ISO AMT calculator: how much will exercising cost?
Your option grants and income in; the AMT-free spread, shares per grant, AMT and cash needed out — under the 2026 rules.
The 2026 AMT rules
When you exercise an incentive stock option and keep the shares, the spread — fair market value minus strike, times shares — isn't regular income, but it is AMT income (IRC §56(b)(3)). AMT income less the exemption ($90,100 single, $140,200 joint) is taxed at 26%, and 28% above $244,500. From 2026 the exemption shrinks by 50 cents per dollar of AMT income over $500,000 ($1,000,000 joint) (Rev. Proc. 2025-32). You owe AMT only by the amount that tentative minimum tax beats your regular tax (IRC §55, Form 6251).
Worked example
The sample is a single filer with $180,000 of 2026 wages and a $12 409A price: a 10,000-share ISO grant at $1.50, a 5,000-share ISO refresh at $4.00 and an NSO grant. Regular federal tax is $31,934.00. About $32,923 of ISO spread fits before AMT — 4,115 shares of the refresh (the lowest spread per share), for $16,460.00 of strike. Exercising every ISO adds $145,000 of AMT income and $29,140.00 of AMT, so $64,140.00 of cash in total.
Your grants stay on your computer
Amtally reads your file with JavaScript in this browser tab. Nothing is uploaded, and the rows are gone when you close the tab.
How it works
- Copy your grants from your equity portal (Carta, Shareworks, E*TRADE, Pulley) into the template, or drop an export that has shares and strike price columns, or try the sample.
- Enter your 2026 income, filing status and the current 409A price.
- See your AMT-free spread and shares per grant — then confirm with Form 6251 or your CPA and exercise before December 31.
FAQ
Is my data uploaded?
No. Your files are read by JavaScript in your browser tab and never sent to us or anyone else. Grants aren't stored, so closing the tab clears them. Settings can be saved in this browser if you ask.
What does Amtally do?
It reads your option grants, works out your 2026 regular federal tax and tentative minimum tax, and finds the largest ISO bargain element (FMV minus strike, times shares) you can exercise this year with no alternative minimum tax. Then it splits that room across your ISO grants and shows the strike cost, and the AMT and cash needed if you exercise everything instead.
How is AMT worked out?
AMT income is your taxable income plus the standard deduction (or the state and local taxes you itemized) plus the ISO spread. Subtract the 2026 exemption — $90,100 single, $140,200 joint, reduced 50 cents per dollar above $500,000 / $1,000,000 — then tax 26% up to $244,500 and 28% above. AMT is the amount by which that beats your regular tax (IRC §55; Rev. Proc. 2025-32).
What changed for 2026?
The One Big Beautiful Bill Act (P.L. 119-21) cut the exemption phaseout start from $626,350 to $500,000 for single filers (and from $1,252,700 to $1,000,000 joint) and doubled the phaseout rate to 50%. High earners and big exercises reach AMT sooner, so the AMT-free amount you used in 2025 may be too high.
Is AMT money lost?
Usually not. AMT caused by ISO exercises becomes a minimum tax credit (Form 8801) you can use in later years when your regular tax is above your tentative minimum tax, for example the year you sell. But you pay it in cash now.
What's free?
Importing any number of grants, the AMT-free number, the three-way comparison, every flag, 3 FMV what-ifs, the plan for the first 2 grants and 12 grants in the table on screen and in print (with a footer). Pro shows everything and adds the Excel workbook and CSV exports.
What isn't covered?
State AMT (California's is 7%), long-term capital gains and qualified dividends at preferential rates, the net investment income tax, credits, NSO wage income from the same year, early exercise with an 83(b) election, and selling ISO shares in the same year (a disqualifying disposition).
Is this tax advice?
No. Amtally applies the rules it describes to the numbers you provide. It is an estimate to plan with — confirm with Form 6251 or a CPA before you exercise. It is not affiliated with the IRS, Carta or any equity platform.