Employee Stock Options: ISO vs. NSO

You join a startup and receive 100,000 stock options with a strike price of $1 per share; five years later the shares are worth $10, and what you owe, and when, depends on decisions most people make without realizing they are making them.

I just received stock options. What do I need to know?

An employee stock option gives you “the right to buy your company’s stock at a set price after a certain period of time” [1]. Your grant paperwork will use a handful of terms. Here is what they mean in practice.

• Strike price (or exercise price). The fixed price you will pay per share. In our scenario, $1.
• Vesting. When you earn the right to exercise your options. A common schedule is four years with a one-year cliff: nothing vests during the first year, then a portion vests at once and the rest vests monthly.
• Exercise. Actually buying the shares by paying the strike price. Until you exercise, you own an option, not stock.
• Spread. The difference between what the shares are worth and what you paid. If shares are worth $10 and your strike is $1, the spread is $9 per share. Almost every tax consequence flows from the spread.
• Fair market value and the 409A valuation. Private company shares have no public price, so the company hires an appraiser to set one. That appraisal, often called a 409A valuation, is the number the IRS will use to measure your spread at exercise [2].
• Expiration. Options do not last forever. Most expire ten years from grant and much sooner if you leave the company.

Options are the most common form of startup equity compensation, and the tax rules were written with public companies in mind, which is why private-company holders run into so many surprises.

The IRS sorts options into two buckets: statutory options, which include incentive stock options, and nonstatutory options, usually called non-qualified stock options [3]. Your grant agreement will say which you have. If it does not, ask before you do anything else.

ISO vs NSO: what is the difference?

The two kinds of option feel identical in your account. They are taxed very differently.

Question	Incentive stock options (ISOs)	Non-qualified stock options (NSOs)
Who can receive them	Employees only [4]	Anyone: employees, advisors, contractors, directors
Tax when granted	None	None, if the strike price is at least fair market value [5]
Tax when exercised	No regular income tax, but the spread counts for AMT [6]	The spread is ordinary income, taxed like salary [5]
Tax when sold	Long-term capital gain on the whole gain, if holding periods are met	Capital gain or loss measured from the value at exercise
Annual limit	$100,000 of grant-date value may first become exercisable each year; the excess is treated as NSOs [7]	None
Biggest risk	An AMT bill with no cash to pay it	A large ordinary-income tax bill in the year of exercise

Sources: [4] [5] [6] [7]

How are NSOs taxed?

NSOs are the simpler of the two. When you exercise, the spread is treated as compensation. If you are an employee, it goes on your Form W-2 and your employer withholds tax.

If you are an advisor or contractor, it is self-employment income [5]. From that day forward your “basis” in the shares, meaning the amount already taxed, is the fair market value at exercise. When you later sell, only the growth above that value is capital gain.

Example B: an NSO exercise

Jordan, a single filer, earns enough that her 2026 taxable income is $150,000 before any option activity. She holds 20,000 NSOs with a $2 strike.

The company’s current 409A valuation is $12 per share. She exercises all of them, paying $40,000.

• Spread: ($12 − $2) × 20,000 = $200,000 of ordinary income, reported on her W-2.
• Taxable income rises from $150,000 to $350,000. Using the 2026 single-filer rate table, federal income tax rises from about $28,600 to about $91,300, an increase of roughly $62,700 [8]. Medicare tax and state tax come on top.
• Her employer must withhold on the $200,000. Many companies require the employee to write a check for the withholding at exercise, so Jordan’s real cash outlay is the $40,000 strike price plus tens of thousands in withholding.
• Her basis is now $12 per share. If she sells two years later at $20, the $160,000 gain is long-term capital gain, taxed at 15 percent for most filers and 20 percent once taxable income passes $545,500 (single) [8].

One trap applies to NSOs specifically. If an NSO is granted with a strike price below fair market value, Section 409A can tax it as the option vests, add a 20 percent penalty tax and charge interest, even if you never exercise [9].

This is why companies obtain independent valuations before granting and why a grant priced “cheap” is a warning sign, not a gift.

How are ISOs taxed?

ISOs were designed to give employees a better deal. If you follow the rules, you pay no regular income tax at exercise, and when you eventually sell, the entire gain from your strike price to the sale price is long-term capital gain [5]. Compared with NSOs, that can cut the tax rate on the spread roughly in half.

The rules are strict. To get the favorable result you must hold the shares at least two years from the grant date and at least one year from the exercise date.

Sell earlier, and it becomes a “disqualifying disposition”: the spread at exercise is taxed as wages, just as if the option had been an NSO [5]. You must also still be an employee, or have left within the last three months, when you exercise [4].

Your company reports every ISO exercise to you and the IRS on Form 3921, which shows the exercise date, strike price and fair market value [10]. Keep every one of them. You will need the numbers years later.

The catch is in the words “no regular income tax.” There is a second tax system, and ISOs feed directly into it.

Why can you owe tax before you receive any cash?

Most people assume tax follows money. You get paid, you owe tax. Stock options break that assumption, and startup employees are the people most often caught by it.

When you exercise an option, the tax law treats you as having received something valuable: shares worth more than you paid.

For an NSO, that value is taxed immediately as wages even though you received stock, not cash. For an ISO, the regular tax system looks the other way, but the alternative minimum tax does not. It counts the spread as income in the year you exercise [6].

At a public company this is manageable: you can sell some shares the same day to cover the tax. At a private company you usually cannot sell at all. You hold shares with a paper value, the IRS computes a real tax on that paper value, and the bill is due the following April whether or not the company ever has a liquidity event. If the shares later fall in value, the tax you already paid does not come back automatically.

What is AMT and why do people fear it?

The alternative minimum tax is a parallel calculation that runs alongside your regular tax return, and AMT on stock options is the reason most startup employees first hear of it. You compute your tax both ways, and you pay whichever is higher [6].

The AMT starts with your regular income, adds back certain items, subtracts an exemption amount and applies two flat rates. For most people the exemption wipes out any AMT. An ISO exercise is the item most likely to change that, because the entire spread is added back on Form 6251 [6].

Source: IRS, Revenue Procedure 2025-32, § 4.10, https://www.irs.gov/pub/irs-drop/rp-25-32.pdf

Example A: an ISO exercise that triggers AMT

Maya is single with 2026 taxable income of $180,000 from salary. Her regular federal income tax is about $35,800 [8]. She holds 50,000 vested ISOs with a $1 strike. The 409A value is $9. In December she exercises all 50,000, paying $50,000, and sells nothing because the company is private.

• Regular tax: unchanged. The ISO exercise is invisible on her Form 1040.
• AMT income: $180,000 + ($9 − $1) × 50,000 = $580,000.
• Exemption: because her AMT income exceeds $500,000, the $90,100 exemption shrinks by half of the $80,000 excess, to $50,100 [8].
• AMT base: $580,000 − $50,100 = $529,900. Tax at 26 percent on the first $244,500 and 28 percent on the rest is about $143,500.
• Because $143,500 exceeds her regular tax of $35,800, she owes the difference, roughly $107,700, as AMT.

Now suppose Maya had exercised only 10,000 shares in 2026. Her AMT income would be $260,000, she would keep the full exemption, and her AMT would be about $8,400.

Spreading the same 50,000 shares over several years, or exercising earlier while the spread was smaller, is the single most effective AMT planning tool available to an employee. These calculations are simplified; the actual computation on Form 6251 includes other adjustments and state tax adds to the total.

Two features soften the blow. First, AMT paid because of an ISO exercise generally becomes a credit, tracked on Form 8801, that reduces your regular tax in later years when regular tax exceeds AMT [11]. It is a timing cost, but the money can be tied up for years.

Second, if you sell the shares in the same calendar year you exercised, the AMT adjustment disappears and the spread is taxed as ordinary income instead [6]. That is why the period between an exercise and December 31 is a decision window, not dead time.

Should I exercise now or wait?

There is no universal answer, but the trade-off is consistent. Exercising early, while the spread is small, minimizes AMT or ordinary income and starts the capital-gain clock sooner.

It also puts your own cash at risk in a company that may fail. Waiting preserves your cash and lets you see how the company performs, but every increase in the 409A value raises the tax cost of exercising later.

Sources: [6] [5]

This annual calculation is the core of stock option tax planning. A practical approach is to calculate, each fall, how many ISOs you can exercise before your AMT exceeds your regular tax, and exercise up to that number.

Many employees can absorb a meaningful number of shares each year at little or no AMT cost.

What happens if I leave the company?

Leaving changes the math in three ways. First, unvested options are forfeited. Second, most plans give you a short window, commonly 90 days, to exercise vested options or lose them; the exact period is in your plan documents, and some companies allow longer.

Third, ISO status is tied to employment. An ISO exercised more than three months after your employment ends is taxed as an NSO, with the spread taxed as ordinary income [4].

That combination forces a compressed decision: pay the strike price and any tax for shares you cannot sell, or walk away from the options.

It is worth modeling before you resign, not after. If a departure is on the horizon, exercising some vested ISOs in the prior calendar year can spread the AMT cost across two tax years.

When does an 83(b) election matter?

Some companies let you exercise options before they vest, a practice called early exercise. When you do, you receive shares that the company can still repurchase if you leave.

The tax law normally ignores such shares until they vest, then taxes the spread at each vesting date as ordinary income [5]. If the company is growing, that means a rising income tax bill every vesting date, with no cash to show for it.

An 83(b) election flips that result. By filing the election within 30 days of receiving the shares, you choose to be taxed now, on today’s spread, rather than later on tomorrow’s.

If you exercise at a strike price equal to current fair market value, today’s spread is zero, so the income is zero. The election is filed on IRS Form 15620 and the deadline cannot be extended [12]. For ISOs, the election also fixes the AMT adjustment at the exercise date [5].

Example C: early exercise with an 83(b) election

Priya joins a seed-stage startup as an early engineer and receives 100,000 options with a $0.10 strike, equal to the current 409A value. The plan permits early exercise. She exercises all 100,000 on day one for $10,000 and files Form 15620 within 30 days.

• Income at exercise: ($0.10 − $0.10) × 100,000 = $0. Nothing is reported on her return.
• Her holding period starts immediately. Her basis is $10,000.
• Over four years the shares vest while the 409A value climbs to $1, $2.50, $4 and $5. Because of the election, none of those vesting dates is a taxable event.
• If she sells six years later at $10 per share, her $990,000 gain is long-term capital gain. If the shares qualify as qualified small business stock, part or all of that gain may be excluded entirely [13].

Compare the result without the election. Each vesting tranche of 25,000 shares would produce ordinary income equal to the spread on the vesting date: $22,500, $60,000, $97,500 and $122,500, a total of $302,500 of wages across four years, taxed at up to 37 percent, before she has sold a single share.

The risk is real. Priya paid $10,000 for shares that may become worthless, and if she had exercised at a spread, the tax paid under the election is not refunded if the company fails. The election fits best when the spread is at or near zero and the strike price is money you can afford to lose.

What happens if the company never goes public?

Most startups do not reach an IPO. Some are acquired, some raise money for a decade, and many close. Each path has tax consequences for option holders.

• Prolonged illiquidity. AMT paid on an ISO exercise sits as a credit until you have regular tax to absorb it, which may take years [11]. Cash used for strike prices and taxes is unavailable for anything else.
• Secondary sales and tender offers. If the company or an investor buys shares from employees, the sale is a taxable event. For ISO shares, check the two-year and one-year holding periods first; a sale that is a few weeks early converts the entire spread to wages [5].
• Acquisition. Options may be cashed out, assumed or canceled. A cash-out of unexercised options is ordinary income. Exercised ISO shares sold in the deal face the same holding-period test.
• Failure. If the shares become worthless, the loss is a capital loss, deductible against capital gains and then against only $3,000 of ordinary income per year. The AMT you paid years earlier may remain as an unused credit.
• Section 83(i) deferral. Some private companies that grant options broadly allow qualifying employees to defer the income from an exercise for up to five years. The election converts an ISO into an NSO and comes with its own conditions [14]. Ask whether your company offers it.

Five Questions to Ask Before Exercising Stock Options

1. Are these ISOs or NSOs, and does the ISO label still hold? Confirm the grant type, whether the $100,000 annual limit pushed any shares into NSO treatment, and whether you are still within three months of employment.
2. What is the current 409A value and therefore the spread? The spread drives either your ordinary income or your AMT. Ask the company for the current valuation and its date.
3. How much can I exercise this year before AMT bites? Run the Form 6251 math, or have someone run it, before you exercise rather than when you file.
4. Where will the cash come from? Add the strike price, any withholding and any AMT, and confirm you can pay it by the following April without selling the shares.
5. What is my plan if the shares cannot be sold for five years, or ever? If the honest answer is that the loss would be painful, exercise fewer shares.

Common Tax Mistakes

• Exercising ISOs without understanding AMT. The most expensive mistake on this list. Once the calendar year closes with the shares still held, the AMT adjustment is locked in [6].
• Missing the 83(b) deadline. Thirty days from the share transfer, measured strictly. A late election is simply not valid, and each later vesting date becomes a taxable event [12].
• Losing ISO treatment after leaving. Exercising in month four after departure, or forgetting that a long leave of absence can end “employment” for this purpose, turns the ISO into an NSO [4].
• Misunderstanding holding periods. The ISO test is two years from grant and one year from exercise, and both must be met. Counting from vesting, or meeting only one, produces a disqualifying disposition [5].
• Failing to maintain basis records. ISO shares carry two different bases, one for regular tax and a higher one for AMT, and brokers track neither reliably. Without your Forms 3921 and exercise confirmations, you may pay tax twice on the same gain [10].

This article is for general informational purposes only and does not constitute legal or tax advice. Examples are simplified and use 2026 federal figures; they omit state tax and other adjustments. Reading it does not create an attorney-client relationship with Dilendorf Law Firm. Attorney Advertising.