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10,000 options at a $5 strike with $40 FMV: NSO costs $172,500 day one; ISO costs $50,000 plus a ~$91,000 AMT shadow. Know which you hold.
Employee stock options split into two tax machines that share a name. NSOs tax the spread as ordinary income at exercise — $350,000 of spread at 35% is $122,500 due whether or not you sell a share. ISOs defer regular tax but feed the spread to the Alternative Minimum Tax — the exercise-and-hold that earns you all-LTCG treatment also hands you a paper-gains tax bill in April. The December mechanic matters: exercise late in the year and the disqualifying-disposition escape hatch (sell before December 31 → ordinary income, AMT evaporates) stays open while you see the real bill. This calculator prices both machines on your grant before you touch anything.
Employees with options use it before exercising: NSO spreads tax at 35% immediately ($172.5k out on a $350k spread) while ISOs defer regular tax but cast a ~$91k AMT shadow — and the December exercise keeps the escape hatch open.
Two clocks, both must run: hold the SHARES at least 1 year after exercise, AND sell at least 2 years after the grant date. Clear both and the entire gain — strike to sale price — becomes long-term capital gain (20% top rate + 3.8% NIIT), the best treatment in equity comp. Miss either and it's a disqualifying disposition: the spread at exercise becomes ordinary income, exactly like an NSO, plus LTCG/STCG on any post-exercise movement. The ISO bargain is real but the holding requirement is the risk: you're concentrating in one stock for a year minimum to earn the rate. The hybrid many planners favor: exercise early in the calendar year, hold 12 months, then sell on the qualifying schedule — and if the stock craters mid-year, the disqualifying sale before December 31 caps the damage at ordinary rates with real cash to pay it.
The AMT on exercise-and-hold is a prepayment, mostly recoverable — but slowly. Mechanism: the spread feeds AMT as a preference item; if AMT exceeds regular tax, you pay the difference (the hit), and generate an AMT credit usable in future years when regular tax exceeds AMT — typically dribbling back over 3–7 years unless the shares sell. The disaster pattern to refuse: exercise-and-hold a volatile stock into a decline — you owe AMT on the $40 phantom FMV while holding $12 shares you may not even be able to sell (private company). That scenario bankrupted people in 2000 and repeats every cycle. The honest controls: exercise in tranches sized to keep AMT under the regular-tax crossover, exercise in December (see the bill while the escape hatch is open), and never exercise-and-hold private shares with money you need. This calculator's flat-26% figure is a planning approximation — real Form 6251 netting deserves a CPA for anything five figures and up.
Everything that makes it harder. FMV is the 409A valuation — an appraisal, not a market — updated annually or on events; your exercise math runs on it, but the shares may be illiquid for years. Liquidity: no same-day sale, so NSO exercise or ISO AMT means real cash out with no way to sell — never exercise illiquid options with money you need, and check whether the company offers tender offers or secondary windows. The 83(i) and QSBS angles: early exercise (if your plan allows pre-vesting exercise) starts the capital-gains and QSBS clocks at near-zero spread — the single most powerful move in startup equity when available, converting future ordinary income into LTCG and potentially QSBS-excluded gain ($10M cap) after 5 years. Leaving the company starts the 90-day exercise clock on most plans — ISOs convert to NSOs after 90 days post-termination, and expired options are simply gone. Read the plan documents before giving notice, not after.