ISO vs NSO Tax Differences

NSO exercise: The spread is compensation. Your employer (or the company at a startup) should report it on a W-2 or 1099-NEC. You owe ordinary income tax and FICA on the spread immediately, even if you can't sell the shares yet.

ISO exercise: No ordinary income for regular tax purposes at exercise if you hold the shares. Instead, the spread is an AMT preference item — it may increase your AMT bill on Form 6251 even with zero cash from a sale.

Both types give you shares with a cost basis equal to what you paid (strike price) plus any amount taxed as ordinary income (NSO spread, or ISO spread if you fail qualifying disposition rules).

To get long-term capital gains treatment on the full amount above strike price, ISO shares must pass two tests: hold at least two years from the grant date and at least one year from the exercise date before selling.

Sell too early (a 'disqualifying disposition') and the spread at exercise becomes ordinary income — up to the lesser of the spread at exercise or the gain at sale. Any amount above that is capital gain.

There's no partial qualifying disposition. One day short of the one-year post-exercise hold triggers ordinary income on the exercise spread.

NSO: After paying ordinary income on the spread at exercise, your basis includes that taxed amount. Gain or loss from exercise FMV to sale price is capital gain — long-term if you held more than one year after exercise.

ISO qualifying disposition: The entire gain above strike price is long-term capital gain (assuming you also held more than one year after exercise). No ordinary income at sale.

ISO disqualifying disposition: Ordinary income recapture at exercise-spread levels, then capital gain on any appreciation after exercise. Your W-2 in the sale year should reflect the ordinary income portion.

NSO exercises at private companies create a tax bill with no liquidity. Companies often require cash payment for the spread tax or offer net exercise (surrendering shares to cover taxes).

ISO exercises don't require withholding for regular tax, but AMT can demand a large payment in April after a year with no salary withholding on the spread. Plan for AMT credit carryforwards if you pay AMT — you may recover some in later years.

Public company ISO holders can sell a portion immediately (if not in a blackout) to cover AMT, but early sales risk disqualifying disposition if the one-year clock hasn't run.

ISOs are limited to employees (not contractors) and have a $100,000-per-year cap based on grant-date FMV of shares that first become exercisable in a calendar year — not the FMV at vesting — which can force NSO treatment on the excess. After termination, ISO tax treatment generally requires exercise within three months; a longer company exercise window does not preserve ISO status.

NSOs work for advisors, contractors, and amounts above the ISO cap. They're easier for companies to administer because tax is due at exercise and reported on payroll.

From the employee side, ISOs are worth more only if you can afford the AMT hit at exercise and meet holding periods. NSOs are often preferable when exercise price is low and FMV is high but the stock is illiquid.

ISO exercise: Form 3921 from employer by January 31. Report AMT adjustment on Form 6251. Track grant date, exercise date, and strike price for qualifying disposition math.

NSO exercise: W-2 or 1099-NEC with spread in Box 1 or nonemployee compensation. Form 3922 if applicable for ESPP-related options.

Sale of either: Form 8949 and Schedule D. ISO sales before qualifying periods need manual adjustment if broker reports full proceeds as capital gain.