Stock Option Tax Calculator
Compare the federal tax on non-qualified stock options (NSOs) versus incentive stock options (ISOs) from exercise through sale — 2025 tax year, simplified, with the AMT trap flagged.
Reviewed by the CalcCafe editorial team · Last updated 18 July 2026 · How we test our tools
Example
You exercise 1,000 options with a $5 strike when the shares are worth $25, and later sell at $40 after holding more than a year. The exercise spread is $20,000. As NSOs, that spread is ordinary income at your 32% marginal rate — $6,400 at exercise — and the further $15,000 of appreciation is a long-term gain taxed at 15%, adding $2,250, for a total of $8,650. As ISOs sold in a qualifying disposition, nothing is due at exercise (before AMT) and the entire $35,000 profit is a long-term gain: $5,250. The ISO advantage is $3,400 — before any alternative minimum tax from the exercise year.
How it works
Spread at exercise = (FMV − strike) × shares, floored at zero. NSO: the spread is compensation, taxed at your marginal ordinary rate in the exercise year; when you sell after holding more than a year past exercise, the gain above the exercise-date FMV is a long-term capital gain at your capital-gains rate. ISO with a qualifying disposition (held over 1 year after exercise and 2 years after grant): no regular tax at exercise, and the whole gain from strike to sale price is a long-term capital gain. ISO advantage = NSO total − ISO total. This is a 2025-tax-year, federal-only, simplified educational model: it assumes long-term treatment, uses the flat rates you enter (for reference, 2025 long-term gains brackets for single filers are 0% up to $48,350, 15% to $533,400, 20% above), and deliberately excludes AMT, the 3.8% net investment income tax, and state taxes.
Good to know
NSOs trigger withholding the moment you exercise: the spread lands on your W-2 (or 1099 for non-employees) and employers typically withhold at the flat supplemental-wage rate — 22% federal up to $1 million of supplemental wages, 37% beyond — plus Social Security and Medicare. If your actual marginal rate is higher than the withholding rate, the difference is due at filing, so a big exercise can leave you owing at tax time even though tax was already withheld. Plan cash for both the strike price and the tax before you exercise.
ISO treatment is earned, not automatic. A qualifying disposition requires holding the shares more than one year after exercise and more than two years after grant; sell earlier and it becomes a disqualifying disposition — the bargain element is recharacterized as ordinary income and the ISO mostly collapses into NSO-like treatment. ISOs also carry a $100,000 limit on the value of options first exercisable in any calendar year; the excess is treated as NSOs from the start.
The AMT trap is the part this calculator deliberately does not model, and it deserves respect: when you exercise ISOs and hold, the spread is an adjustment for the alternative minimum tax even though no regular tax is due. Exercise a large block at a high paper valuation and you can owe six figures of AMT on gains you have never sold — the classic dot-com-era disaster was employees who exercised at the peak, held, watched the stock crash, and still owed AMT on the vanished spread. Run Form 6251 before exercising, consider exercising early in the year (you keep the option to sell before year-end and unwind the hit), and know that AMT paid on ISOs often comes back in later years as a minimum tax credit.
Watch the clock when you leave a job: vested ISOs must generally be exercised within 90 days of termination to keep ISO status — a statutory requirement, and while some companies extend the exercise window, the options legally convert to NSO treatment after those 90 days. Some plans also allow early exercise of unvested options; pairing that with an 83(b) election filed within 30 days starts your capital-gains and ISO holding clocks immediately and can freeze the spread near zero — powerful at low early-stage valuations, but you are buying restricted stock the company can repurchase if you leave before vesting.
Frequently asked questions
Do I pay tax when I exercise stock options?
With NSOs, yes — the spread between fair market value and your strike price is ordinary income at exercise, with withholding taken like a bonus. With ISOs there is no regular income tax at exercise, but the spread counts for the alternative minimum tax, which can produce a real bill in the exercise year if the spread is large.
What makes an ISO sale a qualifying disposition?
Holding the shares more than one year after the exercise date and more than two years after the grant date. Meet both and the entire profit from strike price to sale price is a long-term capital gain. Sell earlier and it is a disqualifying disposition: the bargain element becomes ordinary income, similar to an NSO.
Does this calculator include the alternative minimum tax?
No — and for ISOs that is a major simplification. The ISO spread at exercise is an AMT preference item, and exercising a large block while holding the shares can trigger a substantial AMT bill on paper gains. Use Form 6251 or a tax professional to check your AMT exposure before exercising; this tool is a federal-only, 2025-tax-year educational model.
Is this calculator free, and is my data uploaded anywhere?
Yes, it is completely free with no sign-up, and no — it runs entirely in your browser. Your share counts, prices and tax rates never leave your device.
People also ask
What is the AMT trap with incentive stock options?
Exercising ISOs and holding the shares makes the spread taxable for AMT purposes even though nothing was sold. If the stock later falls, you may have paid tax on value that evaporated. Exercising early in the year preserves the option to sell before year-end and unwind the AMT hit, and AMT paid on ISOs often returns in later years as a minimum tax credit.
What happens to my stock options when I leave my job?
Unvested options are almost always forfeited. Vested ISOs must generally be exercised within 90 days of termination to keep ISO status — many plans cancel them after that window, and even extended windows convert them to NSO treatment. Check your grant agreement before resigning; the exercise cash and the tax bill both come due fast.
What is an 83(b) election for stock options?
If your plan allows early exercise of unvested options, an 83(b) election filed with the IRS within 30 days of exercise lets you pay tax on the spread immediately — often near zero at an early-stage company — and starts your capital-gains holding period right away. Miss the 30-day deadline and there is no do-over.
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Sources & references
These tools follow our methodology and provide educational estimates only — verify important figures with a qualified professional.