What is share dilution? Definition, how it works and examples
Owning a smaller slice of a bigger company: how each new round, option pool and convertible dilutes existing holders, and when anti-dilution protection kicks in.
The short answer
Share dilution, explained plainly, is the fall in an existing holder’s percentage of a company when the company issues new shares — in a funding round, into an employee option pool, or when convertible notes and SAFEs turn into stock. The number of shares you hold stays the same; the total it is divided by grows. A smaller percentage is not by itself a loss. If the new shares are sold above the price you paid, your stake can be worth more after the round than before it.
Stock dilution explained: percentage versus value
The arithmetic is one line. Your new percentage is your shares divided by the old total plus the new shares. A round that issues new shares equal to 25% of the company after it closes leaves every existing holder with 75% of the percentage they had: 10% becomes 7.5%. Whether that is good or bad depends on the price. Hold 10% of a company worth $10M and your stake is worth $1M. If the company then sells new shares at a $15M pre-money valuation, your 10% shrinks, but the shares you hold are now priced at a valuation half as high again, so your stake is valued at $1.5M.
Private company share dilution, round by round
Private company share dilution comes from three sources, and the example below has all three. Two founders hold 8,000,000 shares with a 1,000,000-share option pool. An angel invests $1M on a post-money SAFE with a $10M cap. A Series A raises $7.5M at a $22.5M pre-money valuation, on the condition that the pool is topped up to 15% after the round. A Series B raises $15M at a $60M pre-money valuation. The founders’ percentage falls from 88.9% to 42.7% across two rounds, while the value of the same 8,000,000 shares goes from unpriced to $32M. That is share dilution working as intended.
What is anti-dilution of shares? Full ratchet versus weighted average
Anti-dilution protection is a clause in the certificate of incorporation that lowers a preferred class’s conversion price when the company later sells shares cheaper than that class paid. A lower conversion price means each preferred share converts into more common, so the protected class keeps more of its percentage — and the extra shares come out of everyone else’s. It protects against a down round, not against ordinary dilution at a higher price.
Pro rata rights, and what dilution means through an SPV
The other way to resist dilution is to buy more. A pro rata right lets an investor buy into each new round in proportion to its holding. In the example, the Series A investors hold 25% before the Series B; to stay at 25% they would buy 937,500 of the 3,750,000 new shares, for $3.75M. Under Delaware law no stockholder has a preemptive right unless the certificate of incorporation grants it, so pro rata is usually a contract in the financing agreements, often limited to larger investors.
The rules behind dilution and anti-dilution, as of October 2026
Common mistakes about share dilution
Measuring dilution in percent only A smaller share at a higher price can be worth more. Price decides whether dilution is a loss. Forgetting SAFEs and notes They dilute at the next priced round whether or not they appeared on the last cap table you saw. Taking the pre-money at face value A pool top-up inside the pre-money valuation lowers the price existing holders were really paid. Assuming anti-dilution protects everyone It protects one class, and the shares it adds dilute every holder without it — common first.
The terms this page uses
Fully diluted A share count including everything that could become a share — options, the unallocated pool, warrants and convertibles. Rounds are priced on it. Option pool shuffle Expanding the employee pool inside the pre-money valuation, so the dilution falls on existing holders rather than the new investor. Post-money SAFE A right to future shares, converting at the next priced round; the holder’s ownership is the investment divided by the post-money valuation cap.
Questions about share dilution
Is share dilution always bad? No. If new shares are sold above the price you paid, your smaller percentage can be worth more than before. Share dilution costs you value when new shares are sold cheaper, in a down round, or when the money raised is spent without growing the company. How do I calculate dilution from a funding round? Divide your shares by the total after the round, counting every new share: the investors’ shares, any pool top-up and any SAFEs or notes that convert. Your new percentage is your old one multiplied by one minus the round’s share of the post-round total.
Is share dilution always bad?
No. If new shares are sold above the price you paid, your smaller percentage can be worth more than before. Share dilution costs you value when new shares are sold cheaper, in a down round, or when the money raised is spent without growing the company.
How do I calculate dilution from a funding round?
Divide your shares by the total after the round, counting every new share: the investors’ shares, any pool top-up and any SAFEs or notes that convert. Your new percentage is your old one multiplied by one minus the round’s share of the post-round total.
What is anti-dilution of shares?
It is protection, written into the company’s charter for a class of preferred stock, that lowers that class’s conversion price if the company later sells shares more cheaply. The two main forms are full ratchet and broad-based weighted average.
Does dilution affect investors in an SPV?
Yes, exactly as it affects any holder of the class the vehicle owns. The vehicle’s share count is fixed, so its percentage falls when the company issues new shares. Whether any anti-dilution protection applies depends on the class the vehicle holds.
Where the rules on this page come from
Rules and formulas were last verified on 1 October 2026. The dilution examples are illustrative and describe no real company. The weighted-average formula is quoted from a Delaware charter filed with the SEC; actual charters vary, and the one that governs your shares is the one to read.