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.
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.
The 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.
That bargain element is what makes an ISO exercise expensive in a year you get nothing back.
AMT on a paper gain is the classic equity-comp trap, and the year you exercise is the only year you can control it. The advisers below pay to meet people at exactly this decision. It is free to you, and it is not the only way to find an adviser.
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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.