Pre-money is what the company is valued at before the new investment; the post-money is that figure plus the money coming in. The investor’s ownership is investment divided by post-money, and the price per share is pre-money divided by the fully diluted share count before the new money, including the round’s pool increase and the converting instruments. The two conventions describe the same transaction, but every negotiated number must declare which one it uses.
The classic ambiguities cluster in 3 places. Headline shorthand (“on 12”) omits the convention. The option pool sits in the pre-money by custom, so a pool increase lowers the effective price per share without touching the headline. And converting instruments blur the boundary: a stack of post-money SAFEs converts into the capitalization before the new money, so the founders’ real post-round position can only be read off a fully diluted table that models the conversions explicitly.
In negotiation, the pre-money is the number that carries the story: what the team, the IP and the milestones reached so far are worth. The post-money is the number that carries the math: dilution, ownership targets, and the next round’s reference point. Fluency means moving between the two without rounding errors, and never letting a deck, a model and a term sheet disagree.