Stock Option Exercise Take-Home: NSO vs ISO
Exercise an NSO and payroll withholds 22%. Exercise an ISO and nothing is withheld at all. The 2026 cash math on a $280,000 spread, three ways.
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This article is general information, not tax or financial advice. Tax rules change, individual situations vary, and every figure here is an estimate. Confirm specifics with a qualified tax professional before making money decisions.
Exercising is a payroll event, not a stock purchase
The word “exercise” sounds like a trade. For a non-qualified option it is closer to a bonus that happens to be paid in shares.
Three numbers define the whole thing. Your strike price is what the grant lets you pay per share. The fair market value is what a share is worth on the day you exercise, which at a private company is the 409A valuation. The gap between them, multiplied by the shares you exercise, is the spread. Some documents call it the bargain element or the compensation element. Same number.
That spread is the taxable event. For an NSO it is compensation on the day you exercise, reported on your W-2, whether or not you sell a single share.
How you pay for the exercise changes the cash flow, not the tax:
- Cash exercise, or exercise-and-hold. You wire the strike cost and keep every share.
- Cashless exercise, or same-day sale. The broker sells all the shares at once, pays the strike cost and the tax out of the proceeds, and sends you the rest.
- Sell-to-cover. The broker sells just enough shares to cover the strike cost and withholding, and you keep the remainder as stock.
ISOs and NSOs are twins that split up at exercise and meet again at sale. The NSO gets taxed hard on exercise day and lightly afterward, while the ISO gets taxed almost not at all on exercise day and then hands you a bill from a parallel tax system you may never have filed under. The rest of this piece is what each path does to your bank balance.
NSO exercise: the 2026 withholding stack, line by line
When you exercise an NSO, the spread is a supplemental wage. Payroll treats it the way it treats a signing bonus, with one difference: the strike price you paid comes out of the proceeds first.
What gets pulled off the spread in 2026:
Federal income tax at 22%. The flat supplemental rate. IRS Publication 15 lets payroll choose between the flat method and blending the spread into your regular wages; when it takes the flat method, it is 22 percent, no other percentage allowed. Once your cumulative supplemental wages for the year pass $1,000,000, the excess is withheld at 37% instead.
Social Security at 6.2%, but only up to the wage base. For 2026 the Social Security wage base is $184,500. If you have already earned part of that through salary, the spread only gets charged on the room that is left. This is where a big spread behaves differently from a typical bonus, because the spread is usually large enough to swallow the remaining base in one bite.
Medicare at 1.45%, on every dollar, with no cap.
Additional Medicare at 0.9%. Your employer starts withholding this once the wages it pays you cross $200,000 for the year, regardless of your filing status. The $250,000 married-filing-jointly and $125,000 married-filing-separately figures you may have seen are the liability thresholds you settle at filing, not the trigger payroll uses.
State withholding on top of all of it. Several states apply their own flat supplemental rate, and a few apply a higher rate to stock option income specifically than to ordinary bonuses. That layer is not in the federal-only example below.
On your W-2, the spread lands in box 1, box 3 (capped at the wage base), box 5, and it shows up again in box 12 with code V so the IRS can match it against your broker’s reporting.
If this stack feels familiar, it should. It is the same machinery described in our guides to RSU vesting take-home pay and why your bonus gets taxed so much. What is different here is the strike-price subtraction, and what happens when the option is an ISO instead.
A $280,000 spread, three ways
One grant, one exercise date, three outcomes that are nothing alike. All figures below are federal only; state income tax is a separate layer.
| Input | Value |
|---|---|
| Filing status | Single |
| Salary | $150,000 |
| YTD wages at the June exercise date | $75,000 |
| Options exercised | 10,000 |
| Strike price | $2.00 |
| Fair market value at exercise | $30.00 |
| Spread | $280,000 |
| Gross sale proceeds at $30 | $300,000 |
| Cost to exercise | $20,000 |
Column A: NSO cashless exercise
| Line | Amount |
|---|---|
| Gross proceeds | $300,000 |
| Less strike cost | -$20,000 |
| Federal income tax, 22% supplemental | -$61,600 |
| Social Security, 6.2% on the $109,500 of room left under the $184,500 base | -$6,789 |
| Medicare, 1.45% on $280,000 | -$4,060 |
| Additional Medicare, 0.9% on the $155,000 of wages above $200,000 | -$1,395 |
| Cash to your account | $206,156 |
A pleasant side effect comes with it. After this exercise your year-to-date FICA wages sit at $355,000, far past the $184,500 base, so every remaining paycheck of the year skips the 6.2% Social Security line. On the $75,000 of salary left that is about $4,650 of extra take-home, or roughly $3,975 net once the 0.9% Additional Medicare that now applies to the same salary is subtracted. Our post on why your paycheck gets bigger after the Social Security cap covers that effect on its own.
Timing matters more than people expect. The identical exercise run in December, with $150,000 of salary already on the books, would have cost only $2,139 of Social Security instead of $6,789.
