The definitions
Pre-money valuation is what your company is worth before the investment comes in. Post-money valuation is what it's worth after. The relationship is simple: Post-Money = Pre-Money + Investment Amount.
Example: An investor puts in $1M at a $4M pre-money valuation. Post-money is $5M. The investor owns 20% ($1M ÷ $5M). If the same $1M is invested at a $5M pre-money valuation, post-money is $6M and the investor owns 16.7%. The difference in dilution — 20% vs 16.7% — is the entire negotiation.
Why founders confuse the two
The most common mistake: a founder hears "we'll invest $1M at $5M" and assumes they're giving away 20% ($1M ÷ $5M). But if $5M is the post-money valuation, the pre-money is $4M and they're giving away 20% correctly. If $5M is the pre-money, post-money is $6M and they're giving away 16.7%.
Always clarify in writing: is the stated valuation pre- or post-money? Most term sheets specify "post-money valuation" — which means your ownership percentage is investment ÷ post-money, not investment ÷ stated valuation.
The option pool shuffle
A related trap: investors often require an option pool expansion (typically 10–20% of post-money shares for future employee equity) to be created before the investment closes — which means it comes out of the pre-money valuation, diluting founders before the investor's money arrives.
Example: $5M pre-money agreed, but term sheet requires a 15% option pool created pre-close. If the company has 1M shares at $5 pre-money, the pool creation issues 176K new shares — diluting founders by 15% before the round closes. The effective pre-money valuation paid by the investor is lower than the headline number. Always model the option pool impact explicitly.
Safe notes and convertible notes: the valuation cap
On SAFEs and convertible notes, the valuation cap functions as the effective pre-money valuation for the note holder at conversion. If you raise a $500K SAFE with a $4M cap and later close a Series A at $8M pre-money, the SAFE converts as if the pre-money were $4M — giving the early investor a larger ownership percentage than Series A investors at the same price.
Multiple SAFEs with different caps and discounts create a conversion waterfall that's surprisingly complex to model. CAPLINK's cap table tool handles this automatically — but every founder should understand the mechanics before signing a SAFE.