Roughly a quarter of two-founder VC-backed teams lose a cofounder by the four-year mark (Carta, on teams incorporated 2015–2024), and the rate keeps climbing after that — a cofounder leaves, a fight over control turns ugly, someone stops showing up. What determines whether that event kills the company or is just a bad quarter is almost always decided months earlier, in a five-minute conversation nobody wanted to have: how the equity was split, and whether it vests.

Get this wrong and you can end up with a co-founder who owns 50% of your company and hasn’t worked on it in three years — a “dead equity” problem serious enough that it kills financing rounds. Get it right and a cofounder’s exit is a contract event, not a company-ending crisis.

This guide covers how to split equity fairly, why vesting schedules exist and how they actually work, what a cap table is and how to read your own, and the mistakes that show up over and over in these conversations.

Equal Isn’t Always Fair, But It’s Almost Always the Right Starting Point

There is no formula that objectively prices one founder’s idea against another’s ten years of engineering experience. Every framework — including the popular ones that assign points for the original idea, domain expertise, full-time commitment, and capital contributed — is a structured way to have the conversation, not a calculator that spits out the truth.

What the data on cofounder splits consistently shows: teams that split equal from the start, when everyone is committing full-time from the same starting line, have fewer blowups later than teams that engineer an unequal split based on who talked the loudest in week one. Unequal splits are sometimes correct — they just need a real reason, not a feeling.

Reasons that justify moving off equal:

What doesn’t justify an unequal split: whose idea it was. Ideas are worth close to nothing until execution starts — a startup is what happens after the idea, and the equity should mostly reflect the after.

Why Every Founder’s Equity Should Vest — Including Yours

Vesting is the single most important founder-protection mechanism in a startup’s early paperwork, and it’s the one founders are most tempted to skip because “we’re friends, we don’t need it.”

Here’s the failure mode vesting prevents: two cofounders split equity 50/50 on day one with no vesting. Eight months in, one of them decides the startup isn’t for them and leaves for a job at a big tech company. They keep their full 25–50% stake, forever, having contributed eight months of work to a company that might run for another decade. The founder who stayed is now working for years to build value that a departed cofounder passively owns just as much of. Investors who see this on a cap table during diligence will ask you to fix it before they’ll write a check — an unvested founder stake is a red flag serious enough to stall a raise on its own.

The market-standard schedule: four years, with a one-year cliff.

This applies to every founder, including the CEO and the person with the original idea. It is not a punishment — it’s the mechanism that makes the equity actually mean “reward for building this,” rather than “reward for showing up to the incorporation meeting.”

A few details worth getting right:

Reading a Cap Table

A capitalization table (cap table) is a ledger: who owns what percentage of the company, in what form (common stock, preferred stock, options, SAFEs or convertible notes), and how that ownership changes as new shares are issued.

The concept every founder needs before raising money is dilution: when a company issues new shares — to an investor, to an option pool, to a new cofounder — existing percentages shrink because the total number of shares outstanding grows. Dilution doesn’t reduce the value of your stake if the round grows the pie enough, but it does reduce your percentage, every time.

A simplified example: two founders hold 50% each (1,000,000 shares split evenly, 2,000,000 total). They raise a seed round that issues 500,000 new shares to investors. Total shares outstanding become 2,500,000. Each founder now holds 1,000,000 / 2,500,000 = 40%, even though neither sold a single share.

What to actually check on your own cap table before signing anything:

Cap table software (Carta and Pulley are the two most common at the seed stage) keeps this current automatically and is worth setting up before your first round, not after — reconstructing a clean cap table from a folder of PDFs during diligence is a bad way to spend the two weeks before a term sheet.

The Conversation Framework

For a cofounder team splitting equity for the first time, a workable order of operations:

  1. Default to equal among founders committing full-time from the same starting point.
  2. Name the specific reasons for any deviation — capital, IP, time, risk — and put a number on each one rather than negotiating a gut feeling.
  3. Vest everyone, four years, one-year cliff, no exceptions for the CEO or the “idea person.”
  4. File the 83(b) election within 30 days of the grant.
  5. Put it in writing — a stock purchase agreement and vesting schedule, not a verbal handshake. Verbal equity agreements are close to worthless the moment a relationship turns adversarial, which is exactly when you need the paperwork to hold.
  6. Revisit only at real trigger events — a new round, a cofounder’s role changing materially — not every time someone feels underappreciated.

None of this requires a lawyer for the conversation itself, but the paperwork that codifies it should go through one. A few hundred dollars in legal fees at formation is cheap insurance against a fight that can otherwise cost a company its next round — or its existence.

For founders who want the mechanics of fair worked out in more granular detail — especially in situations where contribution genuinely is uneven and ongoing, like an agency or services business with irregular founder involvement — Mike Moyer’s Slicing Pie lays out a dynamic-equity model built specifically for that case, as an alternative to picking a fixed split upfront and hoping it stays fair.

FAQ

What’s a fair equity split for two cofounders?

Equal (50/50) is the right default when both cofounders commit full-time from day one with comparable risk. Deviate only for specific, named reasons — unequal capital, unequal time commitment, or pre-existing IP — not for who had the idea first.

Should a solo founder’s equity vest too?

Yes. Vesting isn’t about trust between cofounders — it’s about aligning equity with ongoing contribution. A solo founder who brings on a cofounder or key early employee later will want their own stake vested for the same reason: it protects the company (and any future investor) if that person leaves early.

What happens to unvested shares when a cofounder leaves?

Standard vesting agreements let the company repurchase unvested shares at cost (often near zero) when someone departs before their shares fully vest. Vested shares stay theirs. This is exactly why vesting matters — without it, there’s no mechanism to claw back equity from someone who leaves early.

How much should go into the employee option pool?

10–20% of fully diluted shares is typical at the seed stage, sized to cover roughly the next 12–18 months of hiring. Investors usually ask for this to be created before their investment, which dilutes existing founders rather than the incoming round.

Do I need a lawyer to split equity between cofounders?

You don’t need one for the conversation, but you should for the paperwork. A stock purchase agreement, vesting schedule, and 83(b) election filed correctly are inexpensive relative to the cost of an ambiguous verbal agreement unraveling later.

Getting the cap table right early is part of the same discipline as knowing your unit economics or preparing properly to raise a seed round — the founders who treat the boring paperwork seriously are usually the same ones whose companies are still standing when it matters. For more on how today’s builders are structuring their companies, explore more founder stories on FutureSharks.