ISOs and NSOs: How Each One Is Taxed

If your company has granted you incentive stock options (ISOs), non-qualified stock options (NSOs), or both — common once you've moved between employers mid-career — the tax treatment of each is different enough to change when and how much you should exercise. This article walks through what you owe, or don't, at each step: granting, exercising, and holding, for both option types, so you can compare the cash each route requires before you commit to an exercise date. It does not cover what happens if your options are still unexercised when you leave a job — that is a separate decision with its own deadline.

What is the difference between an ISO and an NSO?

Incentive stock options and non-qualified stock options start from the same basic instrument — the right to buy shares at a fixed strike price — but the tax code treats them differently, and only one of them gets the favorable treatment. An ISO is a creature of the tax code itself: it can only be granted to employees, never to contractors, consultants, or outside directors, and it only keeps its special status if the company follows a specific set of statutory requirements, such as a written plan approved by shareholders and a strike price set at or above fair market value on the grant date. An NSO carries none of those restrictions: it can be granted to anyone providing services to the company, priced as the company chooses, subject to the separate §409A valuation rules (a below-market strike price triggers them). Both types can sit side by side in the same startup equity package, and both are silent about tax at the grant date itself — neither one creates income when it is granted, only when you use it. The single sentence that matters for planning purposes is this: the difference between an ISO and an NSO does not show up at grant, and it does not show up while you are simply holding an unexercised option — it shows up the moment you exercise, which is where the rest of this article picks up.

What do I owe when I exercise an NSO?

Exercising an NSO is a taxable event, full stop — there is no way to exercise one without creating income in that tax year if the shares are worth more than your strike price. The taxable amount is the spread: the fair market value of the shares on your exercise date, minus what you paid to exercise them. That spread is treated exactly like a bonus — ordinary compensation income, reported through your employer's payroll rather than as investment income — and your employer is required to withhold on it the same way it withholds on any other supplemental wage payment: federal income tax, Social Security and Medicare tax, and state tax where it applies. You will see this reflected on your W-2 for the year of exercise, not on a 1099. Because the spread is already taxed as compensation at exercise, your cost basis in the shares resets to their fair market value on the exercise date — the same number used to calculate the income you just reported. Any further gain or loss after that point is a capital gain or loss, measured from the exercise-date value forward, not from your original strike price. This is the same basis mechanic that applies to RSUs at vest: the compensation event sets a new basis, and everything after that is simply an investment held from that date.

What do I owe when I exercise an ISO?

Exercising an ISO and simply holding the shares creates no regular income-tax liability in the year you exercise — this is the entire point of the ISO structure, and the main reason employees prefer it when they can get it. Nothing shows up on your W-2, and there is no withholding at exercise, because for regular-tax purposes an ISO exercise is not a taxable event as long as you keep the shares rather than selling them the same year. That does not mean the exercise is invisible to the tax system: the same spread that would have been ordinary income under an NSO — the fair market value of the shares at exercise minus your strike price — becomes an adjustment under the alternative minimum tax, a parallel tax calculation that can produce a real bill even though your regular return shows nothing owed. We cover how that calculation works, and how much it might cost you, in a separate article on AMT after an ISO exercise — it is a big enough topic to deserve its own explanation rather than a summary here. One statutory limit shapes how much of a single grant can even qualify for ISO treatment in the first place: no more than $100,000 worth of ISO stock, valued at the grant-date price, can become exercisable for the first time in any calendar year. Any amount of a grant that crosses that $100,000 threshold in a given year is automatically treated as an NSO instead, taxed the way the previous section describes, even though your option agreement still calls the whole grant an ISO.

How long do I have to hold ISO shares for the best tax treatment?

Getting the full benefit of ISO treatment — no ordinary income at exercise, and long-term capital-gains treatment on the entire gain when you eventually sell — requires clearing two separate holding-period clocks, both of which must be satisfied, not just one. The first clock runs from your grant date: you must hold the shares for at least two years after the option was granted. The second clock runs from your exercise date: you must hold the shares for at least one year after you exercised. Both conditions have to be true on the day you sell; satisfying only the longer of the two is not enough if the shorter one has not also been met. A sale that happens before either clock has run its course is called a disqualifying disposition, and it changes the tax result significantly: some or all of the spread that would otherwise have escaped ordinary-income treatment gets reclassified as ordinary compensation income in the year of the sale, reported through your employer even though you were not paid in cash for it, with any remaining gain or loss treated as capital gain or loss. This is why an early sale of ISO shares — even one that still nets you a gain — can produce a very different, and often larger, tax bill than an identical sale that waits until both clocks have run. Anyone thinking about selling ISO shares soon after exercising should check both dates before assuming long-term treatment applies.

What if my options expire before I can exercise?

Every option, ISO or NSO, has an outer expiration date built into its grant, and for an ISO that date cannot be more than ten years after the grant date — a statutory ceiling on how long the company can let the option stay outstanding at all. In practice, very few employees hold an option anywhere near that long, because leaving the company almost always shortens the clock well before the option's stated expiration. When your employment ends, your option agreement — not the tax code — sets how long you have to exercise before the option is forfeited; this post-termination exercise window is a plan design choice, and it varies from one company's option plan to the next, so the number that matters is whatever your own grant agreement or plan document says, not a general rule. For an ISO specifically, there is also a separate, tax-driven deadline that can be shorter than your plan's stated window: exercising too long after you leave your job can cause an otherwise-valid ISO to lose its favorable tax treatment even if your plan document would still technically let you exercise. That deadline interacts with severance, leave, and disability in ways that deserve their own explanation, which we cover in a separate article about leaving your job with unexercised options rather than compressing it into a footnote here. The short version for now: check your plan document's post-termination window, then check whether the ISO-specific deadline is shorter than that window before you assume you have as long as your plan paperwork implies.

Key numbers (2026)

• $100,000 first-year-exercisable ISO limit — no more than $100,000 of ISO stock, valued at grant, may first become exercisable in a calendar year; the excess is taxed as an NSO (IRC §422(d)).
• ISO holding periods for full tax treatment — at least two years from the grant date and at least one year from the exercise date, both required (IRC §422(a)(1); IRS Pub 525).
• Maximum ISO term — no more than ten years from the grant date (IRC §422(b)(3)).
• ISO post-termination exercise window — to preserve ISO tax treatment, you generally must exercise within three months after your employment ends (IRC §422(a)(2)).
• Form 3921 furnishing deadline — your employer must furnish you a Form 3921 for an ISO exercise by January 31 of the year following the exercise (IRS Pub 525; 26 CFR §1.6039-2).

Current as of 2026-09

Sources

• IRS Publication 525, Taxable and Nontaxable Income — https://www.irs.gov/publications/p525
• Instructions for Forms 3921 and 3922 — https://www.irs.gov/instructions/i3921
• Internal Revenue Code §422 — https://www.law.cornell.edu/uscode/text/26/422
• IRS Publication 15 (Circular E), Employer's Tax Guide — https://www.irs.gov/publications/p15
• 26 CFR §1.6039-2 — https://www.law.cornell.edu/cfr/text/26/1.6039-2
• Treasury Regulation §1.409A-1 — https://www.law.cornell.edu/cfr/text/26/1.409A-1

Sample content for demonstration purposes — not financial advice.