You have stock options. The grant document uses one of two acronyms, and which one determines when you owe tax, how much, at what rate, and whether you can end up owing money on gains you never received in cash.
Most people learn the difference the year they exercise. It is considerably cheaper to learn it beforehand.
What changed in 2026
- AMT exemption amounts continued adjusting with inflation, which shifts where the ISO trap bites but does not remove it.
- Secondary markets stayed more accessible. More private company employees have some route to liquidity, which changes exercise planning — though availability remains company-dependent.
- Extended exercise windows spread further. More companies moved beyond the traditional 90-day post-departure window, which reduces forced exercises at bad moments.
- The rules themselves did not change. ISO and NSO treatment is long-standing. What moves annually are thresholds, exemptions, and rates.
The two types
|
ISO |
NSO |
| Who can receive |
Employees only |
Employees, contractors, advisors, directors |
| Tax at grant |
None |
None |
| Tax at exercise |
No ordinary income; spread is an AMT item |
Spread taxed as ordinary income |
| Withholding at exercise |
None |
Yes, through payroll |
| Tax at sale (qualifying) |
Long-term capital gain on the whole gain |
Capital gain on post-exercise growth only |
| Holding period required |
Two years from grant, one from exercise |
Standard capital gains rules |
| Company deduction |
None if qualifying |
Yes |
The row that generates surprise is withholding. An NSO exercise runs through payroll, so tax is withheld and you see it. An ISO exercise has no withholding at all — nothing appears on your payslip, nothing is taken, and the liability shows up when you file.
The ISO trap
ISOs sound strictly better, and they carry a specific risk worth stating plainly.
At exercise, the spread between the strike price and the fair market value is not ordinary income — but it is an adjustment for alternative minimum tax. AMT is a parallel calculation, and a large ISO spread can push you into owing under it.
The problem is cash. If your company is private and there is no market for the shares, you have exercised, you owe AMT on a paper gain, and you cannot sell anything to pay it. People have owed substantial sums on shares that later became worthless — the tax was assessed on the value at exercise, and a subsequent collapse does not undo it.
This is not hypothetical or rare, and it is entirely avoidable by modelling the AMT before exercising rather than after. ISO AMT explained covers that calculation in more detail.
Two mitigations. Exercise early, when the spread is small — ideally soon after grant, when strike and fair market value are close, so the AMT adjustment is minimal. And exercise in tranches across tax years, keeping each year's adjustment below where AMT bites.
Holding periods and disposition
To get the favourable ISO treatment — the entire gain as long-term capital gain — you must hold the shares for two years from grant and one year from exercise. Both, not either.
Meet both and you have a qualifying disposition: the whole gain from strike price to sale price is long-term capital gain.
Miss either and you have a disqualifying disposition, and the ISO is largely treated like an NSO — the spread at exercise becomes ordinary income in the year you sell. Not catastrophic, and it removes the advantage you were holding for.
The tension is real: holding to qualify means holding a concentrated position in a single company for at least a year after exercise, and that is a genuine investment risk taken for a tax benefit. Plenty of people have watched a qualifying position lose more value than the tax saving was ever worth.
For NSOs it is simpler. Ordinary income at exercise, then the standard clock: hold more than a year after exercise and subsequent growth is long-term capital gain — see capital gains tax explained.
Common mistakes
- Not knowing which type you hold. The grant document says; read it before you need to.
- Exercising ISOs at a large spread without modelling AMT. The most expensive mistake in this area.
- Assuming ISO exercise withholds tax. It does not; the bill arrives at filing.
- Letting a 90-day window force a bad exercise. Plan for departure before departing.
- Holding purely for tax treatment. Concentration risk can exceed the saving.
- Forgetting cost basis is adjusted. For NSOs, basis includes the income you already paid tax on — see cost basis explained.
- Ignoring the ISO annual limit. Beyond a threshold of value vesting in a year, options are treated as NSOs regardless of the label.
FAQ
Which is better?
ISOs have better potential treatment and more complexity and risk. NSOs are simpler and taxed less favourably at exercise. You rarely choose — the company grants what it grants.
What if I leave the company?
The exercise window starts, traditionally 90 days, and unexercised options typically expire. ISOs also lose their status if exercised more than three months after leaving, converting to NSO treatment. Check your specific plan.
Should I exercise early?
It lowers the spread and therefore the tax exposure, and it means spending real money on shares that may end up worthless. It is a risk decision as much as a tax one, and it deserves modelling rather than a rule of thumb.
How does this compare to RSUs?
RSUs are considerably simpler — they are taxed as ordinary income when they vest, with no exercise decision and no purchase. Options require you to buy; RSUs are granted outright.
Where to go next
For the AMT calculation that determines ISO exercise timing, read ISO AMT explained. For the discounted-purchase alternative, ESPP discount explained, and for the rates on eventual sale, capital gains tax explained.
This is general information, not tax advice. Equity compensation is highly situation-specific and mistakes are expensive; consult a qualified tax professional before exercising.