If your company grants stock options, they will be one of two flavors: incentive stock options (ISOs) or non-qualified stock options (NSOs, also written NQSOs). Both give you the right to buy shares at a fixed price. The difference is entirely in how they are taxed, and that difference can be worth a great deal of money. This guide walks through how each works and when each one wins.
The shared vocabulary
Before comparing, a few terms apply to both:
- Strike price (or exercise price): the fixed price you pay to buy a share, set when the option is granted.
- Fair market value (FMV): what the share is worth on a given date.
- The spread: FMV minus strike price. This is your paper profit at the moment you exercise.
- Exercising: actually paying the strike price to convert options into shares.
Both ISOs and NSOs have no tax at grant and no tax at vesting. The action happens when you exercise and when you sell. That is where they diverge.
How NSOs are taxed
NSOs are the simpler of the two and the most common. When you exercise an NSO, the spread is taxed immediately as ordinary income. It is treated like wages and reported on your W-2 if you are an employee, subject to income tax plus Social Security and Medicare.
Worked example: your strike is 5 dollars, the FMV at exercise is 25 dollars, and you exercise 1,000 shares. The spread is 20 dollars per share, so 20,000 dollars is added to your ordinary income that year, regardless of whether you sell.
Your cost basis in the shares becomes the FMV at exercise, 25 dollars. When you later sell, any further gain or loss above 25 dollars is a capital gain or loss, short or long term depending on how long you held after exercising. You can estimate the income hit with our NSO calculator and the resulting sale tax with the capital gains tax calculator.
NSOs can be granted to anyone: employees, contractors, advisors, and board members.
How ISOs are taxed
ISOs receive preferential treatment, but only if you follow strict rules. ISOs can be granted only to employees.
When you exercise an ISO, there is no ordinary income tax on the spread for regular tax purposes. That sounds like a free lunch, but the spread is added back as a preference item under the alternative minimum tax, which we explain below. So exercising an ISO can quietly create an AMT bill even though no regular tax is due.
The payoff comes at sale. If you meet both holding requirements, the entire gain from strike price to sale price is taxed as long-term capital gain, which is a lower rate than ordinary income. The two requirements are:
- Hold the shares more than two years from the grant date, and
- Hold them more than one year from the exercise date.
Meet both and you have a qualifying disposition, taxed entirely at long-term capital gains rates. Miss either and you have a disqualifying disposition, which is taxed largely like an NSO, with the spread becoming ordinary income.
Worked example: strike 5 dollars, FMV at exercise 25 dollars, sale price 40 dollars, both holding periods satisfied. You owe no ordinary income tax along the way for regular tax, and at sale your entire 35 dollar per share gain is long-term capital gain. The catch is the 20 dollar spread was an AMT preference item in the exercise year.
The alternative minimum tax catch
The AMT is a parallel tax system designed to ensure people with certain deductions and preferences still pay a baseline amount. The ISO exercise spread is one of the largest preference items most people will ever encounter.
This creates a real cash trap. You can exercise ISOs, owe no regular tax, hold the shares to chase long-term treatment, and still get a sizable AMT bill in April for stock you have not sold and cannot easily turn into cash. If the stock then falls, you may owe tax on a gain that evaporated.
This is the single most important reason to model ISO exercises before you pull the trigger. Our ISO AMT calculator and the broader AMT calculator help you find the number of shares you can exercise before AMT kicks in.
Side-by-side summary
| Feature | ISO | NSO |
|---|---|---|
| Who can receive | Employees only | Anyone |
| Tax at grant | None | None |
| Tax at vest | None | None |
| Tax at exercise | No regular tax; spread is an AMT preference | Spread taxed as ordinary income |
| Tax at sale | Long-term capital gain if holding rules met | Capital gain or loss above exercise FMV |
| Holding rule for best treatment | 2 years from grant and 1 year from exercise | 1 year from exercise for long-term rates |
| Main risk | AMT bill on unsold shares | Higher ordinary tax, but simpler |
Which is better?
ISOs offer a lower potential tax rate but carry AMT complexity and rigid holding periods. NSOs cost more in tax but are predictable and flexible. Neither is universally better.
For early employees at a startup with a low strike price and large upside, ISOs can be very valuable if exercised thoughtfully, often early when the spread and therefore the AMT exposure is small. For later-stage employees, contractors, or anyone who values simplicity and liquidity, NSOs are perfectly reasonable.
The practical takeaway: the option type determines the tax rules, but your timing of exercise and sale determines the actual bill. Model it before acting.
A note on early exercise and the 83(b) election
Some plans let you exercise options before they vest, known as early exercise. Paired with a Section 83(b) election filed within 30 days, this can start your capital gains holding clock sooner and lock in the spread at today’s low value, which is most attractive when the strike price and fair market value are nearly identical at an early-stage company. The trade-off is that you pay the strike price up front for shares you could still forfeit if you leave, and the cash you spend is at risk if the company fails. Early exercise is a real lever for the right situation, but it is an aggressive move that deserves careful thought and, for large amounts, a conversation with a tax professional. The same caution applies to any decision involving a six-figure spread, because the tax and cash-flow consequences are not easily reversed.
Frequently asked questions
Do I pay tax when stock options are granted?
No. Neither ISOs nor NSOs are taxed at grant, and neither is taxed simply for vesting. Taxable events occur when you exercise and when you sell the resulting shares.
Can incentive stock options trigger taxes even if I do not sell?
Yes, through the alternative minimum tax. The spread at ISO exercise is an AMT preference item, so a large exercise can create an AMT bill in that year even though you have not sold a single share and owe no regular income tax. Model it with the ISO AMT calculator first.
What happens if I sell ISO shares too early?
You trigger a disqualifying disposition. The benefit of long-term capital gains treatment is lost on the spread portion, which is recharacterized as ordinary income, making the result resemble an NSO. The remaining gain above exercise FMV is taxed as a capital gain.
Are NSOs always worse than ISOs?
Not necessarily. NSOs are taxed at higher ordinary rates on the spread, but they avoid AMT complexity, have no two-year grant holding requirement, and can be granted to contractors and advisors. Their predictability is a genuine advantage, especially when you plan to sell shortly after exercising.