Pre-money and post-money valuation differ by exactly one thing, the new investment, and confusing them is one of the most common and expensive mistakes a first-time founder makes at the negotiating table.
Pre-money valuation is what the company is agreed to be worth before the new money comes in. Post-money valuation is simply pre-money plus the amount invested. If a company is valued at a pre-money of a given number and raises new capital, the post-money is the sum of the two, and the new investor owns their check divided by that post-money figure.
Because the investor's ownership is calculated on post-money, the base you agree to matters enormously. Quoting a valuation without specifying pre or post can shift several points of ownership from the founder to the investor on the same headline number. When someone says a valuation, the first question is always: is that pre or post?
Post-money SAFEs, now common, fix the investor's ownership percentage regardless of how many other SAFEs convert, which shifts dilution risk squarely onto the founder. That is not a reason to avoid them, but it is a reason to model every instrument on a post-money basis before signing. This is general information, not investment advice.
Always ask: pre or post?