The spread is income in a second tax system
An incentive stock option lets you buy shares at a strike price. When the shares are worth more than that price, the difference is the bargain element. Exercising 1,000 shares at $10 when their fair market value is $110 creates a $100,000 bargain element—even though you paid cash to buy the shares and received no cash back.
For a normal ISO exercise and hold, regular income tax does not count that spread at exercise. Alternative minimum tax does. Form 6251, line 2i adds it to the AMT calculation if the shares are held past year-end. The standard deduction is also unavailable for AMT. This planner therefore starts AMTI with wages plus the bargain element, subtracts the available AMT exemption, and applies the AMT rates.
You do not pay regular tax plus the entire tentative minimum tax. The added AMT is only the amount by which tentative minimum tax exceeds regular tax. In the example above, a single filer with $200,000 of wages has an estimated $17,840 of AMT under the assumptions here. The $10,000 exercise cost is separate cash out the door.
Headroom is a budget for spread, not a universal share count
Before an exercise, regular tax may already be higher than tentative minimum tax. Some ISO spread can fit in that gap without creating AMT. The calculator finds that dollar headroom, then divides by your spread per share and rounds down. It is a total exercise allowance at the entered price, not an additional allowance on top of the shares you entered.
For the $200,000 single-filer example, the spread budget is about $31,384: 313 whole shares at a $100 spread. Different wages, filing status, deductions, or other AMT adjustments can change that. A new 409A valuation or public-market price can change the share count even if the dollar budget stays the same.
Exercising portions in different tax years can use separate years' headroom. That is a tradeoff, not free tax savings: the option may expire, the strike-to-FMV spread may grow, and company exercise rules may constrain the dates. An estimate for this year is not a promise about next year. The projection deliberately holds 2026 tax parameters flat so you can see the mechanism, not forecast future law.
An early-year exercise leaves more calendar to react
Exercise in January and you have most of the year to watch what happens before December 31. If you sell in the same calendar year, the ISO exercise adjustment drops out of the AMT calculation. This is a disqualifying disposition: the spread generally becomes ordinary income, potentially limited to the gain on a lower-price sale under the applicable rules. It is a different tax outcome, not a tax-free exit.
Exercise in December and that same-year decision window is short. Waiting until January to sell does not undo the prior year's adjustment. The distinction matters especially if the price falls after exercise.
The exit only exists if you can actually sell. Private-company shares may have no buyer, and public-company shares may be subject to a blackout, preclearance, or other trading restrictions. This calculator does not model disqualifying sales; IRS Publication 525's incentive stock option section explains their ordinary-income rules.
The minimum tax credit is a carryforward, not a refund date
The ISO adjustment is a timing difference. That is why the AMT it creates can become a minimum tax credit on Form 8801. In general, each later year can use only the credit that fits between regular tax after nonrefundable credits and tentative minimum tax. Whatever does not fit carries forward.
Wages alone may leave only a narrow gap. At the example's unchanged $200,000 wages and with no other credits, there is room for $8,160 a year before considering a sale, capped by the credit left. Child tax credit and other nonrefundable credits reduce that room dollar for dollar; enter the amount you expect to claim, not a maximum before eligibility limits or phaseouts. The planner holds that amount flat and does not determine eligibility. The headline minimum tax credit balance is not cash available to spend.
A qualifying sale often changes the picture because the shares have two bases. Regular basis is the strike paid; AMT basis is the exercise-date FMV already counted for AMT. Selling the example's shares at $150 produces a $140,000 regular gain but only a $40,000 AMT gain. The smaller AMT gain can widen the gap and allow more credit to be used. A sale in 2028 recovers the rest of the example's credit in this model.
Both calculations preserve preferential long-term capital-gain rates rather than treating the entire sale gain as ordinary income. The sale may also trigger the 3.8% net investment income tax—$5,320 in the example—which is separate and not offset by the minimum tax credit. Keep both basis records; an eventual sale is not the time to reconstruct the exercise.
The crash risk is real cash, not just tax math
AMT can be due on an exercise-date spread that later evaporates. A lower stock price does not automatically reverse a prior year's bill. A future credit may take years to use, while the cash for exercise and tax has already left your account. A later loss can introduce another carryforward; this planner does not model regular or AMT capital-loss carryforwards.
