ISO vs NSO Stock Options: How Executive Options Are Taxed, and How to Handle the AMT

A senior engineer at a public tech company and a vice president at a late-stage private company can hold option grants with identical strikes and identical upside and still face tax bills that differ by tens of thousands of dollars on the same gain. The difference comes down to two letters on the grant agreement and a few dates on the calendar. An incentive stock option held long enough is taxed entirely at long-term capital gains rates, 20% plus the 3.8% net investment income tax at the top. A nonqualified option is taxed as pay at exercise, at up to 37% plus Medicare. And the ISO comes with a catch: the alternative minimum tax, which for 2026 exempts only $140,200 of AMT income for joint filers before the math turns against you. If you hold RSUs rather than options, the issues differ; this guide is about options. For how sold shares are taxed generally, see taxes on sold stock.

What is the difference between ISOs and NSOs?

The difference is when you are taxed and at what rate: an NSO is taxed as ordinary income at exercise, while an ISO is taxed only when you sell, at capital gains rates if you meet two holding periods. Both give you the right to buy a fixed number of shares at a fixed exercise price (the strike) for a set term, usually ten years. The IRS calls ISOs statutory options because they must meet the rules in Section 422 of the tax code; everything else is a nonstatutory or nonqualified option (NSO or NQSO).

Section 422 sets the conditions an ISO has to meet. The exercise price must be at least the stock’s fair market value on the grant date. The option cannot be exercisable more than 10 years after grant. Only employees can receive ISOs; directors, advisers and contractors get NSOs. If you own more than 10% of the company’s voting stock, the price must be at least 110% of fair market value and the term no more than 5 years. Fail any of these and the option is an NSO, whatever the paperwork says.

That last row explains why companies like NSOs: the employer deducts the spread when you recognize it. ISOs are a benefit to you that costs the company its deduction, which is one reason later-stage and public companies grant fewer of them.

When are stock options taxed: at grant, vesting, exercise or sale?

For most employee options, nothing happens at grant or vesting; the tax decision is made at exercise and settled at sale. Chart 1 lays out the four steps side by side.

Grant. An employee option almost never has a “readily determinable fair market value” because it is not traded on an established market, so IRS Topic 427 says there is no taxable event at grant. Section 83(e)(3) keeps the grant itself outside the property-transfer rules for the same reason.

Vesting. Vesting only gives you the right to exercise. Unlike an RSU, a vested option is not income until you use it. Vesting matters for the ISO $100,000 limit, which counts options in the year they first become exercisable.

Exercise. This is where the two types split. The spread, the stock’s fair market value minus the exercise price, times the shares, is wages for an NSO and an AMT adjustment for an ISO. Your employer files Form 3921 for each ISO exercise, reporting the grant date, exercise date, exercise price and fair market value you will need later.

Sale. For an NSO, your basis is what you paid plus the income already taxed, so only the change after exercise is capital gain or loss, long-term if you held more than one year. For an ISO, the outcome depends entirely on whether you met the holding periods.

How is an NSO exercise taxed, and why is withholding often too low?

The full spread at exercise is compensation: it is subject to federal income tax, Social Security and Medicare, and it appears on your Form W-2, even if you keep every share. Exercise 20,000 NSOs at a $25 strike with the stock at $85 and you have $1.2 million of wages that year, whether or not you sell.

The trap is withholding. Option income is a supplemental wage, and IRS Publication 15 lets employers withhold federal income tax at a flat 22% on supplemental wages up to $1 million in a calendar year. Only the amount above $1 million is withheld at the mandatory 37%. An executive whose marginal rate is already 35% or 37% is under-withheld by roughly 13 to 15 percentage points on the first million. On the $1.2 million example, that gap is about $150,000 owed in April, plus possible underpayment penalties if estimated payments do not cover it.

Medicare adds 1.45% on all wages plus the 0.9% Additional Medicare Tax above $250,000 for joint filers ($200,000 single). Social Security applies only up to that year’s wage base, which most executives pass early in the year.

NSOs have an upside: the tax is simple and settled. Once you have paid tax on the spread, your basis equals the value at exercise, and the remaining position behaves like any other stock. That is why public-company NSOs are often exercised and sold together. Holding after exercise gains you nothing on the spread; it only adds single-stock risk.

What makes an ISO sale a qualifying disposition?

A sale is qualifying when you sell no earlier than 2 years after the grant date and 1 year after the exercise date, and you were an employee until 3 months before exercising. Both clocks must run out. The text is in 26 U.S.C. §422(a)(1): no disposition “within 2 years from the date of the granting of the option nor within 1 year after the transfer of such share.”

