Bookmaker Margin Calculator
The margin (overround, vig) a bookmaker has built into a market from the odds on each outcome — and the fair odds and true probabilities once it is stripped out.
Convert each outcome’s decimal odds to an implied probability (1 ÷ odds) and add them up.
How the bookmaker margin calculator works
Convert each outcome’s decimal odds to an implied probability (1 ÷ odds) and add them up. A fair book totals 100%; anything above is the bookmaker’s margin. Two-way markets at 1.91 each imply 104.7% — a 4.7% overround. Dividing each implied probability by the total gives the fair probabilities, and their reciprocals the fair odds.
Formula: overround = Σ(1 ÷ oddsᵢ) − 1; fair pᵢ = (1 ÷ oddsᵢ) ÷ Σ(1 ÷ odds)
Worked examples
| Inputs | Bookmaker margin | Note |
|---|---|---|
| Two-way at 1.91 / 1.91 | 4.71 % | 4.71% |
| Football 1X2: 2.10 / 3.40 / 3.60 | 4.81 % | 5.8% |
| A fair book: 2.00 / 2.00 | 0 % | 0% |
FAQFrequently asked questions
What is a typical margin?
Around 2–5% on major football and racing markets from big bookmakers, 5–10% on smaller markets, and up to 20% or more on exotic bets and some racing books. Betting exchanges charge a commission instead.
Why does it matter?
It is the price of betting. At 5% margin you need to beat the fair probability by five points just to break even over time; at 2% the hurdle is much lower.
Is the margin spread evenly across outcomes?
Usually not — bookmakers often load more on the outsiders (the favorite–longshot bias). The proportional method here spreads it evenly, which is a reasonable first estimate of fair odds.
How do I compare bookmakers?
Compute the overround on the same market at each; the lower, the better the prices overall. For a single selection, simply take the highest odds available.
Where these figures come from
- World Handicap System — Rules of Handicapping — the score differential and handicap index formula
- International Cricket Council — Playing Conditions — over structure and run-rate definitions
- Major League Baseball — Official Rules — the published definitions for baseball statistics
Last checked: September 2026. Formulas follow the governing bodies where one publishes them — the World Handicap System for golf, the ICC playing conditions for cricket run rates.