Incentive stock options (ISOs) and non-qualified stock options (NSOs) are the two forms of employee stock option under United States tax law — an ISO can produce entirely capital-gain treatment but exposes the holder to alternative minimum tax at exercise, while an NSO is taxed as ordinary wage income on the spread at exercise.
In plain terms
Both instruments do the same commercial thing: they let an employee buy company stock later at a price fixed today. The difference is entirely tax. Exercise an NSO and the gap between the strike price and the current value is wages that day — withheld, reported on a W-2, taxed at ordinary rates. Exercise an ISO and nothing appears on the W-2; hold the shares long enough afterward and the whole gain from strike to sale is long-term capital gain. The catch is that the ISO spread is invisible to the regular tax system but perfectly visible to the alternative minimum tax, which can generate a large bill on stock the holder has not sold and may not be able to sell.
How it works
ISOs are creatures of statute and the statute is narrow. They go only to employees, not contractors or directors. They carry a ten-year maximum term and generally lapse ninety days after employment ends — the most consequential deadline in startup equity, because it forces a cash exercise and a possible AMT bill at the worst moment.
They are also capped. Under 26 CFR § 1.422-4, no more than $100,000 of stock, measured at grant-date fair market value, may become exercisable for the first time in any calendar year. Anything above that ceiling is automatically treated as an NSO — so large grants are hybrid instruments from the day they are signed.
To keep ISO treatment, two clocks must both run out: two years from grant and one year from exercise. Meet both and the sale is a qualifying disposition, taxed entirely as long-term capital gain. Miss either and it is a disqualifying disposition, and the spread at exercise reverts to ordinary income.
NSOs have none of this machinery. The spread is compensation, employment taxes apply, and the employer takes a deduction for the same amount — which is why boards grant them more readily than the employee-facing literature implies.
The numbers
- ISO annual limit: $100,000 of grant-date value first exercisable per calendar year; the excess converts to NSO treatment (26 CFR § 1.422-4). Not indexed for inflation.
- ISO holding periods: two years from grant and one year from exercise, both required.
- ISO term: ten years maximum; ninety days after employment ends.
- AMT exemption, 2026: $90,100 unmarried, $140,200 joint (IRS, Rev. Proc. 2025-32).
- AMT phaseout, 2026: begins at $500,000 of AMT income unmarried and $1,000,000 joint, withdrawing the exemption at 50 cents per additional dollar — up from 25 cents in 2025. AMT rates are 26% and 28%.
- NSO withholding, 2026: 22%, or 37% above $1,000,000 of supplemental wages in a year (IRS Publication 15).
- Jurisdiction: United States federal income tax. Several states run their own AMT.
What people get wrong
That an ISO is the better instrument and an NSO is the consolation prize. In 2026 that ranking is weaker than it has been in years: the AMT exemption phaseout thresholds dropped sharply and the withdrawal rate doubled, so the exemption now disappears at exactly the income levels where someone would be exercising a meaningful ISO grant. The ISO advantage is real only for a holder who can pay tax on a paper gain, wait out both clocks, and see the shares hold their value — three conditions a private-company employee controls none of.
The more common misreading is structural. Because the $100,000 ceiling is measured at grant-date value and applied in the order options were granted, a single grant routinely splits into an ISO tranche and an NSO tranche without anyone mentioning it — a fact usually buried in the grant notice and almost never in the offer letter.
Related
Read more: Equity Compensation: RSUs, ISOs, and the Tech Wealth Engine · Tech Wealth: How Founders and Investors Live Differently · Taxes: How Wealth Is Structured and Preserved
See also: RSU · 83(b) election · Alternative minimum tax · QSBS