Incentive Stock Option

An Incentive Stock Option (ISO) is the tax-advantaged stock option granted only to employees under IRC Section 422. It produces no ordinary-income tax at exercise (subject to AMT) and qualifies for long-term capital-gains treatment on the entire spread if held two years from grant and one year from exercise, capped at $100,000 of FMV first-exercisable per employee per year. It is the more tax-favored of the two main option types and the default for employee grants at most C-corp startups.

The ISO mechanic and the tax rules:

• Grant: company grants ISO with strike price equal to fair market value at grant (per 409A valuation), to an employee (W-2, not 1099 contractor or board member), with vesting and expiration terms.
• Exercise: employee pays strike price to convert vested options to common stock. No regular income tax at exercise. The bargain element (FMV at exercise minus strike price) is an "AMT adjustment item" that may trigger Alternative Minimum Tax.
• Qualifying disposition: if the employee holds the shares for both (a) two years from grant date AND (b) one year from exercise date before selling, the entire gain (sale price minus strike price) is treated as long-term capital gain. No ordinary income.
• Disqualifying disposition: if either holding period is missed, the ISO is "disqualified" and treated similarly to an NSO: the bargain element at exercise becomes ordinary income in the year of sale; subsequent appreciation is capital gain.

The $100,000 limit: only $100,000 of ISO FMV (measured at grant) can first become exercisable in any one calendar year per employee. ISOs in excess of this limit are automatically reclassified as NSOs. Example: an employee with a $300,000 ISO grant vesting over 4 years (1-year cliff, monthly thereafter) has roughly $75,000 first-exercisable in the cliff year and ongoing monthly amounts, fitting within the $100k limit. An employee with a $500,000 ISO grant would see some shares automatically convert to NSOs. The AMT trap: at exercise, the bargain element is an AMT adjustment item. If the bargain element is large, the employee can owe significant AMT in the year of exercise, even though regular tax is zero. The AMT is recoverable as a credit in future years against regular tax, but the cash outlay at exercise can be substantial. Founders early-exercising large ISO grants need to model the AMT impact before pulling the trigger.

What founders get wrong: Granting ISOs without explaining the holding-period mechanics or AMT considerations. Employees who exercise at a strong year (high 409A) and then sell in a liquidity event within a year of exercise unknowingly disqualify the ISO and lose the tax-favored treatment. Employees who exercise large amounts without AMT modeling get surprised by tax bills. The right discipline: offer education at grant and at exercise, partner with tax professionals who can help employees model their specific situations, and consider extended PTEW (post-termination exercise window) to give employees time to plan exercises around their personal tax situations.

Related: Stock Option · Non-Qualified Stock Option · ISO AMT Implications · Early Exercise · Option Strike Price

FAQ

What is an incentive stock option (ISO)?
A tax-advantaged stock option granted only to employees (not contractors or board members) under IRC Section 422, characterized by no ordinary-income tax recognition at exercise (subject to AMT) and potential long-term capital-gains treatment on the entire spread if specific holding periods are met (two years from grant AND one year from exercise).

What is the $100,000 ISO limit?
Only $100,000 of ISO FMV (measured at grant) can first become exercisable in any one calendar year per employee. ISOs in excess of this limit are automatically reclassified as NSOs. The limit constrains how much grant value can flow through as ISOs to any single employee in a year.

What is the ISO AMT trap?
At exercise, the bargain element (FMV minus strike) is an AMT adjustment item, potentially triggering Alternative Minimum Tax even though regular income tax at exercise is zero. Employees exercising large ISO grants without AMT modeling can owe significant tax in the year of exercise. The AMT is recoverable as a credit but the cash outlay is real.