Compensatory Stock Options: NSO and ISO Tax Rules, AMT, and Reporting

The tax on compensatory stock options depends almost entirely on which of two categories your grant falls into. Non-qualified stock options (NSOs) generate ordinary wage income the moment you exercise them. Incentive stock options (ISOs) defer all regular income tax until you sell the shares, and if you meet the holding periods your entire profit is taxed as long-term capital gain. The catch on the ISO side is the alternative minimum tax, which can pull the spread back into your income the year you exercise. On the same grant, the difference between the two paths often runs into tens of thousands of dollars.

What Determines Which Rules Apply

The IRS splits compensatory options into statutory options (which include ISOs) and nonstatutory options (NSOs).¹ NSOs are the default. Anything that misses the statutory requirements is taxed as an NSO, and NSOs can be granted to employees, board members, or contractors.

ISOs have to satisfy Section 422 of the Internal Revenue Code. The plan must be approved by shareholders, and grants can only go to employees of the company or a parent or subsidiary. There is also a dollar cap. To the extent ISOs become exercisable for the first time in any calendar year with an aggregate fair market value (measured at grant) above $100,000, the excess is automatically reclassified as NSOs.² When grants straddle that line, the oldest options keep ISO status first.

One more baseline point. Vesting itself is not a taxable event. The tax clock starts when you exercise, or, for ISOs held after exercise, when you sell.

How NSOs Are Taxed

Nothing happens on the grant date for a typical NSO. The taxable event is exercise.³ The spread — the gap between the stock’s market value on the exercise date and the strike price you paid — is ordinary income in that year and is included in your gross income like any other wages.⁴ Your employer treats it as supplemental wages and withholds federal income tax, and it shows up on your Form W-2 for the year.⁵

Because the spread is wages, it is also subject to Social Security tax at 6.2% up to the 2026 wage base of $184,500, and Medicare tax at 1.45% with no cap.⁶ If your total wages for the year clear $200,000 (single) or $250,000 (married filing jointly), the 0.9% Additional Medicare Tax kicks in on the excess.⁷

Withholding is where NSO exercisers get burned. The standard flat rate on supplemental wages is 22%, rising to 37% on supplemental wages above $1 million in a calendar year. If your marginal rate is higher than 22%, the withheld amount will not cover your actual tax, and you make up the difference in April.

Your Basis After Exercise

Your basis in the shares equals the strike price plus the spread already taxed as ordinary income. Strike price $10, exercise-date value $30: you recognized $20 per share as ordinary income and your basis is $30. Any later gain or loss is measured from that $30.

The capital gains holding period starts the day after exercise. Sell inside a year and any gain is short-term, taxed at ordinary rates. Hold more than a year and it is a long-term capital gain, taxed at 0%, 15%, or 20% depending on your income.⁸

How ISOs Are Taxed

Exercising an ISO and holding the shares triggers no regular income tax, and your employer does not withhold income or employment taxes on the spread.⁹ That is the whole appeal. Whether you keep that treatment through to the sale depends on two holding periods.

Qualifying Dispositions

To get the full ISO benefit, you must hold the shares more than two years after the grant date and more than one year after the exercise date.² Meet both and the entire profit — sale price minus strike price — is a long-term capital gain. No portion is ordinary income, and the employer gets no offsetting deduction.⁹

Disqualifying Dispositions

Sell before satisfying either holding period and it is a disqualifying disposition. The spread at exercise (exercise-date market value minus strike price) is reclassified as ordinary income, similar to an NSO.⁹ Any additional gain above the exercise-date value is a capital gain, short-term or long-term depending on how long you held after exercise.

If the stock dropped after exercise, the rule is friendlier. When you sell in a disqualifying disposition for less than the exercise-date value but more than the strike price, the ordinary income is capped at your actual profit. Sell below the strike price and there is no ordinary income at all, just a capital loss.

One more thing that surprises people: no income tax withholding is required on a disqualifying disposition.⁹ You owe the tax, but nothing comes out of the proceeds automatically. Set money aside or adjust your estimated payments, or you can face an underpayment penalty.

The AMT Trap on ISO Exercises

The regular-tax deferral on an ISO exercise does not carry over to the alternative minimum tax. The statute specifically says the favorable Section 421 treatment does not apply for AMT purposes.¹⁰ The spread at exercise is added back to income when you compute AMT, so you can owe tax in a year you thought was quiet.

AMT works as a parallel calculation: certain income items get added, certain deductions get disallowed, and you pay whichever is higher, regular tax or AMT. For 2026, the AMT exemption is $90,100 for single filers and $140,200 for joint filers, with phaseouts starting at $500,000 and $1,000,000 respectively.¹¹ A large batch exercise can blow past the exemption in a single transaction. You calculate the liability on Form 6251.¹² Run the numbers before you exercise. Splitting an exercise across two calendar years can sometimes keep you under the exemption in each.

Recovering AMT Later

AMT paid because of an ISO exercise is not permanently lost. The spread is a deferral item, meaning the timing gap between regular tax and AMT eventually reverses.¹³ You carry the AMT forward as a minimum tax credit and use it in future years when regular tax exceeds tentative AMT. Claim the credit on Form 8801. When you sell the ISO shares at a gain, the regular tax on the sale often opens up enough of a gap to absorb most of the credit. Track Form 8801 every year until the balance is gone.

Two Basis Numbers

Exercise ISOs and hold the shares and you now have two cost bases. For regular tax, basis is the strike price. For AMT, basis is the strike price plus the spread that was already added to AMT income. When you sell, the two bases produce different gain figures on the regular and AMT sides of the return. Getting this wrong is one of the most common mistakes on ISO returns and can cause you to either overpay or lose the minimum tax credit you were owed.

Two Situations That Change the Timing

Early Exercise and Section 83(b)

Some companies, particularly startups, let you exercise options before they vest. The shares stay subject to the company’s right to buy them back if you leave early. Because the shares are restricted, Section 83 normally delays the tax event until vesting, when the spread between then-current value and what you paid becomes ordinary income.

A Section 83(b) election overrides that default. You recognize income immediately at the value on the early-exercise date. If you exercise when the stock is worth essentially the strike price (common early on), the spread may be zero and there is nothing to report. All future appreciation then rides the capital-gains path once you meet the holding-period rules.

The deadline is 30 days from receiving the restricted shares. There is no extension. And there is no refund if the stock goes to zero or you forfeit the shares by leaving; you may claim a capital loss, but the election itself stands.

Discounted Options and Section 409A

Section 409A governs nonqualified deferred compensation, and a stock option is pulled into its scope if the strike price is below the stock’s fair market value on the grant date. If a discounted option fails 409A, the spread is included in income at vesting rather than exercise, plus a 20% additional tax, plus interest at the IRS underpayment rate plus one percentage point running back to the year the compensation first vested.¹⁴ The penalty falls on the employee, not the employer.

This comes up most often at private companies where there is no public market to fix fair market value. ISOs that satisfy Section 422 are generally exempt from 409A. NSOs priced below fair market value are fully exposed.

Extra Surtaxes at Higher Incomes

Two federal surtaxes can layer on top of the regular treatment.

The 3.8% Net Investment Income Tax applies to the lesser of your net investment income or the amount by which your modified AGI exceeds $200,000 (single) or $250,000 (joint). Capital gains from selling shares acquired through options count as net investment income. The NSO spread itself is wages, not investment income, but a large exercise can push your AGI over the threshold and expose your other investment income to the tax.

The 0.9% Additional Medicare Tax applies to wages above $200,000 (single) or $250,000 (joint).⁷ A big NSO exercise clears those thresholds fast. Your employer withholds only on wages it pays you above $200,000 regardless of filing status, so joint filers with the higher $250,000 threshold may owe more at filing, and single filers with two jobs may find they were under-withheld.

Reporting It Correctly

Getting stock option reporting right means reconciling several forms that each capture only part of the picture.

Form W-2 for NSO Exercises

The NSO spread appears on your W-2 in Box 1, Box 3 (Social Security wages, up to the wage base), and Box 5 (Medicare wages), with income tax withheld in Box 2. Federal regulations treat exercise income from nonstatutory options as supplemental wages subject to the flat 22% withholding rate (37% above $1 million in a year).⁵

Form 3921 for ISO Exercises

Your employer files Form 3921 for each ISO exercise.¹⁵ It reports the grant date, exercise date, exercise price per share, fair market value on the exercise date, and the number of shares transferred.¹⁶ Those numbers feed your AMT adjustment on Form 6251. Keep the form permanently. Years later, when you sell, you will need the grant and exercise dates to prove you met the holding periods.

Form 1099-B When You Sell

Your broker reports the sale on Form 1099-B.¹⁷ This is where errors compound. The cost basis on the 1099-B often reflects only the strike price you paid, not your adjusted basis that includes the ordinary income already taxed at exercise. If you report the 1099-B figures as they stand, you will overstate the gain and pay tax on the same spread twice.

Fix it on Form 8949: enter the basis as the broker reported it, then adjust in Column G to reflect the income already recognized. The corrected gain flows to Schedule D. The adjustment is your responsibility. The IRS matching program sees the broker’s numbers; if your return shows a lower gain without the proper adjustment code, expect a notice.

Forms 6251 and 8801 for AMT

If you exercised ISOs during the year, file Form 6251 to see whether you owe AMT.¹² In later years, file Form 8801 to claim any minimum tax credit carried forward.¹³ Running the 6251 calculation is the only way to know for sure whether the exercise pushed you into AMT territory, and the penalty for guessing wrong is an underpayment assessment plus interest.

1
Internal Revenue Service. Topic No. 427, Stock Options
2
Office of the Law Revision Counsel. 26 USC 422 – Incentive Stock Options
3
eCFR. 26 CFR 1.83-7 – Taxation of Nonqualified Stock Options
4
U.S. Government Publishing Office. 26 USC 83 – Property Transferred in Connection With Performance of Services
5
eCFR. 26 CFR 31.3402(g)-1 – Supplemental Wage Payments
6
Social Security Administration. What Is the Current Maximum Amount of Taxable Earnings for Social Security
7
Internal Revenue Service. Topic No. 560, Additional Medicare Tax
8
Internal Revenue Service. Topic No. 409, Capital Gains and Losses
9
Office of the Law Revision Counsel. 26 USC 421 – General Rules
10
Office of the Law Revision Counsel. 26 USC 56 – Adjustments in Computing Alternative Minimum Taxable Income
11
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
12
Internal Revenue Service. About Form 6251, Alternative Minimum Tax – Individuals
13
Internal Revenue Service. Instructions for Form 8801
14
Office of the Law Revision Counsel. 26 USC 409A – Inclusion in Gross Income of Deferred Compensation Under Nonqualified Deferred Compensation Plans
15
Internal Revenue Service. Instructions for Forms 3921 and 3922
16
Internal Revenue Service. Form 3921 – Exercise of an Incentive Stock Option Under Section 422(b)
17
Internal Revenue Service. About Form 1099-B, Proceeds From Broker and Barter Exchange Transactions