The short version
ISOs and NSOs are both rights to buy company stock at a set price, but they are taxed very differently. NSOs are taxed as ordinary income on the spread when you exercise. ISOs are not taxed for regular tax at exercise, though the spread can trigger AMT.
If you hold ISO shares long enough, your gain can be taxed at lower long-term rates. NSOs are simpler and more predictable but give up that potential rate advantage. Which type you hold, and what you do with it, drives the bill.
The quick version of each
An incentive stock option (ISO) is a special, tax-favored option that only employees can receive. Exercise and hold, and you owe no regular income tax at exercise, but the spread counts toward the alternative minimum tax (AMT), and if you meet the holding rules your gain can qualify for long-term capital gains treatment.
A non-qualified stock option (NSO) is the more common, more flexible type. Anyone can receive them, including contractors and advisors. When you exercise, the spread between your strike price and the share value is taxed as ordinary income right away, with payroll taxes and withholding, just like a bonus.
Side by side
Here is the same comparison in one view:
| ISO (incentive stock option) | NSO (non-qualified stock option) | |
|---|---|---|
| Who can receive it | Employees only | Anyone (employees, contractors, advisors, directors) |
| Tax at grant | None | None |
| Tax at exercise (regular tax) | None for regular tax, but the spread is an AMT adjustment | Spread taxed as ordinary income; payroll and withholding apply |
| Tax at sale | Capital gain; long-term if holding rules are met | Capital gain or loss on any change after exercise |
| Best-case rate | Long-term capital gains on the full gain over strike | Ordinary rates on the exercise spread, capital rates only on later growth |
| Main watch-out | AMT at exercise; meeting both holding periods | Withholding may understate the real bill |
The short story: NSOs are simpler and taxed sooner at ordinary rates, while ISOs offer a potential rate advantage in exchange for the AMT wrinkle and stricter holding rules.
How NSOs are taxed
With an NSO, exercising is the taxable event. The spread (share value at exercise minus your strike price) is ordinary compensation income that year, reported on your W-2 with taxes withheld. After that, your cost basis in the shares is their value at exercise, and any later change in price is a capital gain or loss when you sell.
NSOs are predictable, which is their strength. The main watch-out is that withholding on the spread may not fully cover your real rate, so a large exercise can still leave a balance due.
How ISOs are taxed
With an ISO, exercising and holding does not create regular income tax, which is the appeal. But the same spread is an adjustment for AMT, so you can owe tax even without selling. If you hold the shares more than two years from grant and more than one year from exercise, your gain can be taxed at long-term capital gains rates.
ISOs reward patience and planning, but the AMT effect catches people off guard. Because that interaction is the single most important thing to understand about ISOs, it has its own page: see ISOs and AMT.
One more rule worth knowing
There is a limit on how much ISO stock can first become exercisable for you in a single year and still get ISO treatment: $100,000, measured by the value of the stock at grant. Anything that vests above that $100,000 annual limit is generally treated as NSOs instead. This figure is set by statute and is not adjusted for inflation, so it has stayed at $100,000 for years. The practical upshot is that even a grant labeled as ISOs can be part NSO once it gets large, which is one more reason to confirm exactly what you hold.
Where this trips people up
Not knowing which type you actually hold
Grant letters are not always clear, and the tax difference is large. Confirm whether each grant is an ISO or an NSO before you exercise.
Assuming everything gets ISO treatment
Only employees can hold ISOs, and even then the annual limit can push part of a grant into NSO treatment. Contractors and advisors receive NSOs.
Forgetting AMT on ISOs
The headline advantage of ISOs comes with the AMT catch. Exercising and holding can create a tax bill with no sale and no cash.
Underestimating NSO taxes at exercise
The withholding on an NSO exercise may be lower than your real rate. A big exercise can create a shortfall even though tax was withheld.
Missing the ISO holding periods
Selling ISO shares too soon (a disqualifying disposition) pulls some of the gain back into ordinary income and changes the math entirely.
ISO and NSO questions people ask
What is the main tax difference between ISOs and NSOs?
Exercising an NSO creates ordinary income on the spread (value minus strike) right away, reported on your W-2 with withholding. Exercising an ISO creates no regular income at exercise, though it can trigger AMT, and it can qualify for long-term capital gain treatment if you meet the holding periods.
Are ISOs always better than NSOs?
Not necessarily. ISOs offer potential tax advantages but come with AMT complexity and strict holding requirements. NSOs are simpler and more flexible. The better choice depends on your situation, your available cash, and what you plan to do with the shares.
What is the $100,000 ISO limit?
No more than $100,000 of ISOs, measured by the value at grant, can first become exercisable in any one year. Anything above that is treated as an NSO. It is a common reason people end up holding a mix of both.
Can contractors or advisors get ISOs?
Generally no. ISOs are limited to employees. Non-employees such as contractors, advisors, and outside directors receive NSOs instead.
Do I owe tax when my options vest?
Usually not at vesting itself. For NSOs, the tax event is exercise; for ISOs, it is the AMT at exercise and the eventual sale. Vesting mainly determines when you are allowed to exercise.
How are NSOs taxed when I exercise?
The spread between the value at exercise and your strike price is ordinary income, reported on your W-2 and subject to withholding. That value becomes your cost basis, so only later appreciation is a capital gain when you sell.
Keep reading
ISOs and AMT
The full story on why ISO exercises can trigger AMT, and what to consider. →
RSUs and how they're taxed
Have RSUs alongside your options? Here is how those are taxed. →
The 83(b) election
Thinking about early-exercising? The 30-day election may come into play. →
All resources
Browse every equity-comp explainer in one place. →
Not sure what's in your grant?
Figuring out which options you hold and how to time them is exactly the kind of work I do with clients. If you are weighing an exercise or trying to make sense of a grant, I am happy to help you see the trade-offs clearly. Here is how to start a conversation.
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Andrew Sedlacek, CPA
Founder, OGCPA
Andrew is a Certified Public Accountant and the founder of OGCPA. He built his tax career at a local Bend firm and on Deloitte's tax team before founding the firm in 2019, and began professionally as a licensed financial advisor. He focuses on equity compensation, liquidity events, and the tax side of charitable giving and wealth distribution, and serves as the tax subject-matter expert for a venture-backed AI company. He works with clients across Bend, Oregon and the San Francisco Bay Area.
This article is general information, not tax or legal advice for your specific situation. Tax outcomes depend on your individual facts, and the rules change over time. Talk to a qualified professional (I am happy to be that person) before acting on anything here. Reading this page does not create a client relationship.