Every startup offer letter says “stock options,” and almost nobody asks which kind until the exercise window is closing or a tax bill shows up that wasn’t supposed to exist. The two types — incentive stock options (ISOs) and non-qualified stock options (NSOs) — grant the same thing, the right to buy shares at a fixed strike price. What differs is entirely on the tax side, and the difference is large enough to change how much of an exit you actually keep.
The One-Line Version
Both option types work the same way mechanically: you’re granted the right to buy a fixed number of shares at a fixed strike price (usually the fair market value on the grant date), the right vests over time, and you choose when to exercise.
The tax treatment splits at exercise:
| NSO | ISO | |
|---|---|---|
| Who can receive it | Anyone — employees, contractors, advisors, board members | Employees only (IRC §422) |
| Tax at grant | None | None |
| Tax at exercise | Ordinary income on the spread (FMV − strike), plus payroll tax withholding | None for regular tax — but the spread is an AMT preference item |
| Tax at sale (qualifying) | Capital gain on appreciation since exercise | Long-term capital gain on the entire gain since strike price, if holding periods are met |
| Annual limit | None | $100,000 of FMV (at grant) vesting per year |
That single difference — whether the spread at exercise is taxed now, taxed later, or taxed as an AMT preference item that might come back as a credit later — is what the rest of this guide walks through.
How NSOs Are Taxed
NSOs are the simpler case, mechanically, even though “simple” doesn’t mean “cheap.”
At exercise: the spread between what you pay (the strike price) and the stock’s fair market value that day is taxed as ordinary income, reported on your W-2 (for employees) or 1099 (for non-employees), in the year you exercise. If your company withholds taxes on equity compensation, this is the moment it happens — and if it’s private stock with no market to sell into, you can owe real cash tax on paper gains you can’t liquidate.
At sale: whatever additional appreciation happens after exercise is a capital gain — short-term if sold within a year of exercise, long-term (lower rate) if held more than a year past exercise. The exercise-day spread was already taxed as ordinary income; it doesn’t get taxed again at sale.
Worked example: you exercise 10,000 NSOs with a $1 strike price when the stock is worth $6 FMV. You owe ordinary income tax on the $50,000 spread ($5 × 10,000) that year, whether or not you sell a single share. If you later sell at $10, the additional $4/share ($40,000 total) is a capital gain, taxed at short- or long-term rates depending on how long you held after exercise.
How ISOs Are Taxed — and Where AMT Comes In
ISOs are the option type most startup employees actually receive, and the reason people reach for them is the deferral: no regular income tax is due at exercise. That’s the headline benefit.
The catch is the Alternative Minimum Tax (AMT), a parallel tax system that recalculates your liability under a different set of rules and makes you pay whichever number is higher. For AMT purposes, the ISO exercise spread — the exact same FMV-minus-strike number that would be ordinary income on an NSO — is added back as a “preference item.” Exercise enough ISOs at a wide enough spread, and you can owe a real AMT bill on stock you’re still holding and can’t sell.
At exercise: no regular income tax. The spread is added to income for the separate AMT calculation only.
At sale — qualifying disposition: if you hold the shares more than two years from the grant date and more than one year from the exercise date, the entire gain (from strike price to sale price) is taxed as a long-term capital gain, and the earlier AMT preference item is reconciled against actual tax paid (see the AMT credit below).
At sale — disqualifying disposition: sell before both holding periods are satisfied, and the IRS retroactively treats the exercise-to-FMV spread (at exercise, or at sale if the stock dropped in between — whichever is smaller) as ordinary income in the year of sale, with any further gain taxed as a capital gain. This is functionally close to NSO treatment, just recognized a year or two later than it would have been.
The AMT Credit: You Usually Get It Back, Eventually
AMT paid on an ISO exercise isn’t simply lost — in most cases it generates a minimum tax credit that can offset regular tax liability in future years, once your regular tax exceeds your AMT liability again. The problem isn’t that the money disappears forever; it’s timing and liquidity. You can owe AMT cash this year on stock that’s illiquid or, worse, has since dropped in value — a scenario that hit employees at several late-2010s IPO-bound companies hard when the stock fell after exercise but the AMT bill, based on the exercise-day price, didn’t.
A simplified example: you exercise 10,000 ISOs with a $1 strike when FMV is $21 — a $200,000 spread. No regular income tax is due. But that $200,000 is added to your AMT income. For 2026, the AMT exemption is $90,100 (single) or $140,200 (married filing jointly), phasing out above $500,000/$1,000,000 of AMT income, and the AMT rate is 26% up to $244,500 of AMT income above the exemption, 28% above that (IRS, Rev. Proc. 2025-32). Depending on your other income, a $200,000 preference item can easily generate a $40,000–$55,000 AMT bill — due the following April, on stock you may not be able to sell.