Before treating the credit as money coming back soon, compare a hold scenario, a qualifying sale, and a lower sale price. Consider whether the exercise cost and tax reserve can be funded without relying on an unavailable sale. The concentration and liquidity worksheet supplies useful portfolio questions, though its RSU tax timeline is different from an ISO's.
Early exercise and an 83(b) election
Some plans allow exercise before shares vest. Without an election, the AMT adjustment for unvested ISO shares is measured when they vest (using the FMV on the vesting date) and lands in the vesting year, even if FMV equaled strike when you exercised. A section 83(b) election filed no later than 30 days after the transfer instead measures the AMT adjustment at exercise; if FMV equals strike then, there is no spread to add. It is not a general election for RSUs, does not eliminate later tax, and does not make forfeiture risk disappear. Plan eligibility and the election's ISO-specific treatment need professional review; this calculator assumes the entered spread is the applicable exercise adjustment, not a model of vesting or elections.
Six questions about ISO AMT
Do I owe tax just for exercising ISOs?
An ISO exercise generally creates no regular income tax at exercise if you exercise while employed or within 3 months after leaving (1 year if you left because of disability). The ISO limit is $100,000 of grant-date stock value first becoming exercisable in any calendar year, not a cap on how much you exercise; options over that limit are taxed like nonqualified options. If you hold vested shares past year-end, the positive difference between exercise-date fair market value and strike price is an AMT adjustment. Whether it creates a bill depends on the rest of your tax picture.
How many ISOs can I exercise without triggering AMT?
It depends on wages, filing status, and the spread per share. This calculator finds the largest spread that leaves tentative minimum tax at or below regular tax, then divides by the positive per-share spread and rounds down to whole shares. The result assumes wages are your only other income and you take the standard deduction.
Can selling before December 31 remove the ISO AMT adjustment?
Selling in the same calendar year as exercise means there is no ISO exercise adjustment for AMT. It is a disqualifying disposition, however, and generally creates ordinary income from the exercise spread, potentially limited by a lower sale gain. A sale next January is too late to remove the prior year’s adjustment. Company restrictions or a lack of buyers can make a sale unavailable.
Do I get all the AMT back the next year?
Not necessarily. The ISO-related AMT can create a minimum tax credit tracked on Form 8801, but each later year’s use is generally limited to regular tax minus nonrefundable credits minus tentative minimum tax, never below zero. Unused credit carries forward. A qualifying sale may open more room to use it, but neither a sale nor the passage of time guarantees full recovery.
What makes an ISO sale a qualifying disposition?
The sale must be more than one year after exercise and more than two years after grant. This planner assumes both tests are met; selecting a future calendar year does not check the actual dates. Other sales follow different ordinary-income and basis rules and are not modeled.
Does this include California AMT or store my inputs?
Neither. California has its own AMT rules, including an ISO spread adjustment; this calculator estimates federal tax only. Financial inputs stay in the browser, are not stored, and are never added to URLs or analytics events. This is an educational estimate, not tax advice.
Sources and limits
- IRS Form 6251 and instructions: ISO exercise adjustment (line 2i), disposition basis adjustment, and Part III capital-gain rates.
- IRS Form 8801 and instructions: deferral versus exclusion items, credit limits, and carryforward.
- Rev. Proc. 2025-32, section 4.10: 2026 AMT exemptions, phaseout thresholds, and rate breakpoint.
- IRS Publication 525: incentive stock options, qualifying holding periods, and disqualifying dispositions.
- IRS Form 8960 instructions: net investment income tax on a sale.
Updated . Form references describe the currently published instructions; the calculation uses 2026 parameters. Federal only, standard deduction, wages as the only other income, one exercise and at most one qualifying sale. Capital-loss carryforwards are not modeled. The MFS add-back above $640,200 AMTI is not modeled because its 2026 rate is unsettled. See the ISO calculation methodology for the full model and exclusions.
Educational estimates only. Not tax, legal, or accounting advice. A CPA or EA can review your full return, the grant documents, and state rules before you commit cash.