What this calculator does
Decimal odds are a price. Their inverse gives the probability the bookmaker assigns to the outcome. Add those probabilities across every outcome in a market and the total goes above 100%. That excess is the margin: the price you pay to bet.
The calculator does the maths for you, strips the margin out and gives the fair odds of each outcome, the price you would see with no commission.
How to use it
- Choose the format. Decimal (1.90), fractional (10/11) or American (−110).
- Enter every outcome. Two prices for a market without a draw (tennis, over/under), three for a 1X2.
- Read the result. The margin appears at the top, then the implied probability, the no-margin probability and the fair odds of each outcome.
The formula
- Implied probability = 1 / odds.
- Margin = sum of implied probabilities − 100%.
- No-margin probability = implied probability / sum of probabilities.
- Fair odds = 1 / no-margin probability.
Worked example
A tennis match is priced at 1.90 for each player.
| Player | Odds | Implied probability | No margin | Fair odds |
|---|---|---|---|---|
| Player 1 | 1.90 | 52.63% | 50% | 2.00 |
| Player 2 | