Tax

ISO vs. NSO Stock Options: How Each Is Taxed and Why 2026 Raises the Stakes

ISOs and NSOs are taxed very differently. How each works, the ISO AMT trap, and why 2026's OBBBA changes make exercising ISOs riskier for high earners.

By 409.AI Team - 2026-07-09

# ISO vs. NSO Stock Options: How Each Is Taxed and Why 2026 Raises the Stakes

Two engineers join the same startup on the same day. Both get options on 10,000 shares at a $1 strike price. Four years later, both exercise when the shares are worth $11. One writes a check to the IRS that spring. The other pays nothing until she sells, then pays at the lowest rate the tax code offers. Same company, same grant, wildly different outcomes.

The difference is one line in their option paperwork: whether they hold incentive stock options (ISOs) or non-qualified stock options (NSOs). That single distinction decides when you're taxed, at what rate, and whether you can get surprised by a tax you've probably never heard of. And starting in 2026, a change buried in the One Big Beautiful Bill Act makes that surprise more likely for a lot of option holders.

Here's how the two option types actually work, with the numbers.

The one thing both option types share: the strike price

Before the tax treatment diverges, ISOs and NSOs start in the same place. Both are granted with a strike price (also called the exercise price), and for both, that strike has to be set at or above the fair market value of the common stock on the grant date. For a private company, that fair market value comes from a [409A valuation](https://409.ai/articles/what-is-a-409a-valuation-a-comprehensive-guide).

This matters because the whole tax picture is built on the gap between your strike price and what the stock is later worth. A defensible 409A that comes in low is good news for employees: it means a low strike, which means a bigger potential spread that isn't given away to the IRS on day one. That's also why a company's [409A often lands well below its post-money valuation](https://409.ai/articles/why-is-your-409a-valuation-lower-than-post-money-valuation), and why founders shouldn't panic when it does.

Grant a strike below fair market value and you don't just create a bookkeeping headache. You risk blowing Section 409A itself, which carries a 20% federal penalty plus interest for the employee on top of ordinary tax. Both ISOs and NSOs depend on getting that starting number right.

With the strike settled, the paths split.

NSOs: taxed when you exercise

Non-qualified stock options are the simpler of the two, and the more common. Any option granted to a contractor, advisor, board member, or consultant is an NSO by definition, and plenty of employee grants are NSOs too.

The tax event happens at exercise. When you exercise an NSO, the spread between the fair market value and your strike price is treated as ordinary compensation income. It lands on your W-2, and it's subject to income tax withholding plus Social Security and Medicare, exactly like salary.

Take our engineer with 10,000 options at a $1 strike, exercising when the stock is worth $11. The spread is $10 a share, so $100,000 of ordinary income shows up on his W-2 that year, whether or not he sells a single share. At a 35% marginal rate, that's roughly $35,000 in tax owed on paper gains. His cost basis in the shares resets to $11. If he later sells at $20, the additional $9 per share is a capital gain, long-term if he held the shares more than a year after exercise.

The IRS lays out this treatment in its guidance on nonstatutory stock options ([IRS Topic No. 427](https://www.irs.gov/taxtopics/tc427)). The mechanics are predictable, which is the point: with NSOs there's no ambiguity, no separate tax system to worry about, and no holding-period gymnastics required to get the basic treatment. You pay ordinary rates on the spread now, capital gains on any appreciation later.

ISOs: the tax break with a catch

Incentive stock options are the tax-favored cousin, governed by Section 422 of the tax code. They can only go to employees, they have to be granted under a written plan, and no more than $100,000 worth (measured by grant-date value) can first become exercisable in any single year. Anything above that limit is automatically treated as an NSO.

The payoff for those restrictions: when you exercise an ISO, there is no regular income tax. Nothing hits your W-2. If you then meet two holding requirements, your entire gain from strike to sale gets taxed at long-term capital gains rates, which top out at 20% federal rather than 37%.

The two requirements define a qualifying disposition. You have to hold the shares at least two years from the grant date and at least one year from the exercise date. Clear both, and the full spread converts to capital gain. Miss either one, and you've made a disqualifying disposition: the bargain element at exercise flips back to ordinary income, taxed the same way an NSO would have been. This is a place people trip up, and it connects to the broader set of [tax implications that flow from a 409A valuation](https://409.ai/articles/409a-valuation-tax-implications).

So far ISOs sound strictly better. They aren't, because of the catch.

The AMT trap

Exercising an ISO is invisible to the regular tax system but very visible to a parallel one: the alternative minimum tax. The AMT is a separate calculation with its own rules, designed to make sure high-income taxpayers who pile up deductions and preferences still pay a floor amount. You compute your tax the normal way, compute it again the AMT way, and pay whichever is higher.

When you exercise an ISO and hold the shares past year-end, the bargain element (fair market value at exercise minus your strike price) becomes an AMT preference item. It doesn't show up as regular income, but it inflates your alternative minimum taxable income. Our engineer exercising 10,000 ISOs at that $10 spread adds $100,000 to his AMTI. If that pushes his AMT above his regular tax, he owes the difference in cash, in April, on stock he hasn't sold and can't easily turn into money if the company is still private.

That is the classic ISO trap: a real tax bill on paper gains, triggered by an exercise that produced no cash. The AMT rate is 26% on the first slice of AMTI and 28% above a breakpoint (about $239,100 for 2026). There's some relief in that the AMT you pay creates a credit you can recover in later years, but the near-term cash hit is what catches people off guard, and it's a big reason so many employees underestimate [what their options are really worth after tax](https://409.ai/articles/understanding-409a-valuation-for-employees).

What changed for 2026

Here's the timely part. Whether an ISO exercise actually triggers AMT depends heavily on the AMT exemption, an amount you subtract before the AMT rate applies, and on how quickly that exemption phases out as income rises. The One Big Beautiful Bill Act, which reshaped a wide swath of the tax code (it also [expanded the QSBS exclusion](https://409.ai/articles/qsbs-one-big-beautiful-bill-act-section-1202-changes)), tightened both starting in 2026.

For 2026, the AMT exemption is $90,100 for single filers and $140,200 for married couples filing jointly, per the figures reported by the [Tax Foundation](https://taxfoundation.org/data/all/federal/2026-tax-brackets/). The exemption itself edged up. What got worse is the phaseout. In 2025 the exemption started phasing out at $626,350 for singles and $1,252,700 for joint filers, and it shrank by 25 cents for every dollar of income above those lines. Beginning in 2026, the OBBBA drops the phaseout thresholds to $500,000 (single) and $1,000,000 (joint) and doubles the phaseout rate to 50 cents on the dollar.

Translated: high earners lose their AMT cushion sooner and faster. A large ISO exercise stacked on a strong salary year, which is exactly the profile of a senior employee at a company heading toward or just past an IPO, is meaningfully more likely to land in AMT in 2026 than it would have been in 2025. If you're planning a big exercise, the size of the spread you can absorb before AMT kicks in has shrunk.

A side-by-side on the same grant

Put the two option types on identical facts. Strike $1, exercise at $11, 10,000 shares, sold two years later at $20.

With NSOs: $100,000 of ordinary income at exercise (taxed up to 37%, plus payroll tax), basis resets to $11, and the final $9 per share of appreciation is long-term capital gain when sold. Tax is certain and front-loaded.

With ISOs, assuming a qualifying disposition: no regular tax at exercise, but $100,000 of AMT preference that may or may not generate an actual AMT bill depending on the rest of your return and the tighter 2026 phaseout. At sale, the entire $19 per share of gain (from the $1 strike to the $20 sale price) is long-term capital gain, taxed at up to 20%. If everything qualifies and you dodge or recover the AMT, the ISO holder keeps materially more.

That "if" is the whole game. ISOs reward planning and punish improvisation.

So which is better?

For the employee, ISOs offer the better ceiling and NSOs offer the fewer surprises. The right answer depends on the size of your spread, your other income, whether you can hold long enough to qualify, and whether you can cover a possible AMT bill without selling. Early exercising while the spread is small, often paired with an [83(b) election](https://409.ai/articles/the-83b-election-explained-for-founders), is one way ISO holders keep the AMT preference near zero, because there's almost no bargain element when strike and fair market value are close.

For the company, the choice is partly made for you. ISOs are limited to employees and capped at $100,000 a year; everyone else gets NSOs. Many startups grant ISOs up to the limit and NSOs beyond it. Either way, the foundation is the same: a strike price set to a defensible fair market value, which is why the difference between [a 409A valuation and a general FMV assessment](https://409.ai/articles/409a-valuation-vs-fair-market-value) is worth understanding before you issue a single grant.

If you hold options, find out which type you have before you exercise, model the AMT under the 2026 rules rather than last year's, and don't assume the tax-favored option is free. The label on your grant is doing more work than almost anything else on the page, and in 2026 the margin for getting it wrong got a little thinner.