ISO AMT Exposure Calculator

Estimate your Alternative Minimum Tax exposure before exercising Incentive Stock Options, so you can plan for the cash needed and understand the AMT credit you will earn.

Estimated AMT owed

$39,494

Detailed results
AMT preference item (spread)	$40,000
Tentative minimum tax	$39,494
AMT credit to carry forward	$39,494
Effective cost per share (including AMT)	$49.49
AMTI after exemption	$151,900

What this result means

Exercising 1,000 shares creates a $40,000 AMT preference item. You owe an estimated $39,494 in AMT on top of your regular income tax. Your total out-of-pocket to exercise — including AMT — works out to roughly $49 per share. The $39,494 AMT you pay becomes a credit you can use in future years when your regular tax exceeds your tentative minimum tax, so the cash leaves now but the tax cost partially returns later. Consider consulting a tax professional before exercising a large block, especially late in the tax year.

AMT Computation Breakdown

Step-by-step replication of IRS Form 6251 Alternative Minimum Tax computation.

AMT Computation Breakdown. 8 rows, first 8 shown.
Item	Amount
Regular taxable income (other income)	$200,000
ISO spread added (AMT preference item)	$40,000
AMTI before exemption	$240,000
AMT exemption (after phase-out)	$88,100
AMTI after exemption	$151,900
Tentative minimum tax	$39,494
Regular income tax	$0.00
AMT owed (excess of TMT over regular tax)	$39,494

How this is calculated

spread = (fmv_at_exercise - exercise_price) × shares_to_exercise
amti = regular_taxable_income + spread
exemption = max(0, base_exemption - max(0, (amti - phase_out_start) × 0.25))
amti_after_exemption = max(0, amti - exemption)
tentative_minimum_tax = amti_after_exemption × 26% (first $220,700) + 28% (above)
amt_owed = max(0, tentative_minimum_tax - regular_tax_already)

What are Incentive Stock Options?

Incentive Stock Options (ISOs) are a form of equity compensation that receive favorable tax treatment under the Internal Revenue Code. Unlike non-qualified stock options (NQSOs), the spread at exercise is not treated as ordinary income for regular income tax purposes — you do not owe income tax on the day you exercise. That favorable treatment comes at a cost: the spread does trigger Alternative Minimum Tax.

How AMT Works

The Alternative Minimum Tax is a parallel tax system designed to ensure that high-income individuals pay at least some minimum amount of tax, even when large deductions and preferences reduce regular taxable income. You compute your tax under both systems and pay whichever is higher.

The AMT starts with your regular taxable income, adds back certain "preference items" and "adjustments," and then subtracts an exemption amount. The resulting Alternative Minimum Taxable Income (AMTI) is taxed at 26% on the first $220,700 and 28% above that (for 2025, subject to verification). If this tentative minimum tax exceeds your regular income tax, the difference is the AMT you owe.

The ISO Preference Item

When you exercise an ISO, the bargain element — fair market value minus the exercise price, multiplied by the number of shares — is added to your income solely for AMT purposes. This is called the AMT preference item or AMT adjustment. A large exercise, or one where the stock has appreciated significantly above your strike price, can produce a preference item large enough to push you into AMT territory.

The AMT Exemption and Phase-Out

Every taxpayer subject to AMT gets an exemption that shields a portion of AMTI from the 26%/28% rates. For 2025 the exemptions are approximately $88,100 (single) and $137,000 (married filing jointly), but these figures must be verified from the Form 6251 instructions. The exemption phases out at 25 cents per dollar of AMTI above a threshold — roughly $626,350 for single filers and $1,252,700 for married filing jointly — so very high AMTI can eliminate the exemption entirely.

The AMT Credit

Any AMT you pay on ISO exercise does not disappear. It becomes a Minimum Tax Credit (Form 8801) that you can use in future years to reduce your regular income tax when your regular tax exceeds your tentative minimum tax. The credit is limited to the amount by which your regular tax exceeds your TMT in any given year. For employees who sell their ISO shares in a qualifying disposition — holding more than two years from grant and one year from exercise — the stock appreciation becomes long-term capital gain in that later year, typically pushing regular tax above TMT and allowing the credit to flow back.

Timing Strategies

Exercising early in the calendar year gives you time to assess the stock price before year-end. If the stock price falls dramatically after exercise, you may choose to sell before December 31 and trigger a disqualifying disposition, which converts the spread to ordinary income and eliminates the AMT preference item — at the cost of losing ISO treatment. Some employees spread exercises over multiple tax years to stay below the phase-out threshold or to manage the size of the preference item relative to their regular tax.

Disqualifying Dispositions

A disqualifying disposition occurs when you sell ISO shares before meeting the holding period requirements: at least two years after the grant date and one year after the exercise date. In that case the spread at exercise (up to the amount of gain) is taxed as ordinary income in the year of sale, and the AMT preference item is reversed on your return for that year. Disqualifying dispositions remove the AMT advantage but also eliminate the risk of owing AMT on paper gains that later evaporate.

Professional Advice

ISO exercise decisions involve interactions between regular income tax, AMT, capital gains, the AMT credit, state taxes, and the risk of holding concentrated stock. This calculator provides a simplified estimate using federal rules only. Always review your specific situation with a qualified tax professional before exercising a significant number of options.

Assumptions

AMTI is computed by adding only the ISO spread to regular taxable income. Real AMTI requires numerous other adjustments and preferences from Form 6251 that are not modeled here.
The regular income tax entered by the user is taken at face value and is not recomputed from the bracket table.
State Alternative Minimum Tax is not included; several states impose their own AMT on top of federal AMT.
The AMT credit carryforward equals the AMT owed in the exercise year; the pace of credit recovery in future years depends on the gap between regular tax and tentative minimum tax in those years.
Statutory figures marked TODO:VERIFY must be confirmed from IRS Form 6251 instructions for tax year 2025 before relying on this calculator for any tax planning or filing decision.
No other AMT preference items (such as accelerated depreciation or tax-exempt interest on private activity bonds) are included.
The calculator assumes an ISO that has not previously been subject to a disqualifying disposition in the same tax year.

Frequently asked questions

›What is the ISO AMT preference item?

The preference item is the spread — fair market value minus exercise price — multiplied by the number of shares exercised. This amount is not taxable income for regular income tax purposes, but it is added to your income when computing Alternative Minimum Tax. A large spread can create a substantial AMT liability even if you owe nothing in regular income tax on the exercise.

›How is the AMT exemption phased out?

The AMT exemption is reduced by 25 cents for every dollar your AMTI exceeds the phase-out threshold. For 2025 the phase-out begins at approximately $626,350 for single filers and $1,252,700 for married filing jointly (verify from IRS Form 6251 instructions). Once AMTI is high enough, the exemption reaches zero and the full AMTI is subject to AMT rates.

›What is the AMT credit and how does it work?

When you pay AMT because of ISO exercise, that payment generates a Minimum Tax Credit (reported on Form 8801). In future years when your regular income tax exceeds your tentative minimum tax — typically after you sell the ISO shares and recognize long-term capital gain — you can use the credit to reduce your regular tax. The credit effectively means the AMT you pay now is a prepayment of future regular tax, though the timing can span many years.

›Should I exercise all at once or spread it out?

Spreading exercises across tax years can reduce AMT exposure in any single year, because you generate a smaller preference item each year relative to your regular tax and exemption. However, you also extend the holding period clock for each tranche, and the stock price risk accumulates over a longer period. There is no universal answer; the right approach depends on your income level, the stock's trajectory, your cash available for AMT, and your risk tolerance.

›What is a disqualifying disposition and how does it affect AMT?

A disqualifying disposition happens when you sell ISO shares before holding them at least two years from the grant date and one year from the exercise date. The spread at exercise is then treated as ordinary income in the year of sale, your W-2 is adjusted, and the AMT preference item from the exercise year is reversed on your AMT return. You lose the favorable ISO treatment but eliminate the AMT timing mismatch. If the stock has fallen since exercise, a disqualifying disposition before year-end can be a deliberate strategy to avoid paying AMT on gains that no longer exist.

›Is this calculator sufficient for filing purposes?

No. This calculator is a planning tool only. It uses simplified AMT logic (adding just the ISO spread to regular taxable income) and does not account for other AMT adjustments and preferences, state AMT, the alternative minimum tax on net investment income, or the interaction with tax credits. The statutory figures (exemptions, phase-out thresholds, bracket amounts) are provided as estimates and must be verified from the official IRS Form 6251 instructions for the applicable tax year. Always work with a qualified tax professional for actual filing.