Column B: ISO exercise and hold
| Line | Amount |
|---|---|
| Cash out of pocket today (strike only) | -$20,000 |
| Federal income tax withheld | $0 |
| Social Security and Medicare | $0 |
| AMTI (salary plus the $280,000 spread) | $430,000 |
| Less 2026 AMT exemption | -$90,100 |
| AMT base | $339,900 |
| Tentative minimum tax, 26% on the first $244,500 plus 28% on the rest | $90,282 |
| Less regular tax on $133,900 of taxable income ($150,000 less the $16,100 standard deduction) | -$24,734 |
| AMT owed next April, with $0 withheld | $65,548 |
Total cash out across the two dates is $85,548, and you have not sold a share. If the stock falls before you can sell, you still owe that AMT on a valuation that no longer exists. That is the trap.
Column C: ISO exercised and sold the same year
| Line | Amount |
|---|---|
| Gross proceeds | $300,000 |
| Less strike cost | -$20,000 |
| Federal income tax withheld | $0 |
| Social Security, Medicare and FUTA | $0 |
| AMT preference | None |
| Cash to your account | $280,000 |
| Federal income tax owed on the spread, due at filing | $88,900 |
Column C is the one almost nobody writes about, and it is the most dangerous of the three. Selling in the same calendar year as the exercise is a disqualifying disposition, so the AMT preference disappears. The spread becomes ordinary W-2 income instead. But because the shares came from an incentive stock option, the statute does not require federal income tax withholding, and the spread is excluded from FICA entirely.
You take home the most cash of the three paths and carry the largest unprepaid tax bill. Withholding is not required, though some plan brokers will withhold voluntarily if you ask, so ask before you exercise rather than arguing with payroll in February.
Why 22% leaves you $27,300 short
The 22% supplemental rate is a prepayment, not a tax rate. On a spread this size the two numbers are not close.
Rather than being taxed in isolation, the spread stacks on top of your salary and fills the brackets above where your salary stopped. Run the arithmetic on the example above and you get this.
| Line | Amount |
|---|---|
| Regular tax on $133,900 of taxable income (salary only) | $24,734 |
| Regular tax on $413,900 of taxable income (salary plus spread) | $113,634 |
| Federal income tax caused by the spread | $88,900 |
| Withheld at 22% in Column A | $61,600 |
| Shortfall, Column A | $27,300 |
| Shortfall, Column C | $88,900 |
That $413,900 figure walks through the 2026 single brackets like this: 10% on the first $12,400, 12% up to $50,400, 22% up to $105,700, 24% up to $201,775, 32% up to $256,225, and 35% on the remaining $157,675. The top of the spread is taxed at 35% while payroll prepaid 22%. That is where the gap comes from. Only the top slice of your income is taxed at 35%, which is what the marginal versus effective tax rate calculator makes visible for any income figure you enter.
Three ways to close it:
Add extra withholding on your W-4, Step 4(c). Withholding counts as paid evenly across the year no matter when it actually happened, so an amount added after a June exercise can retroactively cure a shortfall in a way an estimated payment cannot. Our guide to adjusting your W-4 shows how to size it, and the W-4 withholding planner does the arithmetic.
Make an estimated payment. A Form 1040-ES payment for the quarter the exercise landed in. Cleaner if you would rather not touch your W-4, but it is timing-sensitive.
Check whether you already sit in a safe harbor. No underpayment penalty applies if your withholding plus estimated payments cover 90% of your current-year tax or 100% of your prior-year tax. That prior-year figure rises to 110% if your prior-year AGI exceeded $150,000 ($75,000 married filing separately). There is also no penalty when the balance due comes in under $1,000.
ISO exercise: no withholding, no FICA, and an AMT bill nobody withheld for
Incentive stock options get favorable treatment on exercise day, and the favor is easy to mistake for a free lunch.
Publication 15-B states it plainly: wages for Social Security, Medicare and FUTA taxes do not include anything resulting from the exercise of an incentive stock option, or from any disposition of the stock acquired by exercising one. No regular federal income tax is due at exercise either. Your employer reports the exercise to you and to the IRS on Form 3921 early the following year.
Nothing on your paycheck changes. That is exactly the problem.
The AMT adjustment
For alternative minimum tax purposes, the spread on an ISO exercise-and-hold counts as income in the year you exercise. It goes on Form 6251 as an ISO adjustment, and the standard deduction is added back, which is why the AMTI in Column B is the full $430,000.
The 2026 AMT exemption is $90,100 for unmarried filers and $140,200 for joint filers. The rate on that base (AMTI after the exemption) is 26% on the first $244,500 and 28% on everything above it. The AMT rate is never 20%, whatever a stale guide tells you.
The exemption itself phases out once AMTI passes $500,000 for unmarried filers or $1,000,000 for joint filers, at 50 cents of exemption lost per dollar of AMTI above the threshold. The One Big Beautiful Bill Act changed both ends of that: the phase-out now begins far lower and runs twice as fast, so a large exercise can lose the exemption entirely and push the effective rate on the spread well above 28%.
You only pay AMT when the tentative minimum tax exceeds your regular tax, and then only the excess. In Column B, $90,282 against $24,734 of regular tax leaves $65,548 due, on stock you cannot sell.
The credit, and the two exits
AMT paid because of an ISO exercise is not gone for good: it becomes a minimum tax credit you claim on Form 8801 in later years, recovered as your regular tax exceeds your tentative minimum tax. That can take several years, and it does nothing for your cash position next April.
Your two ways out of the ISO shares:
Qualifying disposition. Hold at least two years from the grant date and one year from the exercise date, whichever ends later. The entire gain over your strike price is then long-term capital gain, which is the reason ISOs exist.
Disqualifying disposition. Sell before those clocks run out and the exercise-date spread converts to ordinary W-2 income. Sell in the same calendar year as the exercise and the AMT preference vanishes completely, which is Column C.
Check one more thing before you assume any of this applies to you. Under the $100,000 ISO limit, options that first become exercisable in a single calendar year with an aggregate grant-date value above $100,000 are treated as NSOs for the excess. Plenty of people who believe they hold pure ISOs hold a mix, and the NSO portion follows the Column A rules.
What to do before you click exercise
The exercise window is usually the last calm moment you get. Run this list while you are still in it.
- Get the current fair market value. The 409A number at a private company, the market price at a public one. No FMV, no spread, no estimate.
- Confirm the option type for each tranche. Your grant agreement and your Form 3921 (if one arrives) settle it. Mixed grants are common.
- Ask payroll how the money flows. Whether the exercise runs through payroll or entirely through the broker determines what is withheld and when.
- Check your year-to-date FICA-taxable wages against the $184,500 wage base. It decides whether the 6.2% line is a large number or a small one, and it is the single biggest timing lever you control.
- Pick your safe-harbor plan before you exercise, not in April. A Step 4(c) amount or a quarterly payment is a ten-minute job in June and a penalty conversation later.
- Watch the cost basis on your 1099-B. Brokers frequently report only the strike price. Your real basis is the strike price plus the spread already taxed as compensation, adjusted on Form 8949. Miss it and a same-day sale looks like a six-figure capital gain you already paid income tax on.
Then model the paycheck. The Salary Calculator app (Stub44) runs the full federal, state and FICA stack across all 50 states plus DC: enter the spread as a bonus or commission entry, fill in your year-to-date FICA-taxable earnings so the Social Security wage-base truncation is right for a mid-year exercise, and the withholding falls out line by line. Two saved profiles will show you a June exercise against a December one. To weigh the whole grant against a salary number, total compensation versus base salary is the companion read.
The app models W-2 withholding, including federal, state, FICA and your W-4 settings. It does not model AMT. For Column B, the withholding math is the easy half and Form 6251 is the half you should walk through with a professional.
If you have an exercise window open, download Salary Calculator and run the spread against your actual year-to-date numbers before you decide.
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Frequently Asked Questions
How much tax is withheld when I exercise non-qualified stock options?
Federal income tax at the flat 22% supplemental rate on the spread (37% on cumulative supplemental wages above $1,000,000), plus 6.2% Social Security up to the $184,500 wage base for 2026, 1.45% Medicare with no cap, and 0.9% Additional Medicare once your wages for the year pass $200,000. State supplemental withholding comes on top.
Why did I still owe money in April after my employer withheld on my NSO exercise?
Because 22% is a flat withholding rate, not your marginal rate. A $280,000 spread stacked on a $150,000 salary lands in the 32% and 35% brackets, so 22% leaves roughly $27,300 unpaid on that example. The gap shows up at filing unless you make a Q4 estimated payment or raise your W-4 withholding.
Do I pay Social Security and Medicare when I exercise stock options?
On NSOs, yes. The spread is W-2 wages. On ISOs, no. IRS Publication 15-B states that wages for Social Security, Medicare and FUTA do not include anything resulting from exercising an incentive stock option or from any disposition of the resulting stock.
How much cash do I actually get from a cashless exercise?
Gross sale proceeds minus the strike price, minus all withholding, minus the broker's fee. In the worked example, $300,000 of proceeds becomes about $206,156 after a $20,000 strike cost and $73,844 of federal withholding, before any state tax.
What is the ISO AMT trap?
Exercising an ISO and holding the shares creates no regular taxable income, but the spread is an alternative minimum tax adjustment on Form 6251. You can owe a large AMT bill on a gain you never converted to cash, and nothing was withheld for it. The tax generates an AMT credit you claim later on Form 8801, but the cash still has to come from somewhere in April.
Will I owe AMT if I sell my ISO shares the same year I exercise?
No. Selling in the same calendar year as the exercise is a disqualifying disposition that removes the AMT preference entirely. The trade-off is that the spread becomes ordinary income on your W-2, and by statute your employer withholds nothing on it, so plan for the full bill.
How long do I have to hold ISO shares for long-term capital gains?
At least two years from the grant date and one year from the exercise date, whichever ends later. Meet both and the full gain over your strike price is long-term capital gain. Miss either one and it is a disqualifying disposition.
Will I be double-taxed if my 1099-B shows the wrong cost basis?
You should not be, but you have to catch it. Brokers often report only the strike price as your basis. Your real basis is the strike price plus the spread already taxed as W-2 income, so you adjust it on Form 8949. Otherwise a same-day sale looks like a huge capital gain you already paid income tax on.