Equity & legal
Cofounder Equity Split: How to Divide Equity Fairly
How to split equity between cofounders: when an equal split works, when to weight it, which factors matter, and why vesting matters more than the number.
Key takeaways
- Y Combinator's standard advice is "equal (or close to equal) equity splits among co-founders", because the work that creates value is still ahead of you.
- Weight the split only for differences that will last: part-time versus full-time, significant cash invested, or a founder joining much later.
- Vesting matters more than the exact number. With four-year vesting and a one-year cliff, a founder who leaves early leaves most of their equity behind.
- Agree the split in writing before you incorporate, and use the equity split calculator to structure the conversation.
How should cofounders split equity?
Most early founding teams should split equity equally or close to it, then protect the split with vesting. Move away from equal only for differences that will last, such as one founder staying part-time, putting in significant cash, or joining much later. Four-year vesting with a one-year cliff matters more than the exact percentage.
That is also Y Combinator's standard advice. In How to split equity among co-founders, YC's Michael Seibel describes equal or near-equal splits as "what we almost always recommend at YC", and adds: "If you aren't willing to give your partner an equal share, then perhaps you are choosing the wrong partner."
Why equal is the default
Seibel lists the reasons founders give for unequal splits, including "I came up with the idea for the company" and "I am older/more experienced than my co-founder", and argues they fail for four reasons:
- Time. "It takes 7 to 10 years to build a company of great value." A few months' head start is small against that.
- Motivation. "More equity = more motivation." A cofounder who feels short-changed on equity is more likely to drift or leave.
- Signalling. Investors read the split as a sign of how the CEO values the team: "If you only give a co-founder 10% or 1%, others will either think they aren't very good or aren't going to be very impactful."
- Execution. "Startups are about execution, not about ideas." Rewarding whoever had the idea over the people who ship the product gets the incentives backwards.
When a weighted split makes sense
Equal is the starting point. A weighted split is reasonable when a difference is large and will persist:
- Commitment. One founder is full-time and the other is part-time with no date to change that. Ask whether the part-timer is really a cofounder or an adviser.
- Cash. One founder funds the company. It is often cleaner to treat that money as an investment, on the terms an outside investor would get, than to fold it into founder shares.
- Timing. One founder joins after a funding round, a year of full-time work, or real traction. They are taking less risk than the people who started with nothing.
- Existing assets. One founder contributes working code, IP or customers that already exist. Put an explicit value on them.
Things that should move the split very little: the idea itself, age, and a weekend prototype.
Factors to weigh
| Factor | Question to ask | How much it should matter (our view) |
|---|---|---|
| Time commitment | Will each founder be full-time, and from when? | A lot, if the gap persists |
| Cash invested | Is anyone funding the company? | Handle it as an investment where possible |
| Role | How hard would this role be to hire for? | Some |
| Existing work or IP | Is anyone contributing code, IP or customers that already exist? | Some; value it explicitly |
| The idea | Who came up with it? | Little; ideas change |
| Salary forgone | Is anyone taking a bigger pay cut than the others? | Little; address it through salary later |
A worked example
Say three founders, A, B and C, are starting a B2B software company. A and B go full-time on day one. C keeps her job for six months, then joins full-time. A also puts in money to cover the first year of tools and legal fees. This is a hypothetical, and the numbers are not a recommendation.
- Equal option: 34% / 33% / 33%, all on four-year vesting with a one-year cliff. C's vesting starts on her first full-time day, which reflects her later start without changing her percentage. A's cash is recorded as a SAFE or convertible note on the same terms as outside money.
- Weighted option: 36% / 34% / 30%, with the same vesting, if the team feels C's six part-time months should show in the stake as well as the start date.
Either option is defensible. The two choices that cause trouble later are leaving the split undecided and agreeing percentages with no vesting. The equity split calculator lets each founder enter these factors and compare the results side by side.
Why vesting matters more than the exact number
The split assumes everyone stays; vesting handles the case where someone leaves. YC describes the typical setup as "four years of vesting with a one year 'cliff'", and puts the stakes plainly: "while you might own 50% of the company on paper, if you leave or get fired within a year you walk away with nothing."
Say a founder holding 50% leaves after 18 months on that schedule. She has vested 18/48 of her shares, or 37.5%, which is 18.75% of the company. The remaining 31.25% can be bought back and used to recruit a replacement. Without vesting she would keep the full 50%, and the founders who stay would be building for someone who left.
Our founder vesting schedule guide covers cliffs and acceleration. US founders who receive shares subject to vesting should also read about the 83(b) election, which the IRS says "must be filed no later than 30 days after the date the property was transferred."
How to agree the split
- Each founder writes down privately what they think is fair, with reasons.
- Compare, and discuss only the differences.
- Run the numbers through the equity split calculator so the discussion centres on the inputs (hours, cash, timing) that produce the percentages.
- Agree vesting, cliff, acceleration and leaver terms in the same conversation.
- Write it into a founders agreement and have a lawyer draft the share documents.
- Reopen the split only if roles change fundamentally.
If you are still looking for someone to split equity with, BiggMate is a cofounder-matching platform in early access, with free sign-up.
Find a cofounder who fills your gap
BiggMate sends curated, mutually opted-in matches instead of an open directory.
Frequently asked questions
Should cofounders split equity 50/50?
Often, yes. YC's Michael Seibel recommends "equal (or close to equal)" splits because the work that creates value is still ahead. The practical risk of an exact 50/50 split between two founders is deadlock, and a decision-making clause in your founders agreement handles that without changing the split.
How much equity should a technical cofounder get?
In most cases, the same as any other full-time cofounder. Being technical does not by itself justify a bigger or smaller share; commitment, timing and cash matter more. If a developer will only build the first version part-time, they may be a contractor or adviser rather than a cofounder.
How much equity should a cofounder who joins later get?
It depends on how much risk is left. Someone who joins in the first months, before much is built or funded, usually gets close to an equal share. Someone who joins after a funding round or clear traction is taking less risk, so a smaller stake is common. Either way, apply the same vesting.
Does an equity split calculator give the right answer?
No calculator gives a right answer. Its use is in making each founder's assumptions explicit, so the discussion covers hours, cash and timing before it gets to percentages.
Can you change an equity split later?
Yes, but it means issuing, transferring or buying back shares, which has legal and tax consequences. Getting vesting right at the start is much easier than reallocating equity later.
Sources
- Y Combinator (Michael Seibel): How to split equity among co-founders
- IRS: Form 15620, Section 83(b) Election, with instructions (Rev. April 2025)
This guide is general information, not legal, tax or financial advice.