A worked example
For home 2.10, draw 3.30 and away 3.60, the implied probabilities sum to 105.70%. The overround is therefore 5.70 percentage points. Proportional removal gives fair-odds estimates of approximately 2.22, 3.49 and 3.81. The calculator also shows the power-method estimates; these need not be identical.
How the calculation works
For each outcome, implied probability is 1 / decimal odds. Add these probabilities and subtract 1 to obtain the overround. Proportional removal divides each probability by their sum. The power method finds an exponent k so that the adjusted probabilities sum to 1. These are two assumptions about how to remove the margin, not direct observations of the true probabilities.
What this result does not tell you
Use prices from the same bookmaker, market and moment. Mixing historical prices or the best price from different bookmakers does not measure one bookmaker’s margin. Exchange commission is not included. A negative overround is not proof of an executable arbitrage: prices, liquidity and market rules may differ. Margin-free odds are estimates, not a guarantee that a bet has value.
Common questions
Is overround the bookmaker’s actual profit?
No. It describes the quoted market. Actual profit also depends on the bets placed, their distribution, results and costs.
Why do the two methods disagree?
They allocate the margin differently. Compare the assumptions, especially for long-priced selections, instead of treating either estimate as an observed truth.
Can I use just the favourite’s price?
Not for this 1X2 calculator. You need home, draw and away prices from the same market.
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