Your First AMT Bill: Why ISOs Trigger It and How to Model It

The first time most tech employees hear about alternative minimum tax is the April after they exercised ISOs and held. Their CPA runs Form 6251, the line labeled “tentative minimum tax” is larger than the regular tax line, and the bill is a six-figure number nobody prepared them for.

AMT is a separate tax system, running parallel to regular income tax, designed in 1969 to catch high-income filers who used deductions to reduce their regular tax to near zero. Most years it catches almost nobody because of the 2017 TCJA changes that raised the exemption. The exception is ISO exercises, which remain an AMT preference item that cannot be deducted away. Exercise a large spread and hold, and AMT is the result.

This article explains how AMT actually works, walks through a realistic example, and covers the credit recovery path that eventually returns most of the money, usually over many years.

The two-tax system

Every year you owe the greater of (a) regular federal tax and (b) tentative minimum tax. In a normal year, regular tax is higher, and AMT is zero. In an ISO-exercise year, AMT can be many times larger than regular tax, and the difference is what you write a check for.

The mechanics on Form 6251:

1. Start with regular taxable income (Form 1040 line 15).
2. Add back preference and adjustment items. The big one for ISO holders is the bargain element on exercised-and-held ISOs: (FMV at exercise minus strike) times shares.
3. Subtract the AMT exemption. For 2025, single filers get $88,100 and joint filers $137,000. The exemption phases out at 25 cents per dollar once AMTI passes $626,350 single or $1,252,700 joint. At high AMTI, the exemption is fully gone.
4. Apply the AMT rate schedule: 26% on the first $232,600 (2025) of remaining AMTI, 28% above that.
5. Compare that number to your regular tax. Pay the larger one. The excess is your AMT liability for the year.

A realistic worked example

Consider an engineer with the following profile:

• W-2 salary: $220,000
• RSU vests: $80,000
• Filing status: single
• ISOs exercised in December: 40,000 shares
• Strike price: $2
• 409A fair market value at exercise: $22
• Bargain element: ($22 - $2) × 40,000 = $800,000

Regular tax calculation (simplified, 2025):

Line	Amount
Wages	$300,000
Standard deduction	($15,000)
Regular taxable income	$285,000
Regular federal tax	~$71,000

AMT calculation:

Line	Amount
Taxable income	$285,000
Add: ISO bargain element	$800,000
Subtract: standard deduction add-back (already removed)	$0
AMTI	$1,085,000
AMT exemption	$0 (fully phased out above $978,750 single)
26% on first $232,600	$60,476
28% on remaining $852,400	$238,672
Tentative minimum tax	$299,148
Regular tax	$71,000
AMT owed (difference)	$228,148

This person writes a check to the IRS for roughly $228k, on top of normal withholding, by April 15 of the following year. No actual cash has come from the ISO exercise. The shares are sitting in a brokerage account at the 409A price. The tax is real and due.

The AMT credit: the money comes back, slowly

The $228k paid here is not permanently lost. IRC §53 creates a minimum tax credit, which carries forward indefinitely. In any future year when regular tax exceeds tentative minimum tax, the credit can offset the excess.

For a single filer in a normal high-income year, regular tax typically exceeds AMT by $20k to $80k. That means the credit recovers in chunks: $40k this year, $55k next year, $30k the year after. Full recovery of a $228k credit often takes six to twelve years.

The credit is also recovered in the year you sell the ISO shares, because the basis for regular tax purposes equals the strike plus ordinary income recognized (if any), while the basis for AMT purposes equals the FMV at exercise. When you sell, you recognize a larger regular-tax gain than AMT gain, which creates a large negative AMT adjustment and frees up a big chunk of credit. This is where most of the credit recovery happens for people who eventually sell.

There are two planning implications. First, the AMT paid is an unpaid loan to the IRS, not a lost amount. Second, the credit only recovers if your regular tax exceeds AMT in future years, which means continuing to earn at or above your current level. If you retire, move abroad, or shift to low-income work, the credit can sit unused for a long time.

The exercise-and-hold-then-sell cycle

The cleanest use of ISOs combines early exercise (with 83(b)) plus hold-to-qualifying-disposition. The sequence:

1. Exercise early when the spread is small. AMT at exercise is small or zero.
2. File 83(b) within 30 days to lock in the exercise-date FMV.
3. Hold for at least two years from grant and one year from exercise.
4. Sell. Full gain from strike to sale price is long-term capital gain.

If early exercise was not available or the employee missed the window, the second-best path is partial exercise each year, sized to stay under the AMT exemption phase-out cliff. Exercising $100k of spread per year at a modest salary level might trigger $20k-$30k of AMT annually, which is painful but manageable. Exercising $2M of spread in a single year triggers $500k+ of AMT and a liquidity crisis.

The tool for sizing annual exercises is an AMT “crossover” calculation. Find the amount of spread you can add to AMTI this year where AMT equals regular tax, and exercise up to that point. That is your no-AMT exercise budget. Spread beyond it creates current-year AMT that will recover in future years.

Disqualifying dispositions as an AMT safety valve

If AMT is already paid and the stock drops, selling before the one-year holding period is up creates a disqualifying disposition. In the same tax year as exercise, the AMT preference is eliminated because regular-tax treatment takes over: spread at exercise becomes ordinary W-2 income, taxed at regular rates, and the AMT add-back is reversed.

Example: exercise in February with a $500k spread, pay estimated AMT during the year, then the stock drops 40% by November. Selling in December creates a disqualifying disposition. The $500k becomes ordinary income. AMT for the year is recomputed without the preference, and the AMT bill disappears. You still owe ordinary tax on the spread, but at regular rates and with actual cash in hand from the sale.

This is the “exit ramp” on an ISO exercise that went wrong. It only works in the same calendar year as the exercise. Once December 31 passes, the AMT crystallizes.

What AMT does not apply to

NSOs are not an AMT preference. NSO exercises recognize ordinary income at exercise for regular tax, which also flows through AMTI, so there is no preference item and no parallel-calculation gap. AMT rarely matters for NSO holders.

RSUs are not an AMT preference. Vesting RSUs recognize ordinary income at vest for regular tax, identical to AMTI treatment. No gap.

ISOs held for qualifying disposition and never sold generate AMT at exercise and then recovery over many years. ISOs exercised and sold same-year (disqualifying, same-year) generate zero AMT. ISOs exercised and sold in a later year before hitting the holding periods generate AMT at exercise and a partial reversal on sale. ISOs exercised and sold after hitting both holding periods generate AMT at exercise and a large recovery of the credit at sale.

Frequently asked

Do I pay AMT on the shares I already sold through a cashless exercise? No. A cashless exercise is a same-day sale, which is a disqualifying disposition, which eliminates the AMT preference. You pay regular tax on the spread as ordinary income, same as an NSO.

How does AMT interact with state tax? Most states do not have their own AMT. California does, and the California AMT rate is 7% (much lower than federal but still real). A $1M federal ISO spread for a California resident might generate an extra $70k of California AMT on top of the federal number.

Can I make a quarterly estimated payment to cover AMT? Yes. The underpayment-penalty safe harbors apply to total tax liability, which includes AMT. Either pay 110% of last year’s total tax through withholding plus estimates, or pay 90% of the current year’s total tax. Many ISO exercisers make a Q4 estimated payment in December to cover the AMT before year-end.

Does AMT ever go away without being recovered? Only in a few scenarios: death (the credit does not transfer to heirs), expatriation under IRC §877A, or permanent loss of income such that regular tax never again exceeds AMT. For most earners, it recovers eventually.

Can I deduct state AMT paid on federal AMT? No. The state-tax deduction is disallowed for federal AMT purposes (it is an add-back preference). This is one of the reasons high-tax-state residents hit AMT harder than low-tax-state residents on the same spread.

Next step

Before you exercise, model the AMT on Form 6251 line by line. The AMT calculator handles the exemption phase-out and the 26/28% bracket transition. If the projected bill is more than your liquid cash, cut the exercise size or stage it over multiple years. A CPA who specializes in equity comp should see the numbers before you hit the exercise button, not after.