The value of a company after an investment round — calculated as pre-money valuation plus the new investment.
Post-money valuation is the company's value after new investment is added. It equals the pre-money valuation plus the total amount raised. If a company raises $2 million at a $10 million pre-money valuation, the post-money valuation is $12 million, and the new investors own $2M / $12M = 16.7%.
Post-money SAFEs (the current YC standard) specify investment terms based on the post-money valuation, which makes dilution calculations simpler and more transparent. The investor's ownership percentage is simply their investment amount divided by the valuation cap (if the round price is at or above the cap).
Post-money valuations are commonly reported in press coverage and startup databases (Crunchbase, PitchBook). When a startup "raised a $2M seed at a $12M valuation," that's almost always the post-money number. Knowing whether you're looking at a pre-money or post-money number is important for any calculation involving ownership percentages.