Bookmaker Margin Calculator

About this calculator

Every set of decimal betting odds implies a probability, simply 1 divided by the odds, and if a market were perfectly fair, the implied probabilities of all outcomes would sum to exactly 100%. Bookmakers instead price markets so those implied probabilities sum to more than 100%; the excess is the overround, or bookmaker margin, and it is how bookmakers guarantee themselves an edge regardless of the actual result.

Bettors and odds comparison sites calculate the overround to see which bookmaker is offering the tightest, most favorable, margin on a given match, and professional and arbitrage bettors use it to spot mispriced markets where the true, margin-free odds diverge enough from a bookmaker's posted price to represent value.

Enter the decimal odds for each outcome in a two-way or three-way market and this calculator sums the implied probabilities to show the overround percentage along with the fair, margin-free odds and payout ratio.