What is the difference between pre-money and post-money valuation?
Pre-money is what the company is agreed to be worth before the investment; post-money is pre-money plus the money invested. The difference determines what percentage the investor gets, and confusing them is an expensive mistake made surprisingly often.
The arithmetic. A company agrees a £4m pre-money valuation and raises £1m:
Post-money = £5m.
The investor owns £1m ÷ £5m = 20%.
Now suppose the same £1m is raised at a £4m post-money valuation:
Pre-money = £3m.
The investor owns £1m ÷ £4m = 25%.
Same headline number, five percentage points of the company different. Always establish which basis is being quoted before agreeing anything — "we're raising £1m at £4m" is ambiguous and the ambiguity is not always accidental.
The formula. Investor percentage = investment ÷ post-money valuation. Always. Post-money is the denominator, which is why it is the more directly useful figure.
Where the option pool comes in, and why it matters more than the valuation. Investors typically require an option pool for future employees, sized as a percentage of the post-money company. The critical question is whether it is created before or after the investment:
Pre-money pool — created out of the existing shareholders' equity, so founders bear all of it. This is the standard investor request and is sometimes called the option pool shuffle.
Post-money pool — dilutes everyone including the new investor.
A large pre-money pool effectively reduces the valuation without changing the headline number. A £4m pre-money with a 15% pre-money pool is materially worse for founders than £4m with a post-money pool, and the negotiation frequently focuses on the valuation while this does more damage.
Other things to check: whether convertibles are counted in the pre-money, which affects everyone's percentage; and whether the valuation is on a fully diluted basis, which it normally is.
Valuation is not the only term that matters. Liquidation preferences and control provisions can matter more than the number.
General information, not legal or financial advice.