What’s Your AMT Cost Basis After Exercising ISOs? (w/Examples) + FAQs

This article reflects federal rules and selected state rules (notably California) as of June 2026 and covers tax year 2025 (returns filed in 2026), with 2026 figures noted where they differ. Tax law changes — confirm current figures before you file.

Quick Answer

Your AMT cost basis equals your exercise price (what you paid) plus the bargain element you reported on Form 6251 in the year you exercised. For tax year 2025, that AMT basis is higher than your regular-tax basis (the exercise price alone), and you use it to cut your gain when you sell, often unlocking an AMT credit.

When you exercise an incentive stock option (ISO) and hold the shares past December 31, you create two different cost bases for the same stock: a regular-tax basis equal to what you paid, and an alternative minimum tax (AMT) basis that also includes the “phantom” paper gain you were taxed on. Miss this dual-basis rule at sale time and you can pay tax twice on the same dollars — once at exercise as AMT, again at sale as capital gain.

This matters most for startup and tech employees, who file the bulk of ISO-related AMT. The IRS Form 6251 instructions confirm AMT chiefly hits people with large ISO bargain elements, and Carta reports that ISO exercises are the single most common AMT trigger for equity-holding employees.

💵 How to calculate your AMT basis from your exercise price and bargain element, step by step.
🧾 Where the two numbers live on Form 3921, Form 6251, and Form 8949 so you report them right.
🔁 How the negative AMT adjustment and the Form 8801 credit give your money back over time.
⚖️ How qualifying, disqualifying, and same-year sales each change your basis and your bill.
🚫 The seven costliest dual-basis mistakes — and the exact move that prevents each one.

ISO, AMT, and Cost Basis — The Three Pieces You Must Connect

To understand your AMT cost basis, you first need to see how three things fit together: the option itself, the tax it can trigger, and the basis it creates. Each piece changes what you owe and when.

An incentive stock option (ISO) is a right your employer grants you to buy company stock at a fixed strike price (also called the exercise price). ISOs get special tax treatment under Internal Revenue Code Section 422. When you exercise, you do not owe regular income tax — that is the big perk. The consequence of that perk is the catch you are reading about: the gain still counts for a separate tax system.

The alternative minimum tax (AMT) is a parallel tax that runs alongside your regular tax. You calculate your tax both ways and pay whichever is higher. When you exercise an ISO and hold the shares, the difference between the stock’s fair market value (FMV) and your strike price — the bargain element — becomes income for AMT purposes only. The consequence: you can owe a large tax bill on a gain you have not actually cashed in.

Cost basis is what you “paid” for an asset, and it is the number you subtract from your sale price to find your taxable gain. The twist with held ISO shares is that the AMT system and the regular system give the same shares two different bases. You report the lower regular basis to figure your regular capital gain, and the higher AMT basis to figure your AMT capital gain. Keeping them straight is the whole game.

Why Two Bases Even Exist

The dual basis exists because you got taxed on the bargain element under AMT but not under regular tax. Tax law avoids taxing the same gain twice, so it raises your AMT basis by the amount already taxed. The consequence is fairness across time: you pay AMT early, then recover it later through a lower AMT gain and the minimum tax credit. The common misconception is that the AMT at exercise is a permanent extra tax — for most holders it is a prepayment you get back. What you should do: track both bases on a spreadsheet from the day you exercise, because brokers usually report only the regular basis.

The Core Formula (and the Math Behind It)

Your AMT cost basis is built from two numbers you already have. The formula is simple, but every piece must be right because an error here repeats at every future sale.

Regular-tax basis = exercise price × number of shares. This is just the cash you handed over to buy the stock. It ignores the bargain element entirely.

AMT basis = exercise price + bargain element per share, times the number of shares. Put another way, AMT basis = FMV at exercise × number of shares, because FMV equals strike plus bargain element. This is the number you carry on Form 6251 the year you exercise.

Bargain element per share = FMV at exercise − strike price. Your employer reports the FMV and strike on Form 3921, which you should receive by mid-February after the exercise year.

Here is a fully worked example you can copy. Maria exercises 10,000 ISOs at a $1 strike when the FMV is $11, and holds the shares past year-end.

