Bookmaker margin calculator
Enter the decimal odds of every outcome in one market. The calculator shows how far the implied probabilities exceed 100%, what share of stakes a balanced book keeps, and the fair odds once the margin is removed.
Overround and margin are not the same
The overround is how far the implied probabilities add up beyond 100%. The margin is the share of total stakes the book keeps if money arrives in proportion to those probabilities: margin = 1 − 1 / (sum of implied probabilities). For a two-way market at 1.91 on both sides the overround is 4.7% but the margin is 4.5%. For a football market at 2.40, 3.30 and 3.10, the overround is 4.2% and the margin 4.1%.
Fair odds
The calculator removes the margin in proportion to each outcome's implied probability: fair odds = quoted odds × sum of implied probabilities. For the football example that gives 2.50, 3.44 and 3.23. Traders also use other methods that load more margin onto long shots; the proportional method is the standard first estimate.
Why it matters
Margin is the price of a sportsbook's product and the starting point of its hold. Multiple bets multiply the margins of their legs, which is why accumulators earn operators far more per unit staked than singles. See also the sports betting hub and the odds converter.
Updated: