ISO vs NSO: Two Very Different Tax Bills
When you exercise a stock option, you pay the exercise price for shares that are worth more on paper — that gap is the bargain element. What happens to that gap at tax time depends entirely on the option type. Exercise a Non-Qualified Stock Option (NSO) and the bargain element is taxed immediately as ordinary income, no matter how long you hold the shares afterward. Exercise an Incentive Stock Option (ISO) and you generally owe no regular income tax at that moment — but the same bargain element becomes an Alternative Minimum Tax (AMT) preference item, which can still create a real cash tax bill even though you haven't sold a single share.
How the Estimate Works
| Item | Formula |
|---|---|
| Bargain element | Shares × (FMV − exercise price) |
| NSO tax now | Bargain element × marginal ordinary rate |
| ISO AMT estimate | Bargain element × ~26% (simplified AMT rate) |
If you hold ISO shares more than 1 year after exercise and more than 2 years after the grant date, the eventual sale can qualify for long-term capital gains rates instead of ordinary income tax. Sell too early and part of the gain gets reclassified as ordinary income (a disqualifying disposition). Because AMT calculations involve exemptions and phase-outs beyond this simplified estimate, talk to a tax professional before a large exercise.
Frequently Asked Questions
With NSOs, the bargain element (FMV minus exercise price) is taxed as ordinary income the moment you exercise. With ISOs, there's no regular income tax at exercise, but the bargain element counts as an AMT preference item that can trigger the Alternative Minimum Tax.
You need a qualifying disposition: hold the shares more than 1 year after exercise and more than 2 years after the grant date. Sell before that and part of the gain may be taxed as ordinary income instead.
Not regular income tax, but the AMT preference item can push you into Alternative Minimum Tax territory, which may create a real cash tax bill even though you haven't sold any shares.
※ This is a simplified reference estimate. Actual AMT liability depends on your full tax return, exemptions, and phase-outs — consult a tax professional before exercising a large grant.