The real cost of exercising a vested stock option is the tax, not the strike price. Three months after a job ends, an Incentive Stock Option (ISO) loses its special tax status under federal law [4]. In practice, platforms and companies usually give employees just 90 days to make a choice [5].

The big picture:

The part I keep circling is how quietly the tax rules stack up against the option holder.

Exercising an ISO adds nothing to regular taxable income [1][3]. That is the entire point of the label.

But the gap between the strike price paid and the stock’s actual value that day becomes a tax preference item [1]. That spread can trigger the Alternative Minimum Tax (AMT) in the year the shares are bought [3].

Two hidden limits catch most holders off guard. First, the three-month rule turns an ISO into a standard option if the holder waits too long after leaving, taxing that spread as ordinary income [4].

Second, the law caps how much stock can qualify. Only $100,000 of stock value per year can vest as an ISO [4].

Finally, there is the holding test. To keep the profit taxed as a capital gain instead of standard wages, the shares must be held for two years from the grant date and one year from the exercise date [3].

By the numbers

  • 3 months — The ISO cutoff: Any exercise past three months after leaving a job strips the option of its special status, taxing the spread as ordinary income [4].
  • 26% — The AMT rate: The alternative tax hits the first $239,100 of preference items at 26% for tax year 2025, before jumping to 28% [2].
  • 2 years — The holding test: Holders must keep the shares two years from the grant date and one year from the exercise date to secure capital-gains treatment [3].
  • 0 — Taxable regular income: Exercising an ISO adds nothing to standard regular income, leaving many holders surprised by a separate alternative tax bill months later [1][3].

What I’d watch:

What strikes me here is that the tax agency does not care if the private equity is actually worth cash yet. The tax event lands in the year of the exercise, not the year the company sells.

  • The early sale: Selling before the holding periods expire turns the bargain into standard wages, which the company must report to the government [3].
  • The slow refund: Tax paid on a stock spread that later drops in value can return as an AMT credit in future years. It is a slow refund process that few holders track closely [2].
  • The three-month reset: Leaving a company does not just start a countdown clock. It actively converts ISO grants into ordinary-income options, meaning the exact same shares carry a worse tax bill depending on the calendar [4][5].
  • The paper trail: The tax agency tracks the exercise spread on Form 6251, line 2i. The company reports this exact number to the holder on Form 3921 [3].

The catch

On paper, the AMT calculation does not always result in a massive cash bill. It simply raises tentative minimum tax, and only the excess over the regular tax bill is owed.

A holder with a low regular tax base might owe a lot, while someone with a high regular tax base might owe nothing extra.

My read: the 90-day window shown on equity platforms is a software convention, not a law. Carta data shows companies typically give employees about 90 days to decide whether to buy their shares after leaving [5].

The actual law is a strict three-month rule that strips the tax benefit [4]. The two timeframes rhyme, but they are not the exact same deadline.

The ultimate constraint is cash liquidity. Exercising means paying the strike price and the tax bill at the exact same time, often for private shares that cannot be sold for years.

At a glance

  • The Big Shift: The choice to exercise a vested option is often weighed against the strike price, but the real cost is the tax. The AMT on the stock spread and a strict three-month cutoff dictate the true price of buying in.
  • Why It Matters: A holder can owe a large tax bill in the year of exercise on private shares that cannot yet be sold. Missing the three-month deadline or selling early turns a capital gain into ordinary income.
  • What I’d watch: Whether option holders are accurately pricing their exercise against the tax calendar rather than a headline company exit.
  • The three-month rule: Special tax status ends exactly three months after a job ends [4].
  • The $100,000 cap: The law caps these special grants at $100,000 of stock value per year [4].
  • The holding test: Holders must wait two years from grant and one year from exercise to keep the gain as capital [3].
  • The Catch: The alternative tax only bites when it exceeds the regular tax bill, and the common 90-day window seen in equity portals is a platform convention, not federal law.

Related reading

Sources

[1] Internal Revenue Service, “Topic No. 427 Stock Options” — https://www.irs.gov/taxtopics/tc427 [2] Internal Revenue Service, “Instructions for Form 6251 (2025)” — https://www.irs.gov/instructions/i6251 [3] Internal Revenue Service, “Publication 525 — Taxable and Nontaxable Income” — https://www.irs.gov/publications/p525 [4] Cornell Law School, Legal Information Institute, “26 U.S.C. § 422 — Incentive stock options” — https://www.law.cornell.edu/uscode/text/26/422 [5] Carta, “Trends in 409A valuations” — https://carta.com/data/trends-409a-valuations-2023