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Updated July 28, 2026. Quick answer: Exercising an ISO costs you no regular income tax but adds the full spread — shares times (value minus strike) — to alternative minimum taxable income under IRC §56(b)(3). That is phantom income on shares you may not be able to sell.
IRS worksheets read line by line: AMT Exemption Worksheet.
Related reading on this site: IRS Tax Topic 427.
More IRS forms and schedules, read line by line: Form 8801 instructions.
Foreign tax credit (Form 1116) and alternative minimum tax (Form 6251) for 2025, read line by line: Form 6251 line 2i.
Schedule 2 (Form 1040) for 2025, read line by line: Schedule 2 lines 2 and 3.
More IRS forms and schedules, read line by line: Form 15620 instructions.
The IRS form or worksheet behind this, read line by line: Form 6251 instructions.
Exercising an incentive stock option triggers no regular income tax. It does create an alternative minimum tax preference item equal to the spread between what you pay and what the shares are worth — income you never received, on shares you may not be able to sell.
Why this is the number that matters
The bargain element is the whole reason ISOs are complicated. Regular tax ignores your exercise entirely. AMT does not: under IRC §56(b)(3) the spread is added to alternative minimum taxable income in the year you exercise, even though you have received no cash and may hold shares in a private company you cannot sell.
What this tool deliberately does not do
It does not guess your AMT exemption. That figure is indexed annually and phases out at higher income, so a hardcoded number would be wrong for many readers and stale for all of them within a year. It also does not compute whether you actually owe AMT — you pay it only to the extent tentative minimum tax exceeds your regular tax, which requires your whole return.
What it gives you is the input everything else runs on, computed correctly.
Your calculation
Your bargain element
The spread between the fair market value and the strike price you entered, multiplied by the shares. That is the preference item added to alternative minimum taxable income under IRC 56(b)(3). Whether you actually owe AMT depends on your whole return.
Want help sizing the exercise? See the adviser option.Optional next step
Want help sizing the exercise?
How many to exercise, and in which tax year, is the real decision, and it turns on the rest of your return. If you want that conversation, you can request an adviser match.
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How our partner relationships workThe disaster case, and how to avoid it
Exercise early-year at a high valuation, the company falls, you sell the following year for less than the AMT you already owe. The AMT is computed at exercise-date value and does not adjust downward. This is the single most damaging pattern in equity compensation, and the defence is unglamorous: size exercises against the exemption, and consider exercising in tranches across tax years rather than all at once.
Figures verified against 26 U.S.C. §55, §56(b)(3), §422 and §53 and Rev. Proc. 2025-32 section 3.10 (the $140,200 joint exemption and 50% phaseout for 2026) on September 30, 2026.
Sources
IRC §56(b)(3) (ISO spread as AMT preference); IRC §55(b)(1)(A) (26%/28% AMT rates); IRC §422; IRC §53 (minimum tax credit).
This states what the cited authority says. It is not tax advice; AMT in particular is computed across your whole return and cannot be resolved from one page.
Related
The question the other way round: how many shares fit under AMT this year — the free band created by the $140,200 joint exemption, and where the 50% phaseout starts eating it.