Cash paid (regular basis): 10,000 × $1 = $10,000.
Bargain element: 10,000 × ($11 − $1) = $100,000 of AMT income for the exercise year.
AMT basis: 10,000 × $11 = $110,000.

So Maria has a $10,000 regular basis and a $110,000 AMT basis in the very same 10,000 shares. The $100,000 gap is exactly the amount she reported on Form 6251.

Which Situation Applies to You?

Your AMT basis only “pays off” when you sell, and how you sell decides everything. Find your situation below, then read the matching section.

You exercised and are still holding past year-end (no sale yet). You owe AMT now on the bargain element; your AMT basis is set. Read “Exercise and Hold.”
You sold after meeting the holding rules (more than 2 years from grant and more than 1 year from exercise). This is a qualifying disposition. Read “Qualifying Disposition.”
You sold in a later year but before meeting the holding rules. This is a disqualifying disposition in a later year — the case where dual basis matters most. Read “Disqualifying Disposition (Later Year).”
You exercised and sold in the same calendar year. There is no AMT adjustment at all. Read “Same-Year Sale.”
You owe AMT from a past exercise and want it back. Read “Recovering Your AMT With the Credit.”

Exercise and Hold: Setting Your AMT Basis

When you exercise and hold ISO shares across December 31, you lock in your AMT basis and trigger the AMT calculation for that year. Nothing is sold yet, so there is no capital gain — only the bargain element flowing onto Form 6251.

The bargain element lands on Form 6251, line 2i, as a positive AMT adjustment. You add it to your other income to find your alternative minimum taxable income (AMTI). If AMTI minus your exemption pushes your tentative minimum tax above your regular tax, you pay the difference as AMT. The consequence of holding is real cash out the door for a gain you cannot spend yet.

For tax year 2025, the AMT exemption is $88,100 for single filers, $137,000 for married filing jointly, and $68,650 for married filing separately. The exemption phases out at 25 cents per dollar once AMTI exceeds $626,350 (single) or $1,252,700 (joint) for 2025. The AMT rate is 26% on the first $239,100 of the AMT base for 2025 and 28% above that ($232,600 in 2024 figures rose with inflation), per the Form 6251 instructions.

What the Hold Does to Your Two Bases

After an exercise-and-hold, your regular basis stays at the cash you paid and your AMT basis equals FMV at exercise. That gap sits dormant until you sell. The consequence of forgetting the gap is the classic double-tax trap: at sale you compute regular gain off the small basis, fine, but you must also compute a smaller AMT gain off the big basis or you will overpay AMT and lose your credit. A common misconception is that holding “cancels” the AMT — it does not; holding is what creates the AMT in the first place. What to do now: save your Form 3921 and record both bases per lot before you ever place a sell order.

Qualifying Disposition: The Best-Case Basis Story

A qualifying disposition happens when you sell ISO shares more than two years after the grant date and more than one year after the exercise date. Meet both clocks and your entire profit over your exercise price is taxed as long-term capital gain for regular tax — the friendliest result.

For regular tax, your gain is sale price minus your exercise-price basis. For AMT, your gain is sale price minus your AMT basis (the higher FMV-at-exercise number). Because the AMT basis is larger, your AMT gain is smaller, which produces a negative AMT adjustment on Form 6251 line 2k. That negative number is how the system gives back the AMT you prepaid.

Worked example: David grants date is January 2023, he exercised in March 2024 (10,000 shares, $2 strike, $12 FMV), and sells in June 2026 at $30. He met both holding periods, so it is qualifying.

Regular long-term gain: 10,000 × ($30 − $2) = $280,000.
AMT gain: 10,000 × ($30 − $12) = $180,000.
AMT adjustment on line 2k: −$100,000 (the gap reverses).

David pays long-term capital gains rates on $280,000 and books a $100,000 negative AMT adjustment that helps free up his AMT credit.

Disqualifying Disposition (Later Year): Where Dual Basis Matters Most

A disqualifying disposition is any sale that misses the holding periods. When it happens in a year after the exercise year, you face the full dual-basis dance, and getting the basis right is what saves you from double taxation.

In the sale year, part of your profit becomes ordinary compensation income — generally the lesser of the bargain element at exercise or your actual gain at sale, per IRC Section 422(c). That ordinary income adds to your regular basis for the capital-gain piece. Critically, the Fairmark guidance confirms you do not amend the exercise-year return — the AMT you paid then stays, and you recover it through the credit and a line 2k adjustment now.

Worked example: Priya exercised in 2024 (5,000 shares, $4 strike, $20 FMV) and sells in 2026 at $26.

Ordinary income in 2026: lesser of bargain element ($80,000) or actual gain ($110,000) = $80,000, added to her W-2-type income.
Adjusted regular basis: $4 strike + $16 ordinary = $20 per share, so regular gain = 5,000 × ($26 − $20) = $30,000 long-term capital gain.
AMT basis was already $20 (FMV), so for AMT she gets a −$80,000 line 2k adjustment that releases her prior AMT credit.

The consequence of ignoring the line 2k adjustment: Priya would pay AMT on the bargain element twice. What to do: report the ordinary income from your corrected W-2 or Form 3921, then enter the negative AMT adjustment so the prepaid tax unwinds.

Same-Year Sale: No AMT, No Dual Basis

If you exercise and sell in the same calendar year, the dual-basis problem disappears entirely. The IRS treats the bargain element as ordinary income for both regular and AMT purposes, so the two systems match and no Form 6251 adjustment is needed.

Per The Tax Adviser, a same-year disqualifying disposition makes the bargain element ordinary income in the exercise year for regular tax and AMT alike, “so that no AMT adjustment is necessary.” Porte Brown puts it plainly: exercise and sell in the same year and “there are no AMT consequences.”

Worked example: Sam exercises 3,000 ISOs at a $5 strike when FMV is $15, then sells the same year at $18. His ordinary income is 3,000 × ($15 − $5) = $30,000, his basis becomes $15, and his short-term capital gain is 3,000 × ($18 − $15) = $9,000. No AMT, one basis, done. This is the standard escape hatch when a year’s bargain element would otherwise create a crushing AMT bill.

Recovering Your AMT With the Credit

The AMT you pay on an ISO exercise is usually not lost — it becomes a minimum tax credit you claim in future years on Form 8801. This is the payback half of the dual-basis story.

You can use the credit in any year your regular tax exceeds your tentative minimum tax, per IRC Section 53. The IRS Form 8801 instructions state the credit reduces regular tax but cannot create a refund on its own, and any unused amount carries forward with no expiration. The negative line 2k adjustment in your sale year often pushes your AMT down and unlocks more of the credit that year.

Worked example: Maria from earlier paid $26,000 of AMT in her exercise year. In a later year her regular tax exceeds her tentative minimum tax by $9,000, so she claims $9,000 of credit on Form 8801, files it with her Form 1040, and carries the remaining $17,000 forward. The misconception that the credit “vanishes” if unused is false — it waits indefinitely. What to do: file Form 8801 every year after an AMT-triggering exercise, even in years you claim zero, so the carryforward stays documented.

The Forms, Line by Line

Three or four forms carry your ISO basis story, and each line has a job. Knowing where each number goes prevents the most expensive reporting errors.

Form 3921 (Exercise of an Incentive Stock Option) is informational and comes from your employer by mid-February. Box 3 shows your strike price, Box 4 shows FMV at exercise, and Box 5 shows shares — together they give you the bargain element. Keep it forever, because it is your only clean record of AMT basis.

Form 6251 (Alternative Minimum Tax) is where AMT lives. Line 2i carries the positive bargain element in the exercise year; line 2k carries the negative adjustment in the sale year when AMT gain is smaller than regular gain.

Form 8949 and Schedule D (Capital Gains) report the sale. You list the regular-tax gain using your broker’s basis, then, if needed, file a second Form 8949 computation for AMT using the higher AMT basis — the AMT version flows into your Form 6251, not your regular return.

Form 8801 (Credit for Prior Year Minimum Tax) claims the credit you earned by paying AMT and tracks the carryforward.

Three Common Scenarios at a Glance

Each table below shows one situation and the basis result it produces.

Scenario 1: Exercise and Hold Across Year-End

What Happens	Effect on Your Basis and Tax
You exercise and keep the shares	Regular basis = cash paid; AMT basis = FMV at exercise
Year-end passes with shares held	Bargain element hits Form 6251 line 2i; AMT may be due
Future sale	Use both bases; AMT gain will be smaller

Scenario 2: Qualifying Disposition After Holding Periods

What Happens	Effect on Your Basis and Tax
You sell after 2 years from grant and 1 year from exercise	Entire gain over strike is long-term capital gain
AMT basis applies for the parallel calc	Negative line 2k adjustment reverses prior AMT
Credit released	Form 8801 credit becomes usable

Scenario 3: Same-Year Disqualifying Sale

What Happens	Effect on Your Basis and Tax
You exercise and sell in the same year	Bargain element is ordinary income for regular tax and AMT
Bases match	Single basis = FMV; no Form 6251 adjustment
Result	No AMT; simplest reporting

State Conformity: Does Your State Follow the AMT Rule?

Always separate federal from state, because states do not all copy federal AMT. Guessing here can cost you a second basis you never tracked.

California runs its own alternative minimum tax under the California Franchise Tax Board rules, using Schedule P (540). California taxes the ISO bargain element for state AMT much like the federal system, so California ISO holders carry a third basis — a California AMT basis — and may owe California AMT and earn a California minimum tax credit. The consequence of ignoring it: a surprise state bill on top of federal AMT.

Most states without their own AMT — including no-income-tax states like Texas, Florida, Washington, and Nevada — impose no extra ISO AMT layer, so your only AMT basis worry is federal. States that conform loosely or not at all (the majority) simply tax the gain when you sell under ordinary rules. What to do: check your specific state’s instructions, and if you live in California, complete Schedule P alongside Form 6251.

Federal vs. California AMT on ISOs

Feature	Federal / California Treatment
Separate AMT system	Federal: yes (Form 6251); California: yes (Schedule P 540)
ISO bargain element taxed for AMT	Federal: yes; California: yes
2025 exemption	Federal: $88,100 single / $137,000 joint; California: lower, set annually by FTB
Credit for prior-year AMT	Federal: Form 8801; California: Schedule P credit

Mistakes to Avoid

Each error below has cost real filers real money. Read the outcome next to each.

Using only the broker’s basis at sale. Brokers report the regular (cash) basis, so you overstate your AMT gain and overpay AMT — sometimes by tens of thousands.
Forgetting the negative line 2k adjustment. Skip it and you pay AMT twice on the same bargain element, the single most common dual-basis loss.
Amending the exercise-year return after a later disqualifying sale. You are not supposed to; doing so creates a filing mess and can erase your AMT credit.
Tossing Form 3921. Without it you cannot prove your AMT basis or bargain element, and reconstructing it years later is painful.
Ignoring Form 8801 in years with no AMT. Failing to file it can break your carryforward record and delay recovering thousands.
Assuming your state copies federal AMT. California ISO holders who skip Schedule P face a surprise state assessment plus penalties.
Exercising too much in one year. A giant bargain element can trigger AMT far larger than the cash you have, and the credit comes back slowly.

Do’s and Don’ts

Do save Form 3921 and record both bases per lot — it is your only proof at sale time.
Do model your AMT before exercising, because the bill is due even if you never sell.
Do consider a same-year sale when a bargain element would create unaffordable AMT, since it sidesteps AMT entirely.
Do file Form 8801 every year after an AMT exercise to keep the credit alive.
Do track a separate California basis if you live there, because the state runs its own AMT.
Don’t rely on your 1099-B basis for the AMT calculation — it understates your AMT basis.
Don’t amend the exercise year after a later sale — recover through the credit instead.
Don’t assume the AMT is gone forever — for most holders it is a recoverable prepayment.
Don’t exercise late in December without checking the year-end deadline impact on AMT.
Don’t guess your state rule — confirm it in your state’s instructions.

Pros and Cons of Holding ISO Shares (and Paying AMT)

Pro: Holding past the periods unlocks long-term capital gains rates, the lowest rates available, because the gain over strike qualifies.
Pro: The AMT you pay is usually recoverable through the Form 8801 credit, so it is rarely a permanent cost.
Pro: A qualifying disposition produces a negative AMT adjustment that helps release that credit.
Pro: Your AMT basis lowers every future AMT gain, protecting you from double tax if reported correctly.
Pro: Early exercise can start the clocks sooner, advancing the date your gain turns long-term.
Con: AMT is due in cash the year you hold, even though you sold nothing — a real liquidity risk.
Con: The dual-basis tracking is error-prone, and brokers will not do it for you.
Con: The credit can take many years to fully recover, tying up your money.
Con: A stock that drops after exercise can leave you with AMT paid on a gain that vanished.
Con: California and a few states add a second AMT layer and a second basis to track.

What to Do Next

Move through these steps in order to nail your AMT basis and avoid the double-tax trap.

Pull your Form 3921 for every exercise and record strike, FMV, shares, and dates per lot.
Compute both your regular basis (cash paid) and AMT basis (FMV at exercise) for each lot today.
In any exercise-and-hold year, complete Form 6251 line 2i and pay any AMT by the April 15 filing deadline (or your extended date) to avoid penalties.
In any sale year, file a second Form 8949 for AMT using the AMT basis, and enter the negative adjustment on line 2k.
File Form 8801 every year after an AMT-triggering exercise to claim and carry the credit.
If you live in California (or another AMT state), complete Schedule P (540) alongside the federal forms.
Bring a CPA or tax attorney in when your bargain element tops roughly $100,000, when you have multiple exercise years, or when you face a disqualifying disposition — they typically charge a few hundred to a few thousand dollars and can save far more.

This article is educational and is not a substitute for advice from a licensed tax professional about your specific situation.

FAQs

What is my AMT cost basis after exercising ISOs? Your exercise price plus the bargain element you reported on Form 6251. It equals the stock’s fair market value at exercise. You use this higher basis to figure your AMT capital gain when you sell.

Is my AMT basis different from my regular cost basis? Yes. For held ISO shares, regular basis is the cash you paid, while AMT basis is that cash plus the bargain element already taxed under AMT. The two stay different until you sell.

Do brokers report my AMT basis on Form 1099-B? No. Brokers report only your regular (cash) basis. You must track and apply the higher AMT basis yourself, usually on a separate Form 8949 calculation that feeds Form 6251.

How much is the AMT exemption for 2025? $88,100 for single filers and $137,000 for married filing jointly for tax year 2025, per IRS figures. It phases out above $626,350 (single) and $1,252,700 (joint) AMTI.

Do I owe AMT if I exercise and sell in the same year? No. A same-year sale makes the bargain element ordinary income for both regular and AMT purposes, so the bases match and no Form 6251 adjustment or AMT applies.

Where does the bargain element go on Form 6251? Line 2i in the year you exercise and hold. When you later sell, a smaller AMT gain shows as a negative adjustment on line 2k, reversing the earlier income.

Can I get my AMT money back? Yes, usually. Paying AMT on an ISO creates a minimum tax credit claimed on Form 8801 in future years when regular tax exceeds tentative minimum tax. Unused credit carries forward indefinitely.

Should I amend my exercise-year return if I later sell early? No. A later disqualifying disposition is reported in the sale year. You keep the AMT you paid and recover it through the credit and the line 2k adjustment, not by amending.

Does California tax the ISO bargain element for AMT? Yes. California runs its own AMT on Schedule P (540) and taxes the ISO bargain element much like the federal system, creating a separate California AMT basis and credit.

What is a disqualifying disposition? A sale that misses the ISO holding periods — not held more than two years from grant and one year from exercise. Part of your gain becomes ordinary income in the year you sell.

How long can I carry the AMT credit forward? Indefinitely. The minimum tax credit on Form 8801 has no expiration date, so any unused amount carries to future years until you fully recover it.

Which form proves my AMT basis? Form 3921 from your employer. Box 3 (strike), Box 4 (FMV at exercise), and Box 5 (shares) together establish the bargain element and your AMT basis — keep it permanently.

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