NSO vs ISO: When Each Comes Out Ahead for Tech Employees

Both NSOs and ISOs are contract rights to buy a fixed number of shares at a fixed strike for a fixed window. The difference is what happens tax-wise when you exercise. ISOs, if you meet the IRC §422 rules, defer regular-tax recognition until sale and convert the entire spread from strike to sale price into long-term capital gain. NSOs recognize the spread as ordinary W-2 income at exercise, full stop.

That sounds like ISOs win every time. In practice they do not, for three reasons. First, ISOs trigger AMT on the spread, which for large spreads can exceed the tax you would have owed on an NSO. Second, ISOs are limited to $100,000 of first-exercisable value per year (the $100k limitation in IRC §422(d)), so large grants at mature companies get chopped down to NSOs anyway. Third, ISOs have a 90-day post-termination exercise window that often forces a bad-cash decision, while NSOs are easier to hold past employment at some companies.

The honest framing is: NSOs are the default. ISOs are a preferential overlay that helps early employees and founders at pre-IPO companies with low strikes, and hurts or washes at mature companies with large spreads.

The mechanical difference at exercise

NSO exercise: spread (FMV minus strike) is ordinary W-2 income. Your employer withholds at the 22% supplemental rate up to $1M, 37% above. Payroll tax (Social Security, Medicare) applies. Basis in the shares equals FMV at exercise. Subsequent gain or loss is capital.

ISO exercise, qualifying path: no regular-tax income at exercise. Spread is an AMT preference on Form 6251. Basis for regular tax equals strike; basis for AMT equals FMV at exercise. Hold one year from exercise and two years from grant, then sell: full gain from strike to sale is long-term capital gain.

ISO exercise, disqualifying disposition: if sold before hitting both holding periods, lesser of (spread at exercise) or (gain at sale) is ordinary income, rest is capital. Same-year disqualifying disposition eliminates the AMT preference.

Concrete comparison at three stages

Assume the same 10,000 options at $2 strike, single filer, and three scenarios: early-stage (FMV $2.50 at grant, held four years to $20), mid-stage (FMV $10 at exercise, held one year to $30), late-stage (FMV $50 at exercise, sold same day).

Early-stage, exercise at grant, hold to sale at $20:

Item	NSO	ISO
Ordinary income at exercise	$5,000 spread → $1,750 fed tax at 35%	$0
AMT at exercise	$0	~$0 (under exemption)
Capital gain at sale	$175,000 (LTCG at 20%) = $35,000	$180,000 (LTCG at 20%) = $36,000
Total federal tax	$36,750	$36,000

Nearly identical. At low spreads, ISOs offer a small benefit mainly because there is no ordinary income or payroll tax at exercise.

Mid-stage, exercise at FMV $10, sell one year later at $30:

Item	NSO	ISO
Ordinary income at exercise	$80,000 spread → $28,000 fed at 35%	$0 regular tax
AMT at exercise	$0	~$20,800 (26% × $80k, assuming exemption gone)
Capital gain at sale	$200,000 (LTCG 20%) = $40,000	$280,000 (LTCG 20%) = $56,000
AMT credit recovery (later years)	$0	~$20,800 eventually
Total federal tax (long-run)	$68,000	$56,000

ISO wins by about $12,000, but only if the holder has the cash to pay $20,800 of AMT up front and can recover it in future years. If the AMT credit cannot be recovered (low future income, expatriation), ISO loses by $8,800.

Late-stage, exercise at FMV $50, same-day sale:

Item	NSO	ISO (same-day sale = disqualifying)
Ordinary income at sale	$480,000 → $168,000 fed at 35%	$480,000 → $168,000 fed at 35%
Capital gain	$0	$0
Total federal tax	$168,000	$168,000

Identical. Same-day sales of ISOs are disqualifying dispositions, which eliminates the ISO tax benefit. This is why mature public-company employees who exercise-and-sell immediately see no meaningful difference between NSOs and ISOs on the back end.

The $100k limitation

ISOs that first become exercisable in any calendar year above $100,000 of aggregate FMV (measured at grant) are treated as NSOs for the excess. This is IRC §422(d).

A grant of 100,000 ISOs at $10 strike, vesting 25% per year for four years at a grant-date FMV of $12 per share, puts $300,000 of first-exercisable value into each year ($12 × 25,000 shares). Of that $300k, only $100k qualifies for ISO treatment. The other $200k per year is recharacterized as NSO.

The limitation is measured at grant-date FMV, not current FMV, which means the limit is more forgiving for grants issued when the company was cheap. A 100,000-share grant at $0.50 strike with a $0.60 grant-date FMV puts only $15,000 of first-exercisable value per tranche into the bucket, well under the cap.

For most senior engineers at growth-stage companies, a meaningful portion of their “ISO” grant is actually NSOs under the $100k rule. Check the grant agreement or the ISO/NSO statement from the equity administrator to see the actual breakdown.

The 90-day post-termination exercise window

ISOs must be exercised within three months of termination to retain ISO status, per IRC §422(a)(2). Most plans set the exercise window at 90 days for consistency. Miss the window, and the options either expire or convert to NSOs.

This is the operational Achilles’ heel of ISOs. If you leave a company with a $500k ISO spread, you have 90 days to either (a) write a check for the strike price (which can be $50k-$200k), (b) potentially owe AMT on the spread, or (c) lose the options. Many departing employees simply forfeit because they cannot come up with the cash.

A handful of companies (notably Pinterest, Quora, Asana, Coinbase at various points) have extended PTE windows to five or ten years. In those plans, ISOs convert to NSOs on day 91 and can be held as long as the grant allows. This is a real benefit worth negotiating when you join.

Payroll tax treatment

NSO spread at exercise is subject to Social Security (6.2% up to $176,100 of total wages in 2025) and Medicare (1.45% plus 0.9% additional Medicare tax above $200k single). That adds meaningful payroll tax on top of income tax.

ISO spread at qualifying disposition is not subject to payroll tax. The entire gain is capital. This is a genuine advantage for ISOs that gets overlooked. On a $500k ISO spread held to qualifying disposition, the ISO holder saves roughly $2,300 of Medicare tax (capped at the Social Security wage base for that portion) relative to an NSO.

ISO disqualifying dispositions are generally not subject to FICA either, under IRC §3121(a)(22) as amended. This is one of the few cases where the IRS gives a favorable carve-out even on a disqualifying disposition.

When each comes out ahead

ISO wins when: the strike is low, the spread at exercise is modest (under the AMT exemption phase-out), you can hold for qualifying disposition, and your future income will let you recover the AMT credit.

NSO wins when: you plan to exercise-and-sell same-day anyway (no holding-period benefit from ISO), the spread is so large that AMT exceeds the ordinary-income tax, or you need flexibility to hold past 90 days after termination without converting.

Neither matters when: you will not exercise because the stock tanks, or the company is acquired in a cash deal that forces same-day disposition of all options regardless of type.

Frequently asked

Can I convert NSOs to ISOs? No. The grant type is fixed at grant. The reverse can happen: ISOs convert to NSOs if they breach the $100k limit, if they extend beyond the 90-day PTE window, or if the grant terms were not ISO-compliant at issuance.

Which one is better for an early startup employee? Usually ISOs. Early-stage strikes are low, spreads are small, AMT exposure is manageable, and the qualifying-disposition path converts all future appreciation to LTCG.

Which one is better for a late-stage pre-IPO employee? Often NSOs, counterintuitively. The $100k limit already capped ISO treatment, the AMT exposure on a large spread is large enough to force bad cash decisions, and the exercise-and-hold path runs into liquidity risk if the company stumbles.

What does “qualifying disposition” mean exactly? Sale of ISO shares more than two years after grant and more than one year after exercise, per IRC §422(a)(1). Fail either clock and the disposition is disqualifying.

Does 83(b) work with NSOs? 83(b) applies to restricted stock, not to unexercised options. Early exercise of an NSO (buying unvested shares at strike) plus a timely 83(b) election locks in the ordinary-income recognition at the current low FMV, which is the same move that helps on ISOs.

Next step

Check your grant agreement for the actual ISO/NSO split. If your company uses both, the allocation is on your grant documents or on the stock plan administrator’s website (Carta, Shareworks, Morgan Stanley). Then run the NSO vs ISO comparison calculator with your strike, your likely exercise price, and your tax bracket. The answer is rarely ambiguous once the numbers are on the page.