Meet them and the whole gain, from the exercise price to the sale price, is long-term capital gain. No part of it is wages, so there is no Social Security or Medicare tax, and no withholding. Most people with options granted several years ago find the 2-year grant clock has already run; the binding constraint is usually the 1 year from exercise.

Chart 2 runs one position through five outcomes. The gain is the same $700,000 in every case; only the option type and the sale date change.

The qualifying ISO pays about $166,600 in federal tax. The same gain through an NSO held for 13 months costs about $228,800, because $400,000 of it was wages. The gap, roughly $62,000 or 9 percentage points of the gain, is what the ISO rules are worth on this position. Compare the second and third bars, and the fourth and fifth: a disqualifying ISO sale still beats an NSO, because ISO income is exempt from Medicare tax under 26 U.S.C. §3121(a)(22).

What the chart leaves out is the cost of waiting. To earn that $62,000, the ISO holder kept $800,000 of one stock for a year. A 10% fall erases more than the tax saving. The collar strategy is one way to protect shares through a holding period, but a collar on ISO stock can bring in the straddle rules; see how options are taxed, and clear any hedge with your company’s insider trading policy first.

What happens in a disqualifying disposition?

If you sell ISO shares before either holding period ends, the spread at exercise becomes ordinary income, capped at your actual gain, and any gain above the value at exercise is capital gain. Gifts and most other transfers count as dispositions too; transfers at death and transfers to a spouse do not (26 U.S.C. §424(c)).

The cap matters when the stock falls. Under 26 U.S.C. §422(c)(2), the ordinary income “shall not exceed the excess (if any) of the amount realized on such sale or exchange over the adjusted basis.” Take 10,000 ISOs with a $10 strike, exercised when the stock was $50:

The timing of the disqualifying sale relative to December 31 matters as much as the price. Form 6251 instructions say that if you exercise and dispose of the stock in the same year, “the tax treatment under the regular tax and the AMT is the same, and no adjustment is required.” Sell in the same calendar year and the AMT never happens. Sell in January of the next year and you have already incurred AMT on the full spread at the exercise-date price.

How does the $100,000 ISO limit work?

No more than $100,000 of stock, valued at the grant-date price, can become exercisable for the first time under ISOs in any one calendar year; anything above that is treated as an NSO. The rule in 26 U.S.C. §422(d) counts all plans of your employer and its parent and subsidiaries together, and takes options in the order they were granted.

The test uses vesting, not exercise, and the grant-date fair market value, not today’s price. Suppose you were granted 40,000 options at a $10 fair market value, vesting 25% a year. Each year 10,000 options worth $100,000 at grant first become exercisable, exactly the limit, so all are ISOs. If the same grant vested all at once, only 10,000 would be ISOs and 30,000 would be NSOs. A grant with an accelerated or front-loaded schedule, or a refresher that vests in the same year as an older grant, can push options over the line.

Plans usually split a grant automatically and label the overflow as NSOs in your equity portal. Check the type on each tranche before you exercise, because the same grant can contain both and the tax treatment of each lot is different.

How does the AMT hit ISO exercises, and do you get it back?

The AMT is a parallel tax calculation; an ISO spread you hold past December 31 is added to your AMT income, and you pay the larger of regular tax or tentative minimum tax. Line 2i of Form 6251 picks up the spread. The difference between the two calculations is your AMT for the year.

The AMT system has its own exemption and two rates. For 2026, after the changes made by the 2025 tax law (P.L. 119-21), the IRS set these amounts in Revenue Procedure 2025-32:

The gap between the start and end of each phase-out is exactly twice the exemption, which means the exemption shrinks by 50 cents for every dollar of AMT income above the threshold. Inside that band, each extra dollar of ISO spread is effectively taxed at 1.5 times the AMT rate. AMT income above the exemption is taxed at 26% on the first $244,500 and 28% above that for 2026 ($122,250 if married filing separately).

Worked example (illustrative). A married couple with $450,000 of wages and the standard deduction exercises ISOs with a $400,000 spread and holds the shares into the new year:

1. Regular taxable income is $450,000 − $32,200 = $417,800; regular tax is about $86,600. The ISO adds nothing.
2. AMT income is $450,000 + $400,000 = $850,000 (the standard deduction is not allowed for AMT). That is below $1,000,000, so the full $140,200 exemption applies.
3. Taxable excess is $709,800: 26% of $244,500 plus 28% of $465,300 = $193,900 tentative minimum tax.
4. AMT owed is $193,854 − $86,608 = about $107,200, cash due with the return, on shares they have not sold.

Most of that is not gone for good. Because the ISO adjustment is a timing difference, AMT it causes creates a minimum tax credit that carries forward to later years on Form 8801. You use it in any later year when your regular tax exceeds your tentative minimum tax. Your ISO shares also carry two bases: the exercise price for regular tax and the exercise-date value for AMT (Form 6251 tells you to increase AMT basis by the adjustment). In the year you sell, the regular-tax gain is much larger than the AMT gain, which usually opens room to use a large part of the credit.

The better approach is to avoid paying much AMT up front. Chart 3 shows how much ISO spread a joint-filing household can recognize in 2026 before any AMT is due.

The headroom does not rise smoothly with income because the regular brackets and the AMT rates cross at different points, but the range is consistent: roughly $20,000 to $110,000 of spread a year for wage-only households. Large state income tax deductions, which the AMT disallows, shrink it further. Your CPA can run the exact number from a projected Form 6251; you need it before you exercise, not in April.

When should you exercise: early exercise and 83(b), splitting across years, or a same-day sale?

Exercise when the tax cost is lowest relative to the risk you keep: early while the spread is small, in yearly slices sized to your AMT headroom, or in a same-day sale when you cannot afford the AMT or the concentration. Chart 4 turns that into a decision tree.

Early exercise and the 83(b) election. Some private companies let you exercise options before they vest. You receive restricted shares that vest on the original schedule. Filing an 83(b) election within 30 days of exercise (26 U.S.C. §83(b)(2)) fixes the taxable spread at the exercise date. If the exercise price equals the current fair market value, that spread is zero: no wage income for an NSO and no AMT adjustment for an ISO, and the capital gains holding period starts immediately. The risk is real money: you pay the full exercise price for shares that may never be worth anything, and the election cannot be undone. Early-exercised shares in a qualifying small business may also start the 5-year clock for the QSBS Section 1202 exclusion.

Splitting exercises across tax years. Because the AMT exemption resets every year, exercising $80,000 of spread in each of five years can cost nothing in AMT where a single $400,000 exercise costs six figures. December and January exercises fall in different tax years, so a split across a year-end doubles the headroom available within a few weeks. The cost is that later slices may be exercised at a higher stock price, with a larger spread per share.

Exercise in January, not December. If you are going to exercise and hold more than your headroom, doing it early in the year gives you until December 31 to watch the stock. If it falls hard, a disqualifying sale before year-end removes the AMT adjustment entirely. The AMT itself is not due until the following April, so a January exercise also gives you about 15 months to plan the cash.

Same-day sale (cashless exercise). The broker exercises and sells immediately, uses part of the proceeds to pay the exercise price, and sends you the rest. For an ISO this is a disqualifying disposition in the same year: the spread is ordinary income, there is no AMT adjustment, no Social Security or Medicare tax, and no stock risk. For an NSO, a same-day sale simply turns the taxed spread into cash. If you are an officer of a public company, schedule exercises and sales under a 10b5-1 plan so they can happen outside trading windows.

What happens to your stock options when you leave the company?

Vested ISOs must be exercised within 3 months after your last day of employment to keep ISO treatment; later exercises are taxed as NSOs, and the plan may cancel the options entirely. Section 422(a)(2) requires you to have been an employee at all times from grant until 3 months before exercise. The window is 1 year if you leave because of permanent and total disability, and the rules are different after death.

Two different deadlines are at work. The 3-month rule is tax law and only decides whether an exercise counts as an ISO. Your plan’s post-termination exercise period decides whether you can exercise at all. Many plans use 90 days; some private companies have extended theirs to several years, but options exercised after the 3-month mark in those plans are NSOs. Unvested options are normally forfeited on departure unless your agreement accelerates them.

A departure can force a large exercise into one tax year, which is exactly what the AMT punishes. If you are planning to leave, model the exercise before you resign. Spreading exercises over the remaining months of employment, across a year-end if possible, can save more than any negotiation over your start date.

How do you manage the concentration risk that stock options create?

Treat every exercise-and-hold decision as a decision to buy your employer’s stock with cash and tax money, and cap how much of your net worth one company can be. Options already link your paycheck, unvested equity and career to one company. Exercising and holding adds the shares themselves, and with ISOs often an AMT bill that is only recovered if the stock holds up.

The tax saving from a qualifying ISO sale is a one-time gain of about 14 to 16 percentage points of the spread for a top-bracket holder: the gap between ordinary rates plus Medicare and the long-term rate. A single bad earnings report can erase that in one session. Proflex’s working rule for clients is to keep any one stock below 10–20% of liquid net worth, counting vested in-the-money options at their after-tax spread. Our net worth guide explains why a paper net worth built on equity comp overstates what you actually own.

Practical ways to hold the line:

• Sell NSOs at exercise. The tax is the same whether you hold or sell; holding is a new investment decision.
• Diversify the qualifying ISO lots first. Once a lot passes both dates, there is no tax reason to wait, and selling releases AMT credit.
• Stage sales with a plan. A 10b5-1 schedule sells through windows and blackouts on fixed dates.
• Hedge what you must hold. Collars or puts on exercised shares can set a floor while a holding period runs, subject to your company’s hedging policy and the straddle and constructive-sale rules.
• Use the whole tax toolkit. Asset location, harvesting losses elsewhere to offset gains, and gifting appreciated lots are covered in tax-efficient investing for executives.

What should you check before you exercise stock options?

Confirm the option type, the dates, the tax at your real marginal rate, and the cash and concentration you will be left with. Run this list before every exercise:

1. Option type by tranche. ISO or NSO, and whether part of an ISO grant was converted to NSO by the $100,000 limit.
2. Dates. Grant date, vest dates, expiry, and the post-termination exercise period in your plan.
3. Spread. Today’s fair market value minus the exercise price, times shares. For private companies, use the latest 409A valuation your company provides.
4. Tax now. NSO: marginal rate plus Medicare, compared with the 22% withheld. ISO: projected Form 6251 and your AMT headroom for the year.
5. Cash. Exercise price, withholding gap, estimated payments and any AMT due next April.
6. Exit path. Same-day sale, sale after the qualifying dates, or a disqualifying sale before December 31 if the stock falls.
7. Concentration. Your post-exercise exposure to one stock as a share of liquid net worth.
8. Insider rules. Trading windows, 10b5-1 plan, hedging policy and Section 16 reporting if you are an officer.
9. Paperwork. Keep every Form 3921 and your exercise confirmations; your broker’s cost basis for ISO shares is often wrong for AMT and for disqualifying sales.

Frequently asked questions

Key takeaways

1. NSOs are taxed as wages on the spread at exercise; ISOs defer the tax to the sale but add the spread to AMT income in the exercise year.
2. An ISO sale qualifies for all-long-term treatment only after 2 years from grant and 1 year from exercise; anything earlier is a disqualifying disposition.
3. Only $100,000 of ISOs, at grant-date value, can first become exercisable each year; the rest are NSOs.
4. For 2026 the AMT exemption is $140,200 for joint filers and phases out at 50 cents per dollar above $1,000,000; ISO-driven AMT is largely recoverable as a credit.
5. Size each ISO exercise to your AMT headroom, exercise early in the year, and keep the option to sell before December 31 if the stock falls.
6. Supplemental withholding on NSO income is usually 22%, which leaves top-bracket executives owing more in April.
7. Leaving the company gives you 3 months to exercise ISOs as ISOs, and an exercise-and-hold strategy can concentrate your net worth in one stock.

Sources and method

1. IRS Topic 427, Stock options — statutory vs nonstatutory options; tax at grant, exercise and sale
2. IRS Publication 525, Taxable and Nontaxable Income — ISO holding periods, disqualifying dispositions, NSO income
3. IRS Instructions for Form 6251, Alternative Minimum Tax — ISO adjustment on line 2i, AMT basis, same-year disposition rule
4. IRS, About Form 3921 — employer reporting of ISO exercises
5. 26 U.S. Code § 422, Incentive stock options — holding periods, 3-month rule, $100,000 limit, 10-year term
6. 26 U.S. Code § 83, Property transferred in connection with performance of services — 83(b) election and its 30-day deadline

Tax figures are 2026 federal amounts from IRS Revenue Procedure 2025-32 (AMT exemption, phase-out and 28% bracket threshold, regular brackets and standard deduction) and IRS Publication 15 (supplemental withholding). The worked examples and Charts 2 and 3 are illustrative Proflex calculations for a married couple filing jointly with wage income only; they ignore state tax, itemized deductions and other AMT adjustments. Federal general rules only. This is education, not tax or legal advice; confirm with your CPA/counsel.