ISO 90-day exercise window: post-termination decisions on incentive stock options

The 90-day rule under IRC § 422(a)(2)#

IRC § 422 sets out the statutory requirements for an option to qualify as an incentive stock option. Section 422(a) lists the conditions that must be satisfied to receive ISO tax treatment at exercise. Among them, § 422(a)(2) requires that the option be exercised “by the individual within three months after the date such individual ceases to be an employee of the corporation granting the option or a parent or subsidiary corporation thereof.” The three-month period is measured from the date of cessation of employment, not from the date notice was given or the date the grant agreement’s post-termination exercise window expires. In counting, “three months” is interpreted as the date numerically corresponding three months later — effectively a 90-day window.

What happens on day 91 is a tax characterization change, not a contractual change. If the equity plan’s post-termination exercise window is 90 days (the historical standard), the option also lapses contractually on day 90 and there is no later exercise to characterize. If the equity plan extends the post-termination window beyond 90 days — the modern practice at several companies discussed below — the option remains exercisable, but on exercise after day 90 the statutory ISO characterization is lost. The option is treated as a nonqualified stock option for federal tax purposes. The spread at exercise (fair market value minus strike) is ordinary compensation income in the year of exercise, subject to ordinary rates and (since the holder is no longer an employee) reported on Form 1099-NEC or directly on Form 1040 rather than via Form W-2.

Two exceptions extend the three-month period. Under § 422(c)(6), the three-month window extends to one yearif the cessation of employment is due to permanent and total disability within the meaning of § 22(e)(3). Under common practice and Treas. Reg. § 1.421-1(h), employees on certain types of approved leave (sick leave, military leave) are not treated as having ceased employment for purposes of starting the clock. There is no extension for cessation due to death; the three-month rule does not apply when the option holder dies, and the option may be exercised by the estate or beneficiary without ISO disqualification, subject to the option’s own terms.

The cash requirement: strike price times shares#

To exercise an option is to buy stock at the strike price. The exercise cost is the strike price multiplied by the number of shares being exercised, plus any applicable transaction fees. For a holder with 50,000 vested options at a $5 strike price, the cash required is $250,000. For a holder with 100,000 vested options at a $1 strike, the cash required is $100,000. The cash must come from the holder — the company is not paying for the exercise, and unless the company offers a cashless-exercise mechanism (rare for private-company ISOs), the holder must come up with the full strike-price amount.

For public-company ISOs, cashless exercise mechanisms are common: the holder uses a broker to exercise and simultaneously sell enough shares to cover the strike price and applicable taxes, retaining the net shares. Cashless exercise produces a disqualifying disposition automatically (the sale of shares within one year of exercise fails the holding-period test) but solves the cash problem and locks in the spread as ordinary income at known prices. For private-company ISOs, cashless exercise is generally unavailable because there is no public market to sell into. The holder must finance the exercise from personal liquidity or from third-party sources (specialized lenders offer non-recourse exercise loans against the underlying shares, typically at high interest rates).

For holders whose vested option position is large relative to their available cash, the 90-day window often forces a partial exercise — exercising as much as cash allows and forfeiting the balance. Forfeiture is not a taxable event (the holder has no basis in the unexercised options), but it does walk away from the embedded value. The trade-off between exercising and forfeiting depends on the spread, the holding-period plan, and the AMT exposure discussed below.

The AMT trap: ISO exercise as a preference item#

The Alternative Minimum Tax, codified at IRC §§ 55–59, runs a parallel income-tax calculation that disallows or adds back certain “preference items” that are favorably treated under the regular-tax system. IRC § 56(b)(3) treats the bargain element on an ISO exercise — the spread between the strike price and the fair market value of the stock on the exercise date — as an AMT preference item in the year of exercise, unless the shares are sold in the same calendar year as a disqualifying disposition. The result: a holder who exercises ISOs and holds the shares past December 31 of the exercise year picks up the spread as alternative minimum taxable income, even though no regular-tax income is recognized and no sale has occurred.

The AMT rate is 26% on the first $232,600 of AMTI (2026 thresholds; indexed for inflation) and 28% above. For a holder exercising 50,000 ISOs with a $5 strike and a $25 fair market value, the spread is $20 × 50,000 = $1,000,000. Added to the holder’s other income on Form 6251, that spread can produce an AMT liability in the low six figures, owed in cash with the next federal return. The holder has paid $250,000 to exercise and may now owe an additional $200,000+ in AMT, without having sold a single share. If the underlying stock subsequently declines below the exercise-date fair market value, the holder may end up with a tax bill larger than the value of the shares they bought to generate the bill — the canonical AMT trap.

Two mitigations. First, AMT paid on the ISO preference becomes a minimum tax creditunder IRC § 53 that can offset regular tax in future years to the extent regular tax exceeds AMT. The credit eventually recovers the AMT paid, but the recovery can take many years if the holder’s regular-tax liability remains modest. Second, the holder can do a same-year disqualifying disposition: exercising and selling within the same calendar year converts the transaction to a fully regular-tax event (ordinary income on the spread at exercise plus or minus capital gain or loss on the post-exercise movement), eliminating the AMT preference but forfeiting the long-term capital-gain treatment that the qualifying-disposition path would have produced.

The decision tree at termination#

For a holder with in-the-money ISOs and a 90-day clock running, three paths cover the practical decision space.

Path 1: Exercise and hold for a qualifying disposition.Exercise within 90 days, pay the strike price, hold the shares for at least one year from exercise and at least two years from grant, then sell. The entire gain (sale price minus strike) is taxed as long-term capital gain at federal rates of 0/15/20% (plus 3.8% net investment income tax if applicable). Cost: cash to exercise, plus AMT liability on the bargain element in the year of exercise, plus holding risk on the underlying stock for at least one year. Best when the holder has the cash for both exercise and AMT, has confidence in the stock’s long-term value, and the spread between the strike and the long-term capital-gain math justifies the AMT carry.

Path 2: Exercise and sell same year as a disqualifying disposition. Exercise within 90 days, sell the shares in the same calendar year. The spread at exercise is ordinary income (no AMT preference because the sale is in the same year); any post-exercise gain or loss is short-term capital. Cost: ordinary tax on the spread, no AMT exposure, no holding-period commitment. Best when the holder needs liquidity, is concerned about post-exercise downside on the underlying stock, or the math on Path 1 does not produce a materially better after-tax result.

Path 3: Forfeit. Let the 90-day window expire without exercising. No tax impact, no cash outlay, no further exposure. Best when the options are underwater, the spread is small relative to the cash and AMT exposure, the underlying stock is illiquid private-company stock with high downside risk, or the holder simply lacks the capital to exercise.

A hybrid path is to exercise some and forfeit the rest. Many practitioners model an exercise quantity that produces an AMT bill the holder can pay from existing liquidity, with the balance of vested options forfeited. The hybrid is often the correct answer for holders with significant unrealized spreads but limited liquid capital.

Extended-window company policies#

A handful of late-stage and post-IPO technology companies have amended their equity plans to extend the post-termination exercise window beyond the statutory 90-day ISO disqualification deadline. The contractual extension does not change the § 422(a)(2) rule — exercise after day 90 still loses ISO treatment — but it does give departing employees the option to defer the exercise decision (and the associated cash and AMT outlays) past the statutory cliff, accepting the conversion to NSO treatment as the price of the extended window.

Notable adopters have included Coinbase (announced an extended window in its 2018 equity-plan revision), Square (now Block) under its early equity-plan structure, Pinterest, Asana, and several others. Stripe and Quora have offered extended windows in certain grant agreements. The specific windows have varied: some companies extend to seven years from grant (effectively the option’s remaining contractual term), others extend to two or three years from termination, and others tie the extension to a service-anniversary milestone.

The trade-off is real. Inside the 90-day statutory window, ISO treatment is preserved and the qualifying-disposition path is available; outside, the option is an NSO with ordinary-income treatment on the spread at exercise. For departing employees with limited cash and an extended-window grant, the rational play is often to defer exercise into the NSO period and accept ordinary-income treatment in exchange for additional time to assemble the cash or to gain better information about the underlying stock’s trajectory. For employees at companies without extended windows, the 90-day clock is the binding constraint. See the CEO severance memos annotated guide for how equity-plan terms vary across companies, and the acquisition and change of control guide for how equity acceleration interacts with these decisions.

The narrow § 83(i) deferral election#

IRC § 83(i), added by the 2017 Tax Cuts and Jobs Act, allows qualifying employees of qualifying privately-held corporations to elect to defer recognition of ordinary income on the exercise of NSOs or the settlement of RSUs for up to five years. The election is structured to ease the cash-tax burden on employees of pre-liquidity-event private companies, where the spread on exercise is taxable but the underlying shares are illiquid.

The eligibility conditions are narrow. The corporation must have a written plan under which not less than 80% of all U.S. employees who provide services to the corporation receive grants of stock options or RSUs — the “broad-based grant” condition. The employee making the election must not be a 1% owner of the corporation, the CEO or CFO, a family member of any of those individuals, or one of the four highest-compensated officers in the corporation in any of the past 10 years. The election must be made within 30 days of the first date on which the rights of the employee in the qualified stock are transferable or are not subject to a substantial risk of forfeiture, whichever is earlier.

In practice the election is rarely used. The 80%-broad-grant requirement is operationally difficult for many companies (it requires extending option or RSU grants to substantially the entire U.S. workforce, including roles that historically did not receive equity), and most companies whose employee bases would benefit most from the deferral — later-stage private companies with material spreads but no near-term liquidity event — have grant structures that do not satisfy the 80% rule. For employees who are eligible, the election can be valuable, but it does not solve the AMT exposure on ISO exercises (the AMT preference under § 56(b)(3) is a separate regime) and it does not extend the § 422(a)(2) 90-day window.

For the broader context on severance and equity at separation, see the reading your severance agreement guide’s discussion of equity acceleration carveouts, the how severance taxes actually work guide for the federal-tax framework, the methodology page for the calculator’s equity-treatment logic, the FAQ for common questions, the glossary for term definitions, and the scenarios index for fact patterns that involve ISOs at termination.