Founder Equity, Vesting, and What Happens When a Co-Founder Leaves

By Hank FasthoffJuly 5, 2026

Most co-founder relationships end before the company does. Research across venture-backed startups shows that roughly 65 percent of companies experience a co-founder departure before Series B. When that departure happens and there's no vesting schedule, no buyback provision, and no written agreement addressing what the departing founder keeps, the remaining founders discover that the equity structure they skipped at formation is now the most expensive problem in the company.

Vesting is the structural protection that prevents a co-founder who leaves after eight months from walking away with a permanent ownership stake that dilutes everyone who stayed and built the business. Setting up vesting at formation costs a few thousand dollars in legal fees. Cleaning up a cap table after a co-founder departure without vesting can cost months of fundraising delay, tens of thousands in legal fees, and equity concessions that dwarf what proper documentation would have cost.

How Founder Vesting Works

Founders typically receive their shares at formation for a nominal purchase price (often $0.0001 per share). Without vesting, those shares are fully owned from day one. With vesting, the company retains the right to repurchase unvested shares at cost if the founder leaves before the vesting schedule is complete.

This structure is called reverse vesting, because the founder already owns the shares but the company's repurchase right "unvests" over time. It's different from stock options, where an employee earns the right to purchase shares as they vest. Founders get the shares upfront; the question is whether they get to keep them.

The Four-Year Schedule with a One-Year Cliff

Industry standard since the late 1990s, and the structure venture investors expect to see, is a four-year vesting schedule with a one-year cliff.

Nothing vests during the first 12 months. If a founder leaves before the one-year anniversary, the company can repurchase 100 percent of the founder's shares at cost. At month 12, 25 percent of the shares vest at once (the cliff). From month 13 through month 48, the remaining 75 percent vests in equal monthly increments (1/48th of the total grant per month).

A founder who receives 2,400,000 shares under this schedule vests nothing for 12 months. At the cliff, 600,000 shares vest. After that, approximately 50,000 shares vest each month. At month 24, the founder has vested 1,200,000 shares (50 percent). At month 48, all 2,400,000 are fully vested.

A cliff solves the early-departure problem. A co-founder who leaves at month eight walks away with zero equity rather than eight months' worth of ownership in a company they're no longer building. Monthly vesting after the cliff ensures that founders who stay earn ownership proportionally over time.

The Section 83(b) Election

When a founder receives restricted stock subject to vesting, IRC § 83 ordinarily treats each vesting event as a taxable event. Without an 83(b) election, the founder recognizes ordinary income equal to the fair market value of each tranche of shares at the time those shares vest, minus the price originally paid. If the company's value has increased since formation (which it usually has, especially after a priced round), the tax bill on each vesting event can be substantial.

Filing a Section 83(b) election within 30 days of receiving the restricted stock solves this problem. The election tells the IRS that the founder wants to recognize all taxable income at the time of the stock transfer rather than at each vesting event. Because founder stock is typically purchased at nominal value ($0.0001 per share), and because the company's fair market value at formation is minimal, the taxable income recognized under the election is effectively zero.

That 30-day deadline is statutory and absolute. Missing it eliminates the option entirely, and there's no extension, no late filing, and no workaround. In late 2024, the IRS released Form 15620, the first standardized form for making the election. In July 2025, the IRS opened an electronic filing portal for Form 15620, allowing founders to submit the election online for the first time rather than relying on certified mail. Both methods remain valid.

Every founder who receives restricted stock subject to vesting should file the 83(b) election. Failing to file is one of the most expensive and most preventable mistakes in startup formation.

Acceleration Provisions

Vesting schedules can include acceleration clauses that speed up vesting when specific events occur. Two types are common.

Single-trigger acceleration vests all or a portion of unvested shares upon a single event, typically a change of control (an acquisition of the company). If a founder has single-trigger acceleration and the company is acquired at month 24, all remaining unvested shares vest immediately, and the founder participates fully in the acquisition proceeds.

Double-trigger acceleration requires two events before acceleration occurs, typically a change of control followed by involuntary termination of the founder within a defined window (usually 12 months after the acquisition closes). Double-trigger acceleration protects the founder from being acquired and then fired, but it doesn't vest shares just because the company is sold.

Investors and acquirers generally prefer double-trigger acceleration because single-trigger removes the founder's incentive to stay through the post-acquisition integration period. Most term sheets either require double-trigger or prohibit single-trigger entirely. Founders negotiating acceleration provisions should understand that single-trigger may make the company harder to sell, because acquirers don't want to close a deal and immediately vest millions of dollars in founder equity without any retention mechanism.

What Happens When a Co-Founder Leaves

When a founder departs, the vesting schedule and the stock restriction agreement determine what happens to their equity.

Unvested shares are subject to the company's repurchase right. The company can buy back unvested shares at the original purchase price (typically nominal), effectively removing the departing founder's unvested equity from the cap table. Most stock restriction agreements give the company 90 days after departure to exercise this repurchase right.

Vested shares belong to the departing founder. Without a separate buyback provision for vested shares, the departing co-founder retains permanent ownership of every share that vested before departure. A co-founder who leaves at month 30 of a four-year schedule keeps approximately 62 percent of their original grant as vested shares and forfeits the remaining 38 percent as unvested shares.

A departed co-founder who holds 15 or 20 percent of the company's equity, contributes nothing to the business, and has no obligation to support future fundraising or operations creates dead equity on the cap table. Investors discount the company's valuation because a significant ownership stake belongs to someone who isn't working.

Some stock restriction agreements include a buyback provision for vested shares at fair market value, giving the company the right (but not the obligation) to repurchase vested shares when a founder leaves. This provision is negotiated at formation and requires careful drafting, because the valuation method for the buyback (formula, independent appraisal, board-determined FMV) can produce disputes if it isn't specified in advance.

Equity Splits Between Co-Founders

Equal splits between two-person founding teams have grown over the past decade, rising from 31.5 percent in 2015 to 45.9 percent in 2024 according to CRV's data across venture-backed startups. That still means more than half of two-founder teams choose unequal arrangements, and three-founder teams almost always differentiate.

An equal split works when co-founders contribute comparable effort, take comparable risk, and bring comparable value. It doesn't work when one founder has been developing the product for two years, another joins three months before incorporation, and a third contributes a small amount of capital but no operating involvement. Splitting equity equally in that scenario grants the late-arriving founders ownership they haven't earned, and vesting alone doesn't fully correct the problem because vesting applies prospectively (going forward) rather than retroactively accounting for prior contributions.

When contributions are unequal, the equity split should reflect the difference. Some founders negotiate pre-incorporation vesting credit, giving founders who worked on the concept before formation a head start on their vesting schedule. Others use a dynamic equity framework that adjusts ownership based on tracked contributions. Whatever method you use, document the rationale and get the split agreed in writing before filing the certificate of formation, not after.

The Consequences of Skipping Vesting

Without a vesting schedule, every founder owns their shares outright from day one. If a co-founder leaves at month three, they keep 100 percent of their equity. The remaining founders do 100 percent of the work for a fraction of the ownership.

Investors won't fund a company in this position. At Series A, investors routinely require vesting for all founders, and if vesting wasn't in place at formation, the founders have to negotiate retroactive vesting (reverse vesting applied after the fact). Retroactive vesting requires the departing founder's consent if they've already left, and a founder who holds a large equity stake with no obligation to cooperate has maximum bargaining power to extract a buyout payment. The cleanup costs more than proper documentation at formation would have cost, sometimes by orders of magnitude.

Practical Recommendations

Put a vesting schedule in place at formation, before the certificate of formation is filed with the secretary of state. Use the industry-standard four-year vest with a one-year cliff unless specific circumstances justify a different structure. Include reverse vesting provisions in the founder's stock restriction agreement, giving the company the right to repurchase unvested shares at cost upon departure.

File the Section 83(b) election within 30 days of receiving restricted stock. Use IRS Form 15620 and file electronically through the IRS portal or by certified mail. Don't miss this deadline.

Include acceleration provisions. Double-trigger acceleration (change of control plus involuntary termination) protects founders without creating obstacles to an acquisition.

Address buyback rights for vested shares. Decide at formation whether the company will have the right to repurchase vested shares at fair market value upon a founder's departure, and specify the valuation method.

Document the equity split and the rationale behind it. If the split is unequal, record why. If the split is equal, confirm that each founder's expected contribution justifies equal ownership. Put the agreement in the company's operating agreement or shareholders' agreement, signed by all founders before the company begins operating.

Equity disputes between co-founders are among the most destructive problems a startup can face, and they're among the most preventable. The cost of getting the structure right at formation is a fraction of the cost of fixing it after a relationship breaks down.