ISOs and NSOs mainly differ in when and how you pay tax.
- An incentive stock option (ISO) generally creates no regular federal income tax at exercise, but it can trigger alternative minimum tax (AMT).
- A nonqualified stock option (NSO) usually creates compensation income when exercised.
Both let you buy company shares at a fixed strike price, so both require attention to purchase costs and stock risk.
How do ISOs and NSOs compare?
| Question | ISO | NSO |
|---|---|---|
| Who can get it | Eligible employees | Employees and nonemployees |
| Tax at exercise | Generally none | Income on the spread |
| AMT adjustment | Can apply if you hold | No |
| Withholding at exercise | Generally none | Generally yes |
| Later sale | Long-term gain if qualifying; else some ordinary income | Capital gain or loss after exercise |
| Special limits | Eligibility, exercise, holding rules | No $100,000 limit |
For an employee NSO, “compensation income” generally means wages. The spread is the stock’s value when exercised minus the price you pay.
What does each cost in a worked exercise-and-sale example?
Assume you have 10,000 vested options with a $2 strike. The shares are worth $20 when you exercise, and you later sell them at $50, with no fees:
| Item | Math | Amount |
|---|---|---|
| Purchase payment | 10,000 × $2 | $20,000 |
| Exercise spread | 10,000 × ($20 − $2) | $180,000 |
| Sale proceeds | 10,000 × $50 | $500,000 |
With NSOs:
- The $180,000 spread is generally compensation income at exercise.
- Your tax basis becomes $200,000: the $20,000 purchase payment plus the $180,000 already treated as income.
- The later sale creates a $300,000 capital gain. Its short- or long-term treatment depends on how long you held the shares.
With ISOs:
- The $180,000 can enter the AMT calculation if you hold the shares. It is an adjustment, not a $180,000 tax bill.
- If the sale meets the qualifying holding periods, the regular-tax gain is $480,000: proceeds minus the $20,000 purchase price.
- AMT uses a different basis, so the later return and any minimum-tax credit need separate calculations.
What makes an ISO sale qualify?
Generally, you must hold the shares more than one year after exercise and more than two years after grant, while meeting the other ISO requirements. The later deadline controls.
An early sale is called a disqualifying disposition. Part of the result can become ordinary income.
- The amount depends on the exercise value, sale price, and other facts; it is not always the full original spread. IRS Publication 525 explains that split.
- If exercise and disposition occur in the same tax year, the Form 6251 instructions generally require no ISO exercise adjustment. That removes the adjustment, not all tax on the sale.
What other ISO limits matter?
- The $100,000 limit. The $100,000 ISO limit applies to the grant-date value of shares underlying options that first become exercisable in a calendar year. It does not cap eventual proceeds or the amount you can choose to exercise that year. Options above the limit receive nonstatutory tax treatment.
- The post-employment window. After employment ends, you generally must exercise within three months to retain ISO treatment; exceptions include disability. Your contract may give you less time to exercise, or a longer window that changes the tax treatment.
You generally receive the option type specified in the grant. Planning means choosing what to do with that grant, not simply selecting whichever tax label looks better.
How should you choose an exercise size?
Start with the cash required to buy the shares and pay any projected tax. Then check:
- Expiration
- Sale restrictions
- Emergency reserves
- How much of your household wealth already depends on the employer
By option type:
- For NSOs, review withholding against your full tax projection.
- For ISOs, calculate potential AMT before assuming no tax is due at exercise. An NSO does not create the special ISO adjustment, but its income can still affect your overall AMT result.
Use the exercise timing checklist to compare the dates and cash needs. For ISOs, include a projected AMT calculation and a downside stock-price scenario before committing.
