Dilution is the reduction in an existing shareholder’s ownership percentage when a company issues new shares. Each time a startup raises a priced round, creates an option pool, or converts a SAFE, the total share count grows, and every share that existed before now represents a smaller slice of the whole. Founders feel it most, because they start at or near 100% and are diluted at every round that follows.
Dilution is not a sign that something has gone wrong; it is the normal cost of raising capital and granting equity. What matters is understanding how much ownership a round actually moves, how it compounds across several rounds, and why a smaller percentage of a larger company can still be worth far more. This guide covers the math, the typical ranges per round, a worked multi-round example, and the instruments (option pools, SAFEs, anti-dilution) that change the result.
The short version:
- What it is: your ownership percentage falls when new shares are issued, even though your share count is unchanged.
- The math: after a round, existing holders keep (1 − new investor %); per holder, new % = old shares ÷ total shares after issuance.
- How much per round: commonly 10%-25% of the company sold at each early priced round, plus option-pool top-ups; highly variable.
- It compounds: dilution stacks multiplicatively, not additively, across rounds.
- Lower % ≠ less value: if post-money valuation rises faster than you are diluted, your stake is worth more in dollars.
What dilution is, and what it is not
When a company issues new shares, the denominator grows. A founder who holds 4,000,000 shares still holds 4,000,000 shares after the round; what changes is that those shares are now divided into a larger total, so the percentage they represent falls. That percentage drop is dilution.
It is worth separating two things that are often confused. Dilution reduces your ownership percentage. It does not, by itself, reduce the dollar value of your stake, and frequently increases it, because the new money raises the company’s valuation. A founder diluted from 60% to 48% in a round that triples the company’s value comes out ahead in dollars. The percentage is down; the value is up. The two move independently, and a percentage read in isolation says little about whether a holder is better or worse off in dollars.
The dilution math
There are two equivalent ways to compute it.
From the round. A new investor takes a percentage equal to their investment divided by the post-money valuation. Everyone who held shares before keeps the rest, scaled down proportionally:
Existing holders’ retained share = 1 − (Investment ÷ Post-money valuation)
If a round sells 25.0% of the company, every prior holder is multiplied by 0.75: a founder at 60.0% becomes 45.0%, an angel at 8.0% becomes 6.0%.
From the share count. Equivalently, each holder’s new percentage is their share count divided by the total after issuance:
New ownership % = Old shares ÷ Total shares after issuance
Both give the same answer. The first is faster for a quick estimate; the second is what a cap table actually computes, holder by holder, once option pools and convertibles are in the mix. The construction of that fully-diluted denominator is covered in price per share: how to calculate it.
How much dilution per round
There is no fixed rate, but the commonly observed ranges for the share of the company sold at each early priced round are:
| Round | Typical equity sold | Driver |
|---|---|---|
| Pre-seed / seed | 10%-25% | raise size vs. a low early valuation |
| Series A | 15%-25% | larger raise, the round most associated with the “15-25%” rule of thumb |
| Series B | 10%-20% | bigger raise against a higher valuation |
| Series C and later | 5%-15% | large raises, but valuations large enough to keep the percentage down |
These are ranges, not rules. The actual figure is set by how large the raise is relative to the valuation (a founder who raises more at the same valuation sells more of the company), and an option-pool top-up layered into the round adds dilution on top of the investor’s stake. Treat the table as a sanity check, not a prediction; the only reliable number is the one your own round produces.
A worked multi-round example
Dilution compounds across rounds, and the way it compounds surprises people. Take a founding team that owns 100.0% of the company and raises three rounds, selling 20.0%, then 25.0%, then 20.0%:
Start: founders 100.0%
After seed (20%): 100.0% × (1 − 0.20) = 80.0%
After Series A (25%): 80.0% × (1 − 0.25) = 60.0%
After Series B (20%): 60.0% × (1 − 0.20) = 48.0%The founders end at 48.0%, not at 100% − 20% − 25% − 20% = 35%. Dilution stacks multiplicatively, because each round takes its percentage of what remains, not of the original whole, so the founders keep more than naive subtraction suggests, but every round’s bite is taken from a base the previous rounds already shrank.
Now the value side. Suppose the seed closed at a $10,000,000.00 post-money and the Series B at a $100,000,000.00 post-money:
After seed: 80.0% of $10,000,000.00 = $8,000,000.00
After B: 48.0% of $100,000,000.00 = $48,000,000.00Ownership fell from 80.0% to 48.0% across the journey, while the value of the founders’ stake rose from $8,000,000.00 to $48,000,000.00. That is the entire case for accepting dilution: a smaller slice of a much larger pie.
The option pool, and why it dilutes founders specifically
Employee option pools are a second, quieter source of dilution. The subtlety is in the timing. When an investor requires the pool to be created or topped up as part of a round, it is almost always sized against the post-money share count but carved out of the pre-money valuation. The practical effect is that the new pool dilutes the existing shareholders, the founders, rather than the incoming investor, even though the whole company will eventually grant from it.
A “20% post-money option pool” attached to a round is therefore not free; it lands largely on the founders’ pre-money equity. This is one of the most common reasons a founder’s post-round ownership comes in lower than the headline round terms implied, and it is only visible when the pool is modeled inside the cap table alongside the raise.
SAFEs and notes: dilution you cannot see yet
Convertible instruments defer dilution rather than remove it. A SAFE or a convertible note does not change ownership when it is signed. It converts into shares later, at the next priced round, and the dilution arrives then. The risk is that several convertibles stack up unseen: three post-money SAFEs at modest caps can quietly commit 15%-20% of the company before the priced round that converts them even begins, and a post-money SAFE in particular fixes the investor’s percentage and pushes the dilution onto founders. Modeling convertibles on an as-converted basis, before the round, is the only way to know the real pre-round ownership.
Anti-dilution protection
“Anti-dilution” is a contractual protection for preferred investors, not for founders, and in practice it usually increases founder dilution. It adjusts the conversion price of existing preferred stock when a company raises a later round at a lower price (a “down round”), issuing those earlier investors additional shares to compensate. The standard, relatively mild form is broad-based weighted-average; the aggressive form, full-ratchet anti-dilution, reprices as if the earlier investment had been made at the down-round price and can transfer a large block of ownership away from common holders. Either way, the adjustment dilutes the founders further, which is why the anti-dilution clause is worth modeling before it is ever triggered.
This article is general information about startup equity and dilution, not legal, tax, or investment advice. Financing terms are negotiated and fact-specific, so confirm any term sheet, option pool, and cap-table treatment with your counsel and financial advisors before relying on a model.
Frequently asked questions
How much do founders get diluted per round?
Commonly 10%-25% of the company is sold at each early priced round (the Series A is the round most associated with the 15%-25% range), plus any option-pool top-up. The exact figure depends on how large the raise is relative to the valuation, so the ranges are a sanity check rather than a prediction.
How do you calculate dilution?
Two equivalent ways. From the round: existing holders retain 1 − (investment ÷ post-money valuation), so a round that sells 25% multiplies every prior holder by 0.75. From the share count: a holder’s new percentage is their old shares divided by the total shares after the new issuance.
Is dilution bad for founders?
Not by itself. Dilution lowers your ownership percentage, but if the round raises the company’s valuation enough, the dollar value of your smaller stake goes up. The goal is not to avoid dilution but to make sure each round adds more value than it costs in ownership.
Do option pools cause dilution?
Yes. Creating or expanding an option pool issues new shares and dilutes existing holders. When the top-up is part of a round and carved out of the pre-money valuation, it dilutes founders specifically rather than the incoming investor.
Can founders avoid dilution?
Not entirely. Issuing shares to raise money or grant options is what dilutes, and growing companies do both. Founders manage dilution rather than avoid it: raising only what is needed, negotiating valuation and pool size, and weighing each round’s dilution against the value it adds. Pro-rata rights let existing investors maintain their percentage by investing again, but founders generally cannot.
What is anti-dilution protection?
It is a clause that protects preferred investors if the company later raises at a lower price, by adjusting their conversion price and issuing them additional shares. The standard form is broad-based weighted-average; full ratchet is the aggressive version. Both increase the dilution borne by common shareholders, including founders.
The bottom line
Dilution is the normal arithmetic of financing: new shares grow the denominator, existing percentages shrink, and the effect compounds multiplicatively across every round, pool, and conversion. The number to watch is not the percentage in isolation but the percentage against the valuation that produced it, because a smaller share of a more valuable company is the outcome a successful fundraising path is supposed to create.
Waterfalls has a fundraising mode that models a priced round and shows the dilution before you commit to it: enter the pre-money valuation and the raise, and it returns the post-money valuation, the price per share, and every stakeholder’s ownership and dilution before and after, for priced equity rounds. It is a modeling tool, not a system of record, and it does not host your official cap table.