The ISO AMT Trap: How Incentive Stock Options Trigger AMT, and How to Plan Around It

An ISO exercise produces no regular income tax, and that is exactly why it catches people. The alternative minimum tax counts what the regular tax ignores, and it can arrive with no cash to pay it.

Incentive stock options are prized for one reason: exercise, hold and sell in a qualifying disposition, and the entire gain is taxed at long-term capital gains rates rather than as ordinary income. At exercise, there's no regular income tax at all. That combination leads many holders to treat an ISO exercise as a tax-free event.

It isn't. The tax code runs a second, parallel system alongside the regular income tax, the alternative minimum tax, and in that system the spread at exercise counts as income. A large ISO exercise can create a five- or six-figure AMT bill in a year when no shares were sold and no cash came in.

Key points

• The ISO spread at exercise, value minus strike, is an AMT adjustment even though it isn't regular income.

• You pay AMT only when your tentative minimum tax exceeds your regular tax. The gap between the two is your "AMT room."

• AMT paid because of an ISO exercise usually creates a credit you can recover in later years.

• For 2026, the AMT exemption is $140,200 joint ($90,100 single), and it now phases out twice as fast above $1 million joint ($500,000 single).

• California runs its own AMT, with its own rules and its own credit.

How the AMT actually works

Every year, you effectively compute your tax twice. The regular calculation is the one everyone knows. The AMT calculation starts from the same income, adds back certain items (the ISO spread is the big one for executives), subtracts an exemption, and applies flat rates of 26% and 28%. The result is your tentative minimum tax. If it's higher than your regular tax, you pay the difference as AMT.

That framing explains the planning. Most high earners have some room each year: their regular tax exceeds their tentative minimum tax by some amount. An ISO exercise uses up that room, dollar for dollar of spread. Exercise within the room and you pay no AMT at all. Exercise past it, and every additional dollar of spread is taxed at roughly 26% to 28% federally, now.

The first question isn't "should I exercise?" It's "how much AMT room do I have this year, and what else is already using it?"

What changed for 2026

The 2025 federal tax law kept the higher AMT exemption permanently, but reset the income level where the exemption begins to phase out to $1 million for joint filers ($500,000 single) and doubled the phase-out rate, from 25 cents to 50 cents per dollar. For a large exercise, that means the exemption can disappear faster than it used to, and the effective AMT rate inside the phase-out range climbs above 28%.

Why the trap closes

Three features make the AMT from an ISO exercise especially costly when it isn't planned:

1. It's owed in cash, with no sale. An exercise-and-hold produces tax due in April with no proceeds to pay it. Households that haven't reserved for it sometimes have to sell other assets, or the shares themselves, at a bad time.

2. The stock can fall after exercise. AMT is based on the value at exercise. If the price drops before you sell, you may have paid tax on a gain that no longer exists. A disqualifying sale in the same calendar year can undo the AMT adjustment, but once the year ends, that option is gone.

3. The credit comes back slowly. The minimum tax credit is recovered only in years when your regular tax exceeds your tentative minimum tax. Exercise more ISOs in those years, and you block your own recovery.

The planning moves

Measure the room before you exercise. A projection of your regular tax and tentative minimum tax for the year, including salary, bonus, RSU vesting and any other income, shows exactly how much spread you can absorb at no AMT cost.

Exercise in tranches across years. Filling each year's room, rather than exercising everything at once, often converts what would have been a large AMT bill into little or none, while still starting the holding-period clock on a meaningful block of shares.

Exercise early in the year. An exercise in January gives you most of a year to watch the stock. If it falls sharply, a disqualifying sale before December 31 can eliminate the AMT adjustment. An exercise in December gives you no such window.

Choose the lots deliberately. Each grant has its own strike price. A low-strike lot uses far more AMT room per share than a high-strike lot. Which lots you exercise, and in what order, changes how far the room stretches.

Plan the credit recovery. If you do pay AMT, the next question is how to get it back. That usually means keeping later years "clean" of new ISO exercises, and timing other income so regular tax runs high enough to release the credit.

Reserve the cash. Whatever AMT the plan accepts should be funded in advance, from savings, from a planned NSO exercise or share sale, or from estimated payments, so April never forces a sale.

The California layer

California has its own alternative minimum tax, at a flat 7% rate, with its own exemption and its own credit. It treats the ISO spread as an AMT adjustment too, and because California taxes capital gains as ordinary income, the regular-tax advantage of a qualifying disposition is smaller at the state level. The federal and California calculations don't line up neatly, so a plan that fills the federal AMT room exactly can still produce California AMT. Both have to be modeled.

The engineer and the chess player

The AMT calculation is pure engineering: exemptions, phase-outs, rates, dual basis and the credit mechanics. Getting it right is the price of entry. But the value comes from treating it as chess. One year's exercise sets up the next year's position. A year of deliberate AMT starts the long-term clock on a valuable block; the following year has to stay clean so the credit can come back; an RSU vest that fills the bracket shrinks the room for ISOs. Each move is correct only in relation to the moves around it.

What to take from this

An ISO exercise is one of the most valuable tax opportunities an executive will have, and one of the easiest to mishandle. The difference is almost never the decision to exercise. It's how much, which lots, in which year and with what reserved to pay for it, decided from a projection rather than discovered on the return.

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Discuss your situation with Natalie C. Papagni, CPA

If you hold vested ISOs, or have a grant approaching its exercise window, a modeled exercise plan built before you act is the natural first step. A complimentary 30-minute consultation is a good place to start.

Natalie C. Papagni, CPA — Tax, Planning & Advisory Services 4275 Executive Square, Suite 200, La Jolla, CA 92037 (858) 754-8277 | service@lajollataxcpa.com | lajollataxcpa.com

Disclaimer: This material is provided for general educational purposes only. It is not tax, legal or accounting advice, and it does not create a CPA–client relationship. Tax outcomes depend on individual facts and circumstances, and figures and rules change from year to year. Consult a qualified professional about your specific situation before acting. Figures reflect 2026 federal rules under Public Law 119-21; examples are illustrative.