Overround
The overround is the margin a bookmaker builds into its odds — the amount by which implied probabilities across all outcomes exceed 100%.
What it means
Convert every outcome's odds in a market into implied probabilities and add them up. A fair book sums to exactly 100%; a real one sums higher, and the excess is the overround. A two-way market priced at 1.91/1.91 implies 52.4% + 52.4% = 104.7% — a 4.7% overround. That margin is the bookmaker's structural edge, the sportsbook equivalent of the casino's house edge.
Why it matters for operators
The overround is where sportsbook revenue starts, and setting it is a trading decision, not a formula. Price too high and sharp customers and comparison sites route around you; too low and a bad week of results eats the month. Margins also vary wildly by product: main football markets in competitive jurisdictions run tight, while parlays and bet builders carry stacked margins on every leg. Reading your blended overround by product line is one of the fastest health checks a sportsbook operator can run.
Example
A sportsbook prices a tennis match at 1.87 for both players. Implied probability is 53.5% each, so the book sums to 107% — a 7% overround. On balanced action of 100,000, the trading desk expects to keep roughly 6,500 regardless of who wins.