Dilution · 8 min read
How to Calculate Startup Equity Dilution
Equity dilution is what happens to ownership percentages when a startup issues new shares. Every time a company raises money, grants options, or converts a SAFE or convertible note, existing shareholders own a smaller percentage of the total pie — even though their absolute share count stays the same. This guide explains the exact calculation, with formulas and worked examples you can verify in Quozify.
What causes equity dilution?
Dilution occurs any time the total number of shares outstanding increases. The most common causes in a startup lifecycle are:
- Priced funding rounds — new shares issued to investors at a price per share
- Option pool expansion — shares reserved for employee stock options
- SAFE conversions — SAFEs convert to shares at the next priced round
- Convertible note conversions — debt converts to equity at a price with interest and discount
- Warrant exercises — rights to purchase shares at a fixed price
The dilution formula
The core dilution calculation is straightforward:
Total post-round shares = Pre-money shares + New investor shares + New option pool shares
Price per share (PPS) = Pre-money valuation ÷ Pre-money fully diluted shares
New investor shares = Investment amount ÷ Price per share
Step-by-step worked example
Let's walk through a real Seed round scenario:
Starting position
- Two founders: each owns 500,000 shares (1,000,000 total)
- No previous investors or option pool
- Company raises a $1M Seed round at a $4M pre-money valuation
- Investors require a 10% pre-money option pool
Step 1: Calculate the option pool (pre-money basis)
A 10% pre-money option pool means the pool should represent 10% of the post-financing shares, but the shares are issued before investors enter. The calculation requires solving for the pool size such that it equals 10% of the final share count:
X ÷ (1,000,000 + X + investor shares) = 10%
Investor shares = $1,000,000 ÷ PPS
PPS = $4,000,000 ÷ (1,000,000 + X)
Solving: X ≈ 125,000 shares (approx. 11.1% of pre-money cap)
Quozify solves this simultaneously — you just enter the target percentage and it calculates the exact share count.
Step 2: Calculate price per share
PPS = $4,000,000 ÷ 1,125,000 = $3.556 per share
Step 3: Calculate investor shares
Step 4: Final cap table
| Stakeholder | Shares | Ownership % |
|---|---|---|
| Founder 1 | 500,000 | 35.6% |
| Founder 2 | 500,000 | 35.6% |
| Option Pool | 125,000 | 8.9% |
| Seed Investors | 281,250 | 20.0% |
| Total (fully diluted) | 1,406,250 | 100% |
Each founder went from 50% to 35.6% — a dilution of 28.8%. The 20% investor ownership and 10% option pool requirement together drove a meaningful reduction in founder percentage, despite the pre-money valuation being $4M.
How does the option pool affect dilution?
Whether the option pool is created on a pre-money or post-money basis dramatically changes who bears the dilution:
| Pool Basis | Who is diluted? | Investor impact |
|---|---|---|
| Pre-money | Founders and existing investors only | Investors enter after pool — protected |
| Post-money | Everyone including new investors | Investors share in pool dilution |
Institutional investors almost always require a pre-money option pool. This effectively inflates the pre-money valuation that founders must justify while reducing their effective ownership at close.
Cumulative dilution across multiple rounds
Dilution compounds across rounds. A founder who raises a Seed, Series A, and Series B typically retains 15–30% by Series B, depending on the valuations and amounts raised. The key insight: dilution percentage matters less than valuation growth. A founder owning 20% of a $100M company holds more value than 100% of a $1M company.
Quozify's round timeline lets you model multiple sequential rounds and see cumulative dilution at each stage.
Model your own dilution scenario
Enter your founders, valuation, investment amount, and option pool — see results in real time.
Dilution calculation — common questions
- What is the dilution formula for a startup funding round?
- The dilution formula is: New ownership % = Old shares ÷ (Old shares + New shares issued). New shares issued to investors = Investment amount ÷ Price per share. Price per share = Pre-money valuation ÷ Pre-money fully diluted shares. If a pre-money option pool is created, those shares are added to the denominator before investor shares, further diluting existing holders.
- How much do founders typically get diluted in a Seed round?
- In a typical Seed round, founders are diluted by 15–25% of the company. For example, founders who own 100% before the round will own roughly 75–85% after a standard Seed with a 10–15% option pool and 15–20% investor ownership. The exact dilution depends on the pre-money valuation, investment size, and option pool requirement.
- What is fully diluted share count?
- Fully diluted share count is the total number of shares outstanding if all convertible securities, options, warrants, and SAFEs were converted to common shares. It represents the maximum number of shares that could exist and is used to calculate price per share and ownership percentages. Investors almost always negotiate on a fully diluted basis.
- Does dilution always hurt founders?
- Dilution reduces the founder's percentage ownership but not necessarily the value of their stake. If the company's valuation increases faster than the percentage ownership decreases, the absolute dollar value of founder shares grows. The goal is to ensure each dilutive event (raising capital, issuing options) increases the company's value enough to justify the percentage given up.
- How does a pre-money option pool affect dilution differently than a post-money pool?
- A pre-money option pool is created before investors enter, so the dilution falls entirely on existing shareholders (founders and early investors). A post-money option pool is created after investors enter, so the dilution is shared by everyone including new investors. Investors strongly prefer pre-money pools because it protects their ownership percentage at closing.