Founder Vesting Calculator

Background.

Founder vesting is the schedule that decides how much of a founder's stock they actually keep if they leave. The stock is issued up front, but the company holds a right to buy back the unvested portion, and that right lapses month by month across the vesting term. This calculator answers the question that surfaces in a departure conversation, a co-founder dispute or a due-diligence review: as of today, how many shares has this person earned?

The near-universal arrangement is four years with a one-year cliff. Nothing vests during the first twelve months. On the first anniversary the founder vests that entire first year at once — 25% of the grant — and the rest vests monthly until the term ends. The cliff is a probation period, not a forfeiture: it delays the first release, but once reached, the time already served counts in full. That distinction is what this page is built around, because getting it wrong changes the answer materially. At month 18 of a four-year schedule the correct figure is 37.5% of the grant, not the 16.7% you get by spreading the whole grant across only the months after the cliff.

Enter the total shares subject to vesting, months completed since the vesting start, the full term and the cliff length. The vesting start date is often earlier than the date the stock was issued, because founders commonly negotiate credit for time worked before incorporation. Use the date written in the stock purchase agreement rather than the incorporation date, or the result will understate what has vested — sometimes by a full year.

Two things this page deliberately does not model. It ignores acceleration — the single-trigger and double-trigger clauses that vest part or all of the remainder on an acquisition or a termination without cause — because those terms are negotiated per agreement and have to be read from the document rather than derived. And it takes no view on tax: whether an 83(b) election was filed within 30 days of the grant changes the tax treatment completely, but it does not change how many shares have vested.

What is founder vesting calculator?

Vesting is the process by which a founder's stock stops being repurchasable by the company. Under a standard restricted stock purchase agreement the founder holds every share from day one — they can vote them, and the capital-gains holding period starts — but the company may buy back any unvested shares at the original purchase price if the founder departs. Vested shares are the portion that repurchase right no longer reaches.

A cliff is a minimum service period before any shares vest at all. It protects the company and the remaining founders from someone leaving after a few months with a meaningful stake. On reaching the cliff the founder vests everything accrued to that point in one step, and vesting continues on the ordinary monthly schedule from then on.

How to use this calculator.

1. 01Take the vesting start date from the stock purchase agreement, not the incorporation date, and enter whole months completed since then.
2. 02Enter the total shares subject to vesting. If only part of a founder's holding carries a repurchase right, enter that part only.
3. 03Enter the full vesting term and the cliff in months — 48 and 12 for the standard arrangement.
4. 04Read vested shares as of today: that is what the company can no longer repurchase.
5. 05Apply any acceleration clause separately. It is read from the agreement and is not modelled here.

The formula.

Before the cliff, vested shares are zero. On or after it, vesting is the elapsed fraction of the full term:

vested = 0 if months < cliff vested = total x min(months / totalVestingMonths, 1) otherwise

The denominator is the FULL vesting term, not the term remaining after the cliff. That is what makes a cliff a delay rather than a forfeiture, and it is why the schedule steps from 0% to cliff/term — 25% on a four-year/one-year schedule — in a single month instead of restarting from zero. The min() caps vesting at 100% once the term completes, so months beyond the term add nothing. A cliff longer than the total term is rejected as an input error rather than silently clamped, because in practice it means the two fields were transposed.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Securities and Exchange Commission. Investor.gov glossary: Vesting. Retrieved 2026-08-07. independence: primary; access: open.
2. [2]U.S. Internal Revenue Service. Topic no. 427, Stock options and restricted stock. Retrieved 2026-08-07. independence: primary; access: open.
3. [3]U.S. Internal Revenue Service. About Form 15620, Section 83(b) election. Retrieved 2026-08-07. independence: primary; access: open.