ISO AMT: The Tax Bill You Can Owe Without Selling a Share

The regular tax system treats an ISO exercise as a nonevent. Buy shares worth $20 for a $5 strike and no wage income appears, no withholding happens, and if you then hold more than two years from grant and one year from exercise, the entire gain when you sell is long-term capital gain. That is the deal Congress wrote for incentive stock options, and it is genuinely better than the treatment of nonqualified options or RSUs.

But every year, a second calculation runs alongside your regular tax: the alternative minimum tax, computed on Form 6251 with its own income definition, its own exemption, and its own rates. The AMT income definition does not ignore your exercise. The bargain element, market value at exercise minus what you paid, is added to your AMT income in the exercise year. You then pay whichever total is higher: regular tax or tentative minimum tax.

Exercise a small spread and nothing usually happens; the AMT exemption and the fact that regular tax on your salary is already substantial absorb it. Exercise a large spread and the AMT calculation wins, and the excess over your regular tax is due with that year's return, in cash, on shares you may not have sold and, at a private company, may not be able to sell.

One boundary worth knowing before the math: this treatment applies only to true ISOs. Only the first $100,000 of stock (valued at grant) first exercisable in a calendar year qualifies; the overflow is nonqualified options, taxed as wages at exercise. And selling in the same calendar year as exercise (a disqualifying disposition) pulls the spread back into regular wage income, which removes the AMT adjustment entirely, a fact that becomes a strategy in section 4.

The AMT computation, stripped to its skeleton: start with taxable income, add back items the AMT disallows (the standard deduction and state and local tax deductions are the big ones), add AMT adjustments including the ISO bargain element, subtract the exemption, apply the rates, and compare the result to your regular tax.

The 2026 change that matters for ISO holders: the One Big Beautiful Bill Act pulled the exemption phaseout thresholds back to $500,000 single and $1,000,000 joint (they had been over $600,000 and $1.2 million) and doubled the phaseout speed from 25 to 50 cents per dollar. Inside the phaseout zone, every dollar of bargain element removes 50 cents of exemption, making the effective AMT rate on that dollar as high as 39% (26% or 28% on 1.5 dollars of base). High earners exercising large ISO positions feel this directly: an exercise that cleared with modest AMT in 2025 can cost meaningfully more in 2026.

For AMT planning beyond ISOs, including the SALT interaction and deduction timing, see our broader AMT planning guide. This page stays on the ISO-specific machinery.

Notice what drove the result: not the exercise alone, but the ratio of bargain element to regular income. The same $150,000 spread on top of a $700,000 income often produces zero AMT, because regular tax at 35% to 37% already towers over the tentative minimum tax. AMT on ISOs is mostly a middle-to-upper income phenomenon, hitting hardest in the $150,000 to $600,000 income range where the 26% to 28% AMT rates can outrun the regular brackets.

1. Exercise up to your AMT crossover each year.

For every income profile there is a bargain-element amount that raises tentative minimum tax exactly to regular tax, the crossover point. Exercises below it trigger zero AMT. Spreading a large ISO position across several years of crossover capacity, instead of one big exercise, can eliminate AMT entirely at the cost of patience and continued stock exposure. Computing the crossover requires a projection of the whole return, which is a routine year-end CPA exercise.

2. Exercise in January, keep the escape hatch.

An early-year exercise starts the one-year holding clock immediately and preserves an option: if the stock collapses before December 31, you can sell in the same calendar year. That disqualifying disposition converts the income to regular wages measured by the actual (now smaller or zero) gain and erases the AMT adjustment on the vanished spread. A December exercise forfeits the hatch; you learn what the stock does only after the AMT is locked.

3. Exercise when the spread is small.

The AMT cost scales with the bargain element, so the cheapest exercises happen early, when the 409A value sits close to the strike, sometimes at the cost of exercising before you are sure the company will succeed. Employees at very early startups sometimes exercise immediately on grant for exactly this reason. The trade is real money at risk for shares that may never be worth anything, which is a portfolio decision first.

4. Coordinate exercise years with income valleys.

A sabbatical, a job change with a gap, or a year after a big RSU settlement all shift the crossover math. Lower regular income means less room before AMT bites, but a year with unusually high regular income (a double-trigger RSU settlement, say) can be the perfect year for a large ISO exercise, because towering regular tax absorbs the adjustment. The counterintuitive pairing of a big RSU year with a big ISO exercise is one of the most valuable moves in equity-comp planning, and almost nobody finds it without projecting both systems.

Here is the part that reframes the whole topic: AMT paid because of an ISO exercise is largely a timing difference, not a permanent cost. The exercise accelerated income into the AMT system that the regular system will tax later (at sale). To prevent double taxation, Section 53 gives you a minimum tax credit for AMT attributable to deferral items like ISO adjustments. The credit carries forward indefinitely and offsets regular tax in any later year where your regular tax exceeds that year's tentative minimum tax.

The machinery is Form 8801, filed every year after an AMT year. It splits your prior AMT between deferral items (creditable, like ISOs) and exclusion items (not creditable, like the SALT add-back), tracks the carryforward, and computes how much credit each year can absorb. Recovery speed depends on the gap between your regular tax and TMT in the following years: high earners with big regular-tax bills often recover in two or three years, while someone whose income drops may carry the credit for a long time.

The credit's partner is dual basis. Because the AMT system taxed the spread at exercise, your AMT basis in the shares is the exercise-date value while your regular basis is the strike price. In the year you finally sell, your AMT gain is smaller than your regular gain, which pushes tentative minimum tax down, widens the regular-over-TMT gap, and releases credit, often a large chunk of it in the sale year itself. Miss the dual-basis adjustment and you both overstate AMT income in the sale year and strand credit.

1. Private-company AMT with no way to sell.

The bargain element runs off the 409A valuation whether or not any market exists for the shares. An exercise at a hot startup can create a six-figure April tax bill on stock that cannot be sold, and if the company later fails, the AMT was real money paid on imaginary value, recoverable only slowly through the credit against future regular tax. The dot-com era produced people who owed more AMT than their shares were ever worth again. Exercise sizing at private companies should assume the shares might stay illiquid past the tax due date.

2. No withholding means the cash plan is on you.

Nothing is withheld at exercise, and the AMT lands as a balance due. Underpayment penalties stack on top unless you hit a safe harbor: 90% of the current year's tax or 100% of last year's (110% if prior-year AGI topped $150,000). In a big exercise year, the prior-year safe harbor is usually the cheap, certain play.

3. The ISO/NSO split surprises fast vesters.

The $100,000 limit means large grants are often part ISO, part NSO, and the NSO part is taxed as wages at exercise with withholding, a completely different regime covered in our RSU vs stock options comparison and the broader stock options tax guide. Exercising "my options" as if they were all ISOs is a recurring source of surprise W-2 income.

4. Disqualifying a deeply appreciated position out of AMT fear.

Selling early to dodge AMT converts gain that could have been long-term capital gain (top federal rate 20%, plus 3.8% NIIT where it applies) into ordinary income at up to 37%. Sometimes that trade is right, especially when concentration risk is screaming. But AMT that comes back as a credit is often cheaper than a permanent rate difference, and the comparison deserves a spreadsheet, not a reflex.