ISO vs. NSO: how your stock options are taxed
Your offer letter says ISO or NSO. Both give you the same right — to buy shares at a fixed price. The difference is when the IRS taxes you, at what rate, and how much of the upside you keep.
Every option is a bet on one gap: the spread
A stock option is the right to buy shares at a fixed strike price, set on the day of your grant. If the company grows, the shares' fair market value (FMV) rises while your strike stays frozen.
The gap between them is the spread — your paper profit. Every tax rule in this deck is a rule about one question: when does the spread count as income, and what kind?
Same four events, different tax bills
Options move through grant → vesting → exercise → sale. NSOs tax you in the middle, at exercise. ISOs can defer everything to the sale — at a lower rate — if you follow the holding rules.
NSOs: the tax bill arrives the day you exercise
When you exercise an NSO, the spread is treated like salary. It goes on your W-2 as ordinary income, with federal, state, Social Security and Medicare tax withheld — even though you haven't sold a share or seen a dollar of cash.
Whatever the shares do after that is a normal capital gain or loss, measured from the FMV on exercise day. Hold the shares more than a year and further gains qualify for long-term rates.
NSOs can go to anyone — employees, contractors, advisors, board members. They don't have to meet the ISO rulebook's conditions, so they don't get its tax break.
ISOs: no tax at exercise — with two asterisks
Exercise an ISO and, for regular tax purposes, nothing happens. No W-2 income, no withholding, no payroll tax. If you then hold the shares long enough (slide 8), the entire gain from strike to sale price is taxed as long-term capital gains — top federal rate 20%, versus 37% for ordinary income.
Asterisk one: the spread still counts as income under the alternative minimum tax, a parallel calculation that can bill you in the year you exercise (slide 9).
Asterisk two: sell too early and the deal is off — the spread converts to ordinary income, NSO-style. That early sale is called a disqualifying disposition.
Run one exit through both pipes
Take 10,000 options, strike $1, exercised at $10 FMV, sold at $30 after an acquisition. Same shares, same $290,000 profit before tax — routed through NSO rules versus qualifying-ISO rules.
The exercise-and-sale calculator
Drag the sliders to match your own grant. Both columns update live: what each option type costs at exercise, at sale, and what you keep.
Illustrative only — assumes a single filer; it does not model joint filing, state tax, AMT phaseouts, or the rest of your return.
NSO ordinary income at exercise
ISO qualifying disposition
After-tax proceeds, assuming NSO shares also held >1 year after exercise and ISO holding rules met. AMT (amber) is a rough worst case, often recovered later as a credit — it is not subtracted from the ISO total. Very large exercises can exceed it: past $500K single / $1M joint of AMT income, the exemption phases out at 50¢ per dollar and effective rates approach 40% (see slide 9).
ISO's deal has two clocks, and both must run out
A sale is a qualifying disposition — all long-term capital gains — only if you sell more than 2 years after grant and more than 1 year after exercise. Miss either clock and the spread converts to ordinary income.
Two mechanics differ from a true NSO: a disqualifying sale has no withholding (you settle it through estimated tax), and if you sell in the exercise year for less than the exercise-day FMV, the ordinary income is capped at your actual gain.
AMT: the parallel tax that can hit ISO exercises
The alternative minimum tax is a second income-tax calculation that runs alongside the regular one. You pay whichever is higher. For most people the two never diverge — until an ISO exercise, because the AMT counts your untaxed spread as income.
The math, roughly: add the spread to your income, subtract the AMT exemption ($90,100 single / $140,200 joint for 2026), and apply 26–28%. Big exercises got harsher in 2026: above $500,000 of AMT income ($1,000,000 joint) the exemption phases out at 50 cents per dollar, pushing the effective marginal rate to roughly 40% — on shares you cannot sell yet.
Two softeners. A small exercise often triggers nothing — AMT only bites once the parallel calculation overtakes your regular tax, the crossover in the chart. And AMT paid on an ISO exercise usually becomes a credit you recover against future years' regular tax — a prepayment rather than a pure loss, but the cash is due now.
Three rules that quietly reshape your grant
- The $100K rule caps ISOs
Only $100,000 of options (valued at grant-date FMV) may become exercisable as ISOs in any calendar year. Large grants routinely split: the first $100K per year stays ISO, the overflow is automatically NSO. Check your grant paperwork — many "ISO grants" are mixed.
- Leaving starts a 90-day fuse
ISO status survives only 3 months after you leave the company (12 months if you leave on disability; waived at death). Exercise later than that — even under a company that offers a 5- or 10-year extended window — and the option is taxed as an NSO. The extended window is real; the ISO treatment is what expires.
- ISOs are for employees only
Contractors, advisors, and non-employee directors cannot receive ISOs at all. If you consult before converting to full-time, that early grant is an NSO by law, whatever the size.
The questions that actually decide it
You usually don't choose which type you get — the company does. What you control is when to exercise and when to sell. These are the questions a good advisor will ask.
Holding ISOs? Ask:
- Can I afford the exercise cost and a possible AMT bill, in cash, on stock I can't sell?
- Does exercising early — while the spread is small — keep me under the AMT exemption?
- Can I realistically hold 1 year past exercise (and 2 past grant) given my exit timeline?
- Am I near the 90-day post-departure fuse?
Holding NSOs? Ask:
- Do I have cash for the strike price plus withholding on the spread (often 30% or more), immediately?
- Is the spread small enough today that exercising now beats a bigger ordinary-income hit later?
- Will I hold the shares a year after exercise so further gains go long-term?
- Does a cashless / sell-to-cover exercise at exit make more sense than fronting cash?
The common thread: taxes reward exercising when the spread is small and holding long — and punish waiting until the spread is huge and the timeline is short. A tax professional can run your real numbers before you commit either way.
Four questions before you go
Where this comes from
- IRS Publication 525 — Taxable and Nontaxable Income
The primary rules for statutory (ISO) and nonstatutory (NSO) option taxation. - 26 U.S. Code §422 — Incentive stock options
The statute: holding periods, the $100,000 limit, the 3-month post-employment rule. - IRS — Tax year 2026 inflation adjustments
2026 AMT exemption ($90,100 single / $140,200 joint) and phaseout thresholds. - IRS Form 6251 instructions — Alternative Minimum Tax
How the ISO spread enters the AMT calculation, and the minimum tax credit. - Tax Foundation — 2026 federal tax brackets
Ordinary and capital-gains rate schedules used in the illustrations.