The $100,000 ISO Limit
IRC §422(d) caps how much ISO treatment you can get in a single calendar year: only the first $100,000 of fair market value (priced at grant, not at vesting) that becomes exercisable for the first time in a given year keeps ISO status. Anything vesting past that $100,000 threshold is automatically treated as an NSO — even if your grant letter calls the whole thing an ISO (26 CFR §1.422-4).
This mostly matters for early employees at companies with large grants and standard four-year vesting: a big enough grant, vesting evenly, can push later tranches over the $100K line without anyone flagging it until the exercise paperwork shows a mix of ISO and NSO shares from the same original grant.
Early Exercise and the 83(b) Election
Some companies let employees exercise unvested options early — buying the shares before they’ve vested, subject to a right of repurchase that lapses on the normal vesting schedule. Doing this and filing an 83(b) election within 30 days locks in the ordinary-income (NSO) or AMT-preference (ISO) calculation at the current, presumably low, spread — often close to zero right after a fresh 409A valuation — instead of at each future vesting date when the spread may be much larger.
This is the same 83(b) mechanism covered in our founder equity and vesting guide: miss the 30-day window and the option to make this election is gone permanently for that grant, no exceptions. Early exercise also starts the capital-gains holding clock earlier, which matters for QSBS eligibility if the company is a qualifying C-corp — one more reason the exercise decision and the tax-structuring decision aren’t separable.
Common Mistakes
- Exercising a large ISO grant near year-end without checking AMT exposure first. The AMT preference item is calculated based on FMV on the exercise date — waiting for a lower 409A valuation, or splitting a large exercise across two calendar years, can meaningfully change the bill.
- Assuming a “disqualifying disposition” is a penalty, not just a reclassification. It converts ISO treatment to something close to NSO treatment retroactively — often still the right call if you need liquidity or the stock has dropped and holding for the qualifying period would cost more than it saves.
- Not knowing which type of option you hold. A single employee grant can straddle both — the first $100K tranche as ISO, the rest as NSO — and the tax treatment for each portion has to be tracked separately at exercise and sale.
- Letting the post-termination exercise window lapse. Most option grants require exercise within 90 days of leaving the company (a small number of employers now offer extended windows). Missing it forfeits the options entirely, regardless of how much they’d vested.
- Treating AMT as money lost instead of a credit to plan around. The AMT credit generated by an ISO exercise can offset regular tax in future years — worth discussing with a tax preparer before assuming the AMT payment disappears.
FAQ
Which type of option do most startup employees get?
ISOs, when the recipient is a W-2 employee — companies generally prefer to grant ISOs to employees because of the favorable tax treatment, subject to the $100K annual limit. Anyone who isn’t a common-law employee (contractors, advisors, board members, most consultants) can only legally receive NSOs, since IRC §422 restricts ISOs to employees.
Do I owe tax just for having unexercised options?
No. Neither ISOs nor NSOs trigger any tax at grant or during vesting — tax only enters the picture when you exercise (both types, for AMT on ISOs and ordinary income on NSOs) or sell (capital gains on the appreciation, plus ordinary income on a disqualifying ISO disposition).
Can my company convert my ISOs to NSOs, or vice versa?
A company can choose to grant NSOs from the start, and ISO tranches automatically become NSOs once they exceed the $100K annual limit. Voluntarily converting existing ISOs to NSOs is unusual but can happen around an acquisition, when the surviving company’s option plan doesn’t support ISOs — read any acquisition-related option paperwork carefully, since this changes the tax treatment of shares you may have held for years already.
Does exercising options early always make sense?
Not automatically. Early exercise plus an 83(b) election locks in a low tax basis while the spread is small, but it also means paying real cash (the strike price, plus any AMT on an ISO exercise) for stock in a company that might fail — money you don’t get back if the company doesn’t work out. It’s a bet on the company, made earlier than you’d otherwise have to make it.
None of this is a substitute for running your actual numbers with a tax preparer before a large exercise — the dollar thresholds above are 2026 figures and shift with inflation adjustments most years, and state tax treatment (particularly for AMT, which several states calculate differently from the federal version) adds another layer this guide doesn’t cover. For the fuller mechanics of vesting, 83(b) elections, and reading a cap table before any of this becomes a live decision, see our founder equity and vesting guide. Kaye Thomas’s Consider Your Options remains the most complete plain-English reference on the exercise and disposition mechanics above, updated for current tax law and worth having open the week an exercise decision is actually due. For more on structuring founder and employee finances deliberately, see the finance section on FutureSharks.
