Bookmaker margin calculator
Enter the decimal odds for every outcome of one market. The tool adds the implied probabilities, shows how far the total exceeds 100% (the margin, or overround) and what share of stakes the prices pay back on average. It measures prices; it cannot predict a result.
Margin (overround)
- Total implied probability
- –
- Average payback of the prices
- –
- Implied probabilities
- –
- Margin-free probabilities
- –
- Margin-free decimal odds
- –
The formula
For a market with outcomes priced at decimal odds D1, D2, ... Dn:
If the prices carried no margin, S would be exactly 1 (100%). The excess is what the bookmaker keeps on average when stakes are spread across the outcomes in proportion to the prices. The average payback, 1 ÷ S, is the share of those stakes returned to bettors.
Illustrative example
A three-way market is priced 2.10, 3.30 and 3.60. The implied probabilities are 47.62%, 30.30% and 27.78%, which add up to 105.70%. The margin is 5.70% and the average payback is 1 ÷ 1.0570 = 94.61%.
Scaled to 100%, the probabilities become 45.05%, 28.67% and 26.28%, and the margin-free odds are 2.22, 3.49 and 3.81. These figures are invented to show the method.
How the same margin looks at different sizes
| Two-way prices | Total implied probability | Margin | Average payback |
|---|---|---|---|
| 2.00 / 2.00 | 100.00% | 0.00% | 100.00% |
| 1.95 / 1.95 | 102.56% | 2.56% | 97.50% |
| 1.90 / 1.90 | 105.26% | 5.26% | 95.00% |
| 1.85 / 1.85 | 108.11% | 8.11% | 92.50% |
| 1.80 / 1.80 | 111.11% | 11.11% | 90.00% |
The rows are arithmetic on an even two-way event, not a survey of real prices.
Limits of this tool
A calculator cannot predict results. The margin tells you the average cost built into a set of prices; it does not tell you which outcome will happen, and a low margin does not make a bet a good idea.
The proportional method assumes the margin is spread evenly across outcomes, which a bookmaker is not obliged to do. Treat the margin-free figures as an approximation. The step-by-step reasoning is in the guide on calculating the bookmaker's margin, and the short definition is in the entry on overround.
If you have prices in fractional or American form, change them to decimal first with the odds converter. Regulation 23(1) of the Gambling Control (Conduct of Gambling Operations) Regulations, 2026 requires a licensee to clearly display betting odds, rules and terms applicable to each bet, so the inputs for this sum should always be available before a bet is accepted.
Questions and answers
What is a normal margin?
This site does not publish typical figures, because it has no verified data on operators' prices. The calculator lets you measure the margin of any market from the odds in front of you.
Why must I enter every outcome?
The margin is a property of the whole market. Leaving out an outcome, for example the draw in a football match, makes the total implied probability too small and the result meaningless.
What are margin-free probabilities?
They are the implied probabilities scaled down so that they add up to exactly 100%. This proportional method is one common way to remove the margin; it is an estimate of what the prices imply, not the true chance of the event.
Can the margin be negative?
With prices from a single bookmaker it should not be. If the total implied probability comes out below 100%, check that you typed the odds correctly and that all outcomes belong to the same market.