Equity & Compensation

How ESOP Vesting Works, and What Happens if You Leave Early

Silicon Fund Editorial Updated August 23, 2026 8 min read

An employee stock option pool (ESOP) is how startups share ownership with the people who build them. But a granted option is not the same as an owned share, and the gap between the two is governed entirely by vesting. Understanding vesting mechanics matters just as much for the person joining a startup as it does for the founder designing the pool in the first place.

Grant versus vesting: two different moments

When a company gives someone stock options, that is the grant: a fixed number of options at a fixed strike price, documented in an offer letter or grant agreement. Vesting is separate. It is the schedule by which that grant is earned over time. Until an option vests, the employee has no right to exercise it or claim it, regardless of how long they have held the grant paperwork.

The standard four-year, one-year cliff structure

The most common default in startups is a four-year vesting schedule with a one-year cliff. Under this structure, no options vest at all until the employee has completed twelve months of service. At that point, roughly 25 percent of the total grant vests all at once. The remaining 75 percent then vests in smaller increments, commonly monthly, over the following three years.

The cliff exists to protect the company (and, by extension, existing shareholders) from someone leaving after a few months while still walking away with meaningful equity. It also means the first year at a new company carries real risk from an equity perspective: leave in month eleven, and the standard structure means none of the grant has vested yet.

What happens to unvested options when someone leaves

This is the part most people get wrong. Leaving a company, voluntarily or not, does not typically forfeit options that have already vested. What is forfeited is only the unvested portion, the part of the grant the person had not yet earned as of their last day. A person who leaves after two years of a standard four-year schedule generally keeps the roughly 50 percent that vested, and loses the other 50 percent that had not.

Some companies negotiate acceleration clauses that vest some or all of the remaining unvested options automatically under specific conditions, most commonly an acquisition of the company (sometimes called single-trigger or double-trigger acceleration, depending on whether it requires just the sale or also a termination event). These are negotiated terms, not a universal default, and the details are worth understanding at the same level of care founders apply when reading a term sheet.

Vesting doesn't ask "how long have you held this grant." It asks "how much of it have you actually earned so far."

Exercising: turning vested options into actual shares

Vested options are not automatically shares. To convert them, the holder has to exercise: pay the strike price set at grant time for each option, which actually purchases the underlying shares. Exercising can trigger a tax event depending on jurisdiction and option type, even if the shares themselves cannot yet be sold, which is a detail that catches people off guard when a company's valuation has risen substantially since grant.

After leaving a company, vested-but-unexercised options usually come with a post-termination exercise window, often a standard 90 days, though some companies have moved to offer longer windows. If a former employee does not exercise within that window, the vested options typically expire and are forfeited entirely. This window is one of the most consequential and least understood parts of an option grant, since it can force a decision to pay real money to exercise options in a private company with no guaranteed path to liquidity.

Why the size and structure of the pool matters beyond any one grant

The option pool itself is a shared resource that dilutes all existing shareholders, including founders. Investors negotiating a new round frequently require the pool to be topped up before the round closes, which affects how dilution compounds across the company's history. And if the company later raises money at a lower valuation than before, a down round, existing option strike prices and the pool's economics can shift in ways that affect everyone holding options, not just new hires.

What to actually check before you accept, or exercise, options

• The exact vesting schedule and whether there's a cliff, and how much time you have already completed toward it.
• Whether there is any acceleration in the event of an acquisition, and whether it's single-trigger or double-trigger.
• The post-termination exercise window, since a short window can force a costly decision quickly after leaving.
• The current strike price relative to the company's most recent valuation, since that spread is what determines your actual cost and tax exposure to exercise.

None of this is exotic once it's laid out, but it is rarely explained clearly at the point someone is handed an offer letter. Understanding the mechanics up front avoids the two most common surprises: losing unvested options on an unlucky exit timing, and missing a short exercise window on options that were actually earned.