Stock options are not all taxed the same way. Incentive stock options (ISOs) and non-qualified stock options (NSOs) follow entirely different tax rules at exercise, at sale, and for purposes of the alternative minimum tax. Getting these wrong -- or simply not knowing which type you hold -- can cost tens of thousands of dollars in unnecessary taxes or missed planning opportunities.
Getting ISO and NSO tax treatment wrong -- or not knowing which type you hold -- can cost tens of thousands of dollars in unnecessary taxes. Check your stock option grant agreement or your equity management platform (Carta, Shareworks, E*Trade) to confirm the option type before making any exercise decisions.
The fundamental divide between ISOs and NSOs is timing. With NSOs, you owe ordinary income tax the moment you exercise. With ISOs, you owe nothing at exercise for regular tax purposes -- but the AMT may still apply, and you must meet strict holding period requirements to keep the favorable treatment.
NSOs are governed by IRC Section 83, which treats the spread at exercise -- the difference between the stock's fair market value and your exercise price -- as compensation income. Your employer reports this spread on your W-2 in Box 12 (Code V), withholds federal income tax, and deducts FICA taxes. You have a taxable event the day you exercise, whether or not you sell a single share.
ISOs are governed by IRC Sections 421 and 422. No regular income tax is owed at exercise, and no amount appears on your W-2. But the spread does count as an adjustment for the alternative minimum tax, reported on Form 6251. Your employer issues Form 3921 after the year you exercise, documenting the exercise details for your records and the IRS.
How ISOs Are Taxed
The tax treatment of ISOs depends entirely on whether you meet the holding period requirements. If you do, the entire profit is taxed as a long-term capital gain. If you do not, part of the profit is recharacterized as ordinary income.
Qualifying disposition. To qualify for long-term capital gains treatment, you must hold the stock for at least one year after the exercise date and at least two years after the grant date. IRC Section 422(a)(1) specifies both holding period requirements. Meet both, and when you sell, the difference between your sale price and exercise price is taxed entirely as long-term capital gains -- currently 0%, 15%, or 20% depending on your taxable income.
Disqualifying disposition. Sell before satisfying either holding period, and the IRS treats the transaction differently. The spread at exercise (the FMV on the exercise date minus your exercise price) is taxed as ordinary income. Any additional gain above the exercise-date FMV is capital gain, short-term or long-term depending on how long you held the shares after exercise. Technical detail
No payroll taxes (Social Security or Medicare) apply to ISO exercises or qualifying dispositions. This is one of the advantages over NSOs.
How NSOs Are Taxed
NSOs are more straightforward but typically more expensive at exercise.
At exercise: The spread is ordinary income, period. If your company's stock is worth $50 per share and your exercise price is $10, you have $40 per share of ordinary income. On 1,000 shares, that is $40,000 of W-2 income, subject to federal income tax, Social Security tax (up to the wage base), and Medicare tax (including the 0.9% Additional Medicare Tax if your wages exceed $200,000 for single filers). Your employer withholds taxes and takes a corresponding compensation deduction. Technical detail
At sale: Your cost basis in the stock is the FMV at the time of exercise (not the exercise price). Any subsequent gain or loss from that basis is capital gain or loss -- short-term if you hold for one year or less after exercise, long-term if you hold for more than one year.
The AMT Trap With ISOs
ISOs do not trigger regular income tax at exercise, but they can trigger the alternative minimum tax. This is the issue that catches the most people off guard.
When you exercise ISOs and hold the shares (rather than selling in the same year), the spread at exercise is added to your AMT income. Technical detail
- Exercises all 5,000 shares in a single year
- AMT adjustment of $200,000 added to income
- Tentative minimum tax exceeds regular tax
- Owes ~$37,620 in additional AMT on stock not sold
- Spreads exercises across 3 tax years to stay below AMT threshold
- Models AMT breakeven point each year
- Sells some shares in the same year to eliminate AMT on those shares
- Uses AMT credit carryforward from any remaining AMT paid
The 2026 AMT exemption for single filers is $90,100. The exemption phases out at 25 cents per dollar of AMTI above $500,000. Rev. Proc. 2024-40 (inflation-adjusted AMT amounts for 2025); 2026 amounts per OBBBA-adjusted thresholds. Sarah's AMTI of $350,000 is below the phaseout threshold, so she gets the full $90,100 exemption. Her AMT base is $350,000 - $90,100 = $259,900. At the 26% AMT rate (the rate on AMT income up to $244,500 for 2026) plus 28% on the excess, her tentative minimum tax is roughly $67,720. Her regular tax on $150,000 of income is approximately $30,100. Because the tentative minimum tax exceeds her regular tax, she owes roughly $37,620 in additional AMT -- on stock she has not sold and may not be able to sell.
The AMT paid generates a credit (Form 8801) that can be used in future years when her regular tax exceeds the AMT, but the cash is gone now. This is why exercise timing matters.
The Section 83(b) Election
Section 83(b) allows you to elect to be taxed on the value of restricted property at the time of transfer rather than when it vests. This election is most relevant for early-exercise stock options (where you exercise before the shares are fully vested) or restricted stock grants. It does not apply to standard vested option exercises.
- 30-day deadline. You must file the election with the IRS within 30 days of receiving the property. Not 30 business days -- 30 calendar days.
Technical detail
If Day 30 falls on a weekend or federal holiday, the deadline extends to the next business day. The election must be mailed to the IRS office where you file your return. - Irreversible. Once filed, you cannot undo the election. If the stock later becomes worthless or you forfeit the shares, you do not get a refund of the tax you paid.
- Send a copy to your employer. You must also provide a copy of the election to your company.
When 83(b) makes sense. If you can exercise options early (before vesting) and the current FMV is close to your exercise price, the taxable spread is small or zero. By filing an 83(b) election, you pay minimal tax now and start the capital gains holding period clock immediately. All future appreciation is taxed as capital gain rather than ordinary income when the shares vest.
When 83(b) is risky. If the stock price drops or you leave the company and forfeit unvested shares, you have paid tax on income you never realized, and there is no mechanism for recovery.
When you exercise matters almost as much as what type of option you hold.
Exercise early in the year (ISOs). If you exercise ISOs in January and the stock drops by December, you can sell before year-end and eliminate the AMT adjustment. The sale would be a disqualifying disposition (you have not met the one-year holding period), but you avoid the AMT problem entirely.
Exercise and sell in the same year (ISOs). Same-day or same-year sales of ISO shares eliminate the AMT issue because the preference item is zero when you sell in the same tax year. The trade-off: the gain is taxed as ordinary income (disqualifying disposition), so you lose the capital gains benefit.
Spread exercises across tax years (ISOs). Rather than exercising all your ISOs at once, exercise in batches sized to keep your AMT exposure manageable. A CPA can model the AMT breakeven point for your specific income level.
Exercise when the spread is small (NSOs and ISOs). The lower the FMV relative to your exercise price, the less ordinary income (NSOs) or AMT exposure (ISOs) you trigger. Exercising early -- when the company's 409A valuation is still low -- is one of the most effective tax strategies for startup employees, especially when paired with an 83(b) election.
Cashless Exercise vs. Exercise-and-Hold
Cashless exercise (sell-to-cover). You exercise and immediately sell enough shares to cover the exercise cost and taxes. You receive the remaining shares or cash. No out-of-pocket cost, but you realize a taxable event on the shares sold. For ISOs, this is a disqualifying disposition on the sold shares. For NSOs, the ordinary income tax is covered by the proceeds.
Exercise-and-hold. You pay the exercise price out of pocket and keep all the shares. For ISOs, this preserves the potential for qualifying disposition treatment (long-term capital gains) but exposes you to AMT risk on the spread. For NSOs, you have already paid ordinary income tax on the spread at exercise; subsequent appreciation is capital gain.
The decision depends on your cash position, your confidence in the stock's future, and your AMT exposure. Many employees use a blended approach: cashless exercise on a portion to generate liquidity, exercise-and-hold on the rest to pursue long-term capital gains treatment.
When Each Type Is Better
Neither ISOs nor NSOs are universally superior. The advantage depends on your circumstances.
ISOs favor employees who:
- Can afford to hold the stock for the required holding periods (one year from exercise, two years from grant)
- Have AMT exposure that is manageable relative to the expected tax savings
- Are in a high ordinary income tax bracket, making the spread between ordinary and capital gains rates significant
- Work at companies with high growth potential where exercising early (small spread) and holding is viable
NSOs favor employees who:
- Plan to sell immediately at exercise (the ISO holding period advantage disappears)
- Want certainty about the tax bill at exercise rather than navigating AMT calculations
- Are contractors or advisors (ISOs are only available to employees under IRC Section 422(b))
- Receive options with high spreads where the AMT cost of ISOs would be prohibitive
The Net Investment Income Tax
One more layer: the 3.8% Net Investment Income Tax (NIIT) under IRC Section 1411 applies to capital gains from stock sales if your modified adjusted gross income exceeds $200,000 (single) or $250,000 (married filing jointly). These thresholds are not indexed for inflation. The NIIT thresholds were set by the Affordable Care Act and have not been adjusted since 2013.
For ISOs with a qualifying disposition, the entire gain is subject to NIIT (it is a capital gain). For NSOs, the ordinary income portion at exercise is W-2 wages and is not subject to NIIT, but any subsequent capital gain on the shares is. In some scenarios, NSOs produce a slightly lower combined tax burden because the exercise spread avoids NIIT, even though it is taxed at ordinary income rates. The math depends on your marginal bracket, the size of the spread, and your total investment income.
The single most consequential stock option decision for startup employees is whether to exercise early -- when the 409A valuation is still low -- and pair it with an 83(b) election. The numbers are worth modeling with a CPA before you commit, especially before an IPO or funding round that will increase the valuation.
| Feature | ISO | NSO |
|---|---|---|
| Tax at exercise | None for regular tax; AMT adjustment on spread | Ordinary income on spread (W-2) |
| Payroll taxes at exercise | None | Social Security + Medicare |
| Tax at sale (qualifying) | Long-term capital gains on full gain | Capital gain on appreciation above exercise-date FMV |
| Holding period for LTCG | 1 year from exercise + 2 years from grant | 1 year from exercise |
| Employer deduction | None (qualifying disposition) | Yes, equal to employee's ordinary income |
| Available to | Employees only | Employees, contractors, advisors |
| 83(b) election | Not applicable to standard ISOs | Available for early-exercise/restricted stock |
| AMT risk | Yes, on exercise-date spread | No |
| NIIT on gain | Yes (capital gain) | Only on post-exercise appreciation |