Calculate bookmaker overround, true probabilities and fair odds
The bookmaker margin (also called "overround" or "vig") is the percentage by which the bookmaker inflates the true probabilities to guarantee a profit. A margin of 0% would mean perfectly fair odds. Most recreational bookmakers have margins of 5-10% on football 1X2 markets, while sharp books like Pinnacle typically offer 2-3% margins.
For a 3-way market (Home/Draw/Away), the margin formula is:
Margin = (1/Home + 1/Draw + 1/Away - 1) × 100%For example, odds of 2.10 / 3.40 / 3.50 give: (1/2.10 + 1/3.40 + 1/3.50 - 1) × 100 = 5.63% margin