Use this free overround (book percentage) calculator to convert betting odds into implied probabilities, reveal bookmaker margin and estimate fair, no-margin prices.
Calculate Overround & Margin
Enter the odds for every possible outcome in a sports betting market. The calculator works out the total overround, bookmaker margin and true probabilities after removing the built-in margin.
Results
Total Overround (%):
Bookmaker Margin (%):
Fair Book (No Margin)
Overround above 100% indicates a bookmaker margin built into the odds. True probabilities are calculated by removing this overround proportionally from each selection.
Overround is a key concept in sports betting. It shows how bookmakers price markets, build in their profit and adjust odds away from the true, fair probabilities.
How Overround Works in Odds
Each set of decimal odds implies a probability: 1 / odds. In a perfectly fair market, all implied probabilities would sum to 100%. Bookmakers shade the odds so the total is above 100%, creating an overround and therefore a margin. A 105% book means a 5% theoretical margin for the bookmaker.
Why Calculate Overround?
Compare value across bookmakers - lower overround usually means sharper, more competitive odds.
Convert odds into true probabilities by stripping out the margin and see what the market 'really' thinks the chances are.
Spot potential value bets where your own probability assessment is higher than the true probability implied by the adjusted odds.
Practical Uses for Bettors
Line shopping: find bookmakers with the lowest overround in the leagues and markets you bet on most.
Value betting: compare your model or personal estimates against the true probabilities derived from the market.
Market analysis: track how overround and margins change between sports, competitions and bet types to understand where the best long-term value lies.
For mainstream sports and top leagues, 102-106% is considered competitive. Betting exchanges can be very close to 100-101%. Higher overround (110%+) usually means poor value for bettors.
Overround is the total book percentage (for example 105%), while margin is the amount above 100% (5% in this case). They describe the same idea from two angles: the full book vs. the profit slice.
Yes, but it's rare. It can happen with aggressive promotional odds, pricing errors or when combining prices from multiple bookmakers to create an arbitrage. An overround below 100% means you can, in theory, back every outcome and lock in a profit.
