The ISO Three-Month Rule and Your Exercise Deadline

Two Different Deadlines Often Get Confused

If you are leaving a startup, check your equity plan, grant agreement, and separation documents immediately. Those documents determine how long your vested options remain exercisable. The deadline may be shorter or longer than three months, and different grants can have different terms.

Separately, federal tax law generally requires continuous employee status through the day three months before an ISO exercise for the favorable statutory-option rules to apply. That tax condition does not itself set the contractual expiration date, and “three months” should not be assumed to mean exactly 90 calendar days. Disability and death can also change the tax treatment. Confirm the actual dates and terms with the company and a qualified adviser.

Why Three Months Matters for ISOs

Section 422 generally requires the option holder to have remained an employee from grant through the day three months before exercise. If that condition is not met, the exercise does not receive statutory ISO treatment. Whether the option can still be exercised after that point—and on what terms—depends on the plan and grant documents.

NSOs do not use this ISO employment condition, but their contractual exercise deadline still matters.

What the Clock Looks Like

If your documents actually give you a 90-day contractual window, an illustrative planning timeline might look like this:

Day 0 — Last day of employment The clock starts. Your vested options are intact, but the countdown begins.

Days 1-30 — Assessment phase Gather your option agreement, check vested shares, calculate exercise cost and tax burden. This is when you should run our calculator.

Days 31-60 — Decision and funding phase Decide how many shares to exercise. Arrange funding — personal savings, financing, or a combination. Start the exercise paperwork with your company.

Days 61-80 — Execution phase Confirm the company's required exercise notice, payment method, receipt rules, and processing time. Do not assume submitting a form alone completes the exercise.

Days 81-90 — Emergency zone If you haven't started the exercise process by now, you're in dangerous territory. Company processes, bank transfers, and administrative delays can eat up these final days.

Contractual deadline — Verify completion Confirm in writing that the company received and accepted every required item before the deadline stated in your documents.

What You Need to Exercise

To exercise your stock options, you typically need:

1. Your stock option agreement — this documents your strike price, number of shares, vesting schedule, and PTEP
2. Cash for the exercise cost — number of shares x strike price per share
3. Cash for taxes — AMT (ISOs) or ordinary income tax + FICA (NSOs)
4. A completed exercise notice — your company's stock administrator will provide this form
5. Payment — wire transfer, ACH, or check depending on the company

The Hidden Cost: Taxes

The exercise cost is just the tip of the iceberg. Here's what catches many employees off guard:

For ISOs: The spread (FMV minus strike price) is an AMT adjustment. If you're exercising a significant number of options, the AMT liability can be larger than the exercise cost itself. You won't owe this until you file your tax return, but you need to plan for it.

For NSOs: The spread is immediately taxable as ordinary income. Your company may withhold taxes at exercise, or you may need to make estimated tax payments. Either way, you need the cash.

Use the calculator for an educational estimate based on your inputs, then have a qualified professional review the result. Actual withholding, AMT, state sourcing, and filing consequences depend on facts the calculator does not collect.

Companies With Extended Exercise Windows

Not all companies use the 90-day standard. Some companies have adopted extended post-termination exercise periods:

• 10 years: Some companies allow ex-employees to exercise for up to 10 years, giving much more breathing room
• 7 years: A middle ground that some companies have adopted
• 1 year: More generous than 90 days but still creates urgency

If your company has an extended PTEP, you have more time to plan — but don't procrastinate. Tax laws change, company valuations change, and your financial situation changes.

Important note about ISOs: An extended contractual PTEP does not extend the federal ISO employment requirement. An exercise that does not meet Section 422's employment condition is generally taxed under the nonstatutory-option rules. Ask the company how it records the option and obtain tax advice for your dates and circumstances.

Strategies for the 90-Day Window

Start Planning Before You Leave

If you know you're leaving (by choice), start planning your exercise strategy while you're still employed. Calculate your costs, explore financing, and save up if possible.

Exercise Partially

You don't have to exercise all or nothing. If you can only afford to exercise 2,000 of your 10,000 vested options, that's better than exercising zero.

Consider Early Exercise

Some companies allow you to exercise options before they vest (early exercise). Combined with an 83(b) election, this can significantly reduce your tax burden. But you need to do this while you're still employed.

Explore Financing

Some third parties offer stock-option financing. Availability, recourse, fees, collateral, and repayment terms vary; review definitive documents and do not assume a structure is non-recourse.

Talk to a Tax Professional

The tax implications of exercising are complex, especially with ISOs and AMT. A tax professional who specializes in stock compensation can help you optimize your strategy.

Verify the Date, Then Plan Backward

Do not rely on a generic “90-day rule.” Obtain the governing documents, ask the company to confirm the last date and completion requirements in writing, and leave time for payment and administration.

Our calculator provides an educational estimate of exercise price and selected federal and state tax components. It is not a filing calculation, legal advice, or a guarantee of the cash you will need.