ISO vs. NSO

The two types of employee stock options — Incentive Stock Options (ISOs) receive preferential tax treatment, while Non-Qualified Stock Options (NSOs or NQSOs) do not.

Stock options granted to employees come in two forms: Incentive Stock Options (ISOs) and Non-Qualified Stock Options (NSOs, also called NQSOs or NQOs). The key difference is tax treatment. ISOs receive preferential capital gains treatment if holding period requirements are met: you don't pay regular income tax when you exercise, only when you sell — and if you hold the shares long enough, gains are taxed at the lower long-term capital gains rate. NSOs are taxed as ordinary income at exercise on the spread between your exercise price and the current fair market value (the 409A valuation), with the employer also withholding payroll taxes.

ISOs sound clearly better, but they come with important restrictions and risks. ISOs can only be granted to employees (not contractors or board members), there's a $100,000 annual limit on the value of ISOs that can become exercisable in any calendar year, and ISOs can trigger the Alternative Minimum Tax (AMT) at exercise — a parallel tax system that ignores the ISO preference and treats the spread as income. AMT can result in a significant cash tax bill even before you've sold a single share, which is a real risk for employees holding large ISO grants at high-valuation companies.

The 90-day exercise window is one of the most consequential — and least understood — provisions in startup equity. When you leave a company, your vested stock options typically expire 90 days after your last day unless exercised. For ISOs, this window is a legal requirement of the tax code; after 90 days, ISOs automatically convert to NSOs. For early employees with a large spread and a high exercise cost, this can be a genuinely difficult decision: exercise and pay potentially significant taxes on paper gains, or let the options expire. Some companies have extended exercise windows to 5 or 10 years as a retention and employee-friendly policy — worth asking about before accepting an offer.

Key Differences at a Glance

  • ISOs: employees only, $100K annual limit on exercisable value, no regular tax at exercise (but AMT may apply), favorable long-term capital gains if holding periods met.
  • NSOs: anyone can receive (employees, contractors, board members), no dollar cap, taxed as ordinary income at exercise on the spread, employer withholds payroll taxes.
  • Both types: same mechanics (exercise price, vesting, expiration), same 10-year option term, same 90-day post-termination exercise window by default.
  • ISO holding period: must hold shares 2 years from grant date AND 1 year from exercise to get long-term capital gains treatment.
  • Disqualifying disposition: selling ISO shares before meeting holding periods converts the gain to ordinary income — the ISO tax benefit is lost.

AMT and ISOs: What You Need to Know

  • Exercising ISOs creates an AMT preference item equal to the spread (FMV minus exercise price) at the time of exercise.
  • If your AMT liability exceeds your regular tax liability, you pay the difference — a real cash cost even though you haven't sold shares.
  • AMT paid generates a credit you can use in future years when regular tax exceeds AMT — but this is cold comfort if the stock declines.
  • The risk is highest when the 409A is high relative to your exercise price and you're exercising a large number of shares.
  • Run the AMT calculation before exercising — or hire a CPA to do it. The surprise AMT bill is one of the most painful outcomes in startup equity.

The 90-Day Exercise Window

When you leave a company — voluntarily or not — your vested stock options typically expire 90 days after your last day unless you exercise them. For ISOs, this is a tax code requirement: options held beyond 90 days after termination automatically lose their ISO status and become NSOs. For early employees with a low exercise price and a high current 409A, this can mean forfeiting significant paper value — or writing a large check to exercise options on shares you can't yet sell. Some employee-friendly companies have extended their post-termination exercise window to 5 or 10 years, which dramatically reduces this pressure. Always check your option agreement for the exact window, and factor it into any decision about leaving the company.

Example

An early startup employee receives 100,000 ISOs with a $0.10 exercise price when the 409A is $0.10. Three years later, the 409A is $5.00. She exercises all 100,000 shares, paying $10,000. She doesn't owe regular income tax yet — but the $490,000 spread is an AMT preference item and may trigger a significant AMT bill. If she holds for 2 years from the grant date and 1 year from exercise, her eventual gain is taxed at long-term capital gains rates rather than as ordinary income.