The calculator
Score each founder on six factors. The weights favour future work heavily, because that is where almost all of the value comes from. Move the sliders until the output matches something you could both defend out loud — the disagreement it surfaces is the real output.
Suggested split
- Founder A50%
- Founder B50%
That's close to even. Even splits are fine when the commitment really is even — write down what happens if it stops being even.
Weights used: time commitment counts four times the idea, ongoing responsibility three times, and risk, skills and cash twice. Argue with those weights — the argument is the useful part.
Whatever number you land on, vest it. Four years with a one-year cliff is the norm, and it is the only thing that protects the team from a co-founder who leaves in month three holding a third of the company.
This is a thinking tool, not legal or tax advice.
What each factor is actually worth
1. Time commitment — the heaviest factor
Full-time from day one is worth several times evenings-and-weekends. Almost all of a startup's value is created after the split is agreed, so the person doing that work should hold most of the equity.
2. Ongoing responsibility
Who carries the outcome: fundraising, hiring, firing, the difficult customer call. This is distinct from hours worked and is the factor founders most often forget to price.
3. Risk and the pay cut taken
Salary given up, savings burned, a visa or a mortgage put at stake. Real, quantifiable and fair to reward — but it is compensation for the past, so keep it below commitment.
4. Rare, needed skills
Ability the company cannot easily hire right now. Not a generally impressive CV, and not a skill that matters in year four but not this year.
5. Cash actually wired in
Money in the bank account. Often better handled as a convertible loan or a separate share class than as founder equity, so that an early cheque doesn't permanently outvote the work.
6. The idea — worth less than you think
An idea with no execution attached is worth a few points, not a controlling stake. Courts, investors and your own co-founders all converge on this; founders who insist otherwise usually end up renegotiating anyway.
Six mistakes that cause a dispute later
Noam Wasserman's The Founder's Dilemmas found that most founding teams split equity within a month of forming, before they knew what they'd signed up for, and that rushed splits with no vesting were strongly associated with later conflict. These are the recurring ones.
1. Agreeing 50/50 to avoid the conversation
An even split chosen because it felt awkward to discuss is a deferred argument, not a decision. It's the right answer often enough — just make sure you got there by reasoning, not by flinching.
2. No vesting
Without a four-year vest and a one-year cliff, a founder who leaves in month three keeps their whole stake and every future investor inherits the problem. Vesting protects whoever stays, including you.
3. Equal equity mistaken for equal authority
Equity is ownership; decision rights are a separate document. Name who decides on product, hiring, spend and fundraising when you genuinely disagree.
4. Paying for past work with permanent ownership
Six months of prior building is worth a bump, not a majority. Consider cash, a bonus, or a modest founder-shares grant instead of a structurally unequal cap table.
5. Forgetting the option pool and future founders
Splitting 100% between yourselves and then discovering you need a 10–15% employee pool means diluting from a number you already emotionally banked.
6. Nothing in writing
A handshake split reconstructed from memory eighteen months later is where co-founder disputes actually happen. A short signed document beats a long remembered conversation.
Have the conversation before the spreadsheet
The number is the easy part. The hard part is that "fair" means different things to each of you — one person is counting hours, another is counting risk, another is counting the years they spent learning the thing that makes this possible. A calculator can't resolve that, and a friendly chat usually papers over it, because you both perform reasonableness.
What works better is answering the awkward questions separately and comparing afterwards. That's what the two free games here do: ten minutes on separate phones, private answers, everything revealed at once, and a report showing where the gaps are. Play it before you fill in a cap table, not after.
Frequently asked
- Is a 50/50 co-founder split a bad idea?
- No — it's a bad default. It works when both founders are genuinely full-time with comparable risk and responsibility. It fails when one of you is part-time, or when you chose it to avoid a difficult conversation, because the resentment shows up later with no mechanism to fix it.
- How much equity should a part-time co-founder get?
- Far less than a full-time one — often under 10%, and sometimes an advisor grant rather than founder equity. Alternatively, agree a trigger: they move to a founder-level stake when they go full-time by an agreed date.
- How much equity is the idea worth?
- Typically in the range of a few percent up to about 10% as a founder premium. Value in a startup is created by the years of execution that follow, and every subsequent hire and investor prices it that way.
- What is standard co-founder vesting?
- Four years with a one-year cliff, sometimes with acceleration on an acquisition. Apply it to all founders, including yourself, and start the clock on a stated date.
- Should a co-founder who invests cash get more equity?
- Usually the cleaner route is a convertible loan or preferred shares for the money, and founder equity for the work. That keeps one early cheque from permanently outweighing four years of building.
- Can we change the split later?
- Legally yes, practically it is painful — whoever is asked to give up points feels accused. Better to agree review triggers in advance: a named date, a fundraise, or a change in someone's commitment.