exceeds $500,000 (single) or $1,000,000 (joint), so a large exercise can eliminate it entirely. The remaining amount is taxed at 26% up to $244,500 and 28% above that.⁴

A Worked Example

Single filer, $150,000 regular taxable income, exercises 10,000 ISOs at a $5 strike when the stock is $55. The $500,000 bargain element pushes AMTI to roughly $650,000. That’s $150,000 above the $500,000 phaseout, cutting the exemption by about $75,000 and leaving roughly $15,100 of it. Taxable excess for AMT is about $634,900: 26% on the first $244,500 is $63,570, plus 28% on the remaining $390,400 is $109,312, for a tentative minimum tax around $172,882. If regular tax is about $30,000, the AMT owed on top is roughly $142,882.

That’s due April 15, and no shares have been sold to produce the cash.⁵

Paying the Bill on Time

Nothing is withheld from an ISO exercise. Your employer does not take taxes out of your paycheck or the exercise itself. You are responsible for delivering the money to the IRS, and waiting until you file the following April can trigger underpayment penalties.

Federal estimated payments are due April 15, June 15, September 15, and January 15 of the following year.⁶ Exercise in July and you need to bump the September 15 payment to cover the added AMT. You can sidestep the penalty by paying at least 110% of your prior-year total tax through withholding and estimates if your prior-year AGI was over $150,000 ($75,000 if married filing separately). Schedule AI of Form 2210 (the annualized income installment method) lets you match payments to when the income actually hit, which helps when a single exercise concentrates your income in one quarter.

When the Stock Falls Before Year-End

This is where ISO holders get hurt. You exercise in March at a $55 fair market value, the stock drops to $12 by December, and your AMT bill is still calculated on the March spread. The tax on paper gains that no longer exist can exceed what the shares are worth.

Before December 31, there is one way out: sell the shares in the same calendar year you exercised. A same-year disqualifying disposition eliminates the AMT adjustment because the exercise and sale are treated identically for regular and AMT purposes in that scenario.² You’ll pay ordinary income tax on the actual gain you realized, not the phantom gain from March, and you won’t face a tax bill larger than what the shares are worth. Once January 1 arrives, this escape closes.

Watch the wash sale rule if you plan to buy back in. Selling at a loss and repurchasing within 30 days disallows the loss and undoes the benefit of the same-year disposition.

Same-Year Sale as a Deliberate Strategy

Even without a price drop, exercising and selling in the same year on purpose is a legitimate way to avoid the AMT trap. You give up long-term capital gains treatment and pay ordinary rates on the bargain element, but you skip the AMT adjustment, avoid paying tax on illiquid shares, and don’t tie up cash in a multi-year credit recovery.²

This can be the right call when the bargain element is large enough that AMT would eat most of the capital gains rate savings, when you don’t have cash to cover the AMT while holding, or when the stock is volatile. Splitting the exercise, holding some shares and selling others in the same year, lets you hedge between the two outcomes.

Two Cost Bases and the AMT Credit

After a hold-past-year-end exercise, each share carries two cost bases. Regular tax basis is your strike price. AMT basis is the fair market value on the exercise date.² In the earlier example, that’s $5 versus $55. When you sell, the regular-tax gain will be $50 per share larger than the AMT gain, and that gap is what unwinds the AMT you already paid.

The mechanism is the AMT credit, tracked on Form 8801 and carried forward indefinitely.⁷ In any future year where your regular tax exceeds your tentative minimum tax, you can claim a credit equal to that gap, up to your remaining balance. The usable amount lands on Schedule 3 of Form 1040. The ISO bargain element is a “deferral” item, which is why it generates a recoverable credit; AMT paid on “exclusion” items does not.

The credit doesn’t expire, but it doesn’t earn interest either. Large exercises often take five or more years to fully recover, and a dollar recovered eight years from now is worth less than the dollar you paid today. Model recovery time when you’re deciding whether to exercise and hold.

What Happens When You Finally Sell

The tax result at sale depends on holding periods. A qualifying disposition requires more than two years from the grant date and more than one year from the exercise date, both.⁸

Meet both and the entire gain from strike to sale price is long-term capital gain. The 2026 top rate is 20%, hitting single filers above $545,500 of taxable income and joint filers above $613,700; most sellers pay 15%.⁹ The AMT gain on the same sale is smaller because AMT basis is higher, which is typically the year you start pulling down the credit.

Miss either holding period and it’s a disqualifying disposition. The bargain element (the lesser of the spread at exercise or the actual gain at sale) becomes ordinary income taxed at rates up to 37%, with any additional gain above exercise-date value treated as capital gain.⁴

Long-term capital gain from a qualifying disposition can also be hit with the 3.8% net investment income tax, which applies to the lesser of net investment income or the amount your modified AGI exceeds $200,000 single or $250,000 joint. Those thresholds have not been indexed since 2013.¹⁰ The real ceiling on long-term ISO gains is 23.8%, not 20%.

The $100,000 First-Exercisable Limit

Not every option your employer labels as an ISO will actually be treated as one. If ISOs with aggregate fair market value at grant of more than $100,000 first become exercisable in the same calendar year, the excess is automatically reclassified as nonqualified stock options.¹¹ The NSO portion generates ordinary income and employment taxes on the spread at exercise and creates no AMT adjustment, because it’s already taxed under the regular system.

The trigger is when the options first become exercisable, not when you exercise them. If $150,000 becomes exercisable in 2026, the first $100,000 (in grant-date order) stays ISO and the remaining $50,000 is NSO. Employers usually track this, but verify the classification on your Form 3921 before you file.

Forms and Records

An exercise that triggers AMT requires Form 6251 in the year of exercise. The bargain element is added there, the tentative minimum tax is computed, and the AMT amount flows to Schedule 2 of Form 1040.¹² Every year afterward until the credit is used up, you file Form 8801 to track and claim the carryforward, with the usable amount landing on Schedule 3.⁷

Keep the following until the credit is fully recovered or you’ve sold all the shares, whichever comes later:

Form 3921 from your employer, showing grant date, exercise date, strike price, fair market value, and share count.³
Exercise confirmations from your brokerage or plan administrator.
Both cost bases for every lot: regular tax (strike) and AMT (fair market value at exercise).
Every prior-year Form 8801, showing the running credit balance.

Losing these documents years later makes it nearly impossible to prove your AMT credit balance or your basis at sale. Multiple exercises across years each create their own pair of bases and their own contribution to the credit pool, so a per-lot spreadsheet is the minimum level of organization this deserves.

1
Office of the Law Revision Counsel. 26 USC 55 – Alternative Minimum Tax Imposed
2
Office of the Law Revision Counsel. 26 USC 56 – Adjustments in Computing Alternative Minimum Taxable Income
3
Internal Revenue Service. Form 3921 – Exercise of an Incentive Stock Option Under Section 422(b)
4
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
5
Internal Revenue Service. About When to File
6
Internal Revenue Service. Estimated Tax
7
Internal Revenue Service. Form 8801 – Credit for Prior Year Minimum Tax for Individuals, Estates, and Trusts
8
Office of the Law Revision Counsel. 26 USC 422 – Incentive Stock Options
9
Internal Revenue Service. Topic No. 409 – Capital Gains and Losses
10
Internal Revenue Service. Topic No. 559 – Net Investment Income Tax
11
eCFR. 26 CFR 1.422-4 – $100,000 Limitation for Incentive Stock Options
12
Internal Revenue Service. Form 6251 – Alternative Minimum Tax, Individuals