A dilution calculator shows how much each round reduces existing shareholders' ownership. Every round creates new shares. Those new shares increase the total count, so existing percentages fall even when no shares are taken from anyone. This calculator models the full stack. Founders, options, converting instruments and priced rounds are all included. Founders see the real outcome before negotiating.
Why dilution matters more than people expect
Dilution changes over time because it compounds. A founder losing four percent at Seed looks acceptable on its own. This same four percent multiplied across three rounds removes twelve percentage points, which shifts board control at Series B.
Three patterns compound most often.
| Pattern | Why it hurts | Example |
|---|---|---|
| Multiple SAFE rounds | Each converts into fresh shares | Three £200k SAFEs at different caps can cost ten percent of ownership |
| Option pool top-ups | Investors add a pool mid-round if the first one runs out | A 10% pool becomes 15% after two years of hires, diluting founders again |
| Pro-rata rights ignored | Early investors refuse to top up unless they pay cash | An investor declining to participate drops from 8% to 4%, leaving a gap founders must fill |
Understanding dilution protects negotiating power. A founder who knows the exact outcome can push back against vague promises like "you will still own most of the company." Ownership numbers do not lie.
The math behind dilution
Dilution happens because each round adds shares without removing any. The formula tracks total share count after every issuance, divides each holder's count by the total, and reports the result as a percentage.
Consider a company with 1 million shares held equally by two founders. Each owns 50%.
Seed round creates 200k preference shares at pre-money valuation £800k. Total shares become 1.2 million. Each founder holds 500k of 1.2m, or 41.67%. Both founders lost 8.33 percentage points simultaneously.
Series A creates 400k preference shares at £4.8m pre-money. Total shares become 1.6m. Each founder holds 500k of 1.6m, or 31.25%. Cumulative dilution from the original 50% is now 18.75 percentage points.
Each founder lost almost a third of their original ownership through two rounds alone. Shares left existing shareholders' pockets entirely, replaced by new ones created at the round.
The unallocated option pool adds another layer. If the Series A round requires a 15% post-money pool, additional shares get created specifically for employee grants. This pool dilutes all holders including the newly invested Series A backers, but typically falls hardest on founders at the margin.
Typical dilution ranges per round
Founders searching for dilution numbers need benchmarks, not absolutes. These figures come from actual UK Seed and Series A rounds.
| Round | Typical fund size | Typical dilution to founder | Common range |
|---|---|---|---|
| Pre-seed / angel | £50k–£200k | 10%–20% | Depends heavily on instrument mix |
| Seed (single close) | £200k–£500k | 20%–30% | Single SAFEs or a single priced close |
| Seed (multiple closes) | £500k–£1m | 30%–45% | Two or three close rounds compound quickly |
| Series A | £1m–£4m | 25%–40% | Usually includes a mandatory option pool |
| Series B and beyond | £4m+ | 20%–30% | Growth-stage rounds tend cleaner |
These ranges assume standard UK terms. Non-standard provisions like accelerated vesting, drag-along thresholds, or unusual class structures shift outcomes meaningfully. The calculator above models those specifics.
Reducing dilution as a founder
Rounds always create shares, and shares change ownership percentages. Founders still control the dilution rate for each round.
Five approaches ranked by practical impact.
- Raise larger amounts upfront. Fewer rounds means fewer dilution events. A £500k Seed round dilutes once. Two £200k closes dilute twice with compounding losses.
- Push back on pre-money option pools. A pool created at the round price dilutes existing holders. A pool added post-close dilutes only the new investors. UK practice allows both structures, but the post-close variant favours founders.
- Negotiate higher valuations. Even small improvements in pre-money valuation reduce dilution non-linearly, since valuation determines how many new shares the round buys.
- Consolidate convertible instruments. Splitting a SAFE into two separate closings costs more in dilution. Merge rounds where practical to reduce events.
- Use pro-rata commitments. Getting early investors to commit participation rights before the next round prevents surprise dilution from absent early backers.
None of these replace having the numbers in front of you during negotiation. The calculator above produces the output needed for each term sheet review.
Frequently asked questions
Dilution happens because each funding round creates new shares. Existing shareholders keep their original share count. The total increases through new round issuances. Each owner's percentage falls alongside the larger denominator, so everyone holds less of the expanded company.
UK Seed rounds typically dilute founders by 20% to 30%. Series A rounds typically dilute by 25% to 40%. These ranges depend on deal size, instrument mix and whether the round creates an option pool.
SAFEs do not dilute until they convert at a future priced round. At conversion they become shares, increasing the total share count and reducing all existing percentages simultaneously.
A pre-money option pool is a block of shares created as part of the round terms before the money counts. It dilutes existing shareholders, including founders. Post-money pools dilute the new investors instead. UK negotiations usually target pre-money.
Multiple SAFE rounds compound. Each converts separately into new shares, and each conversion adds to the base before the next calculation. Three £200k SAFEs at different caps can produce more dilution than a single £600k priced round.
Generally no. Share counts never decrease, so percentages never rise again. Pro-rata investments extend a holder's absolute percentage but require cash. Secondary sales, where an existing shareholder sells shares, slightly alter relative percentages without creating new shares.
Next step
A companion cap table template is coming soon, letting you add founders, options, SAFEs and a priced round to see live fully diluted ownership across the whole company.
Founders raising up to £600,000 can work with Lucy Colson through Capital Studio, at £500 a month, with no equity and no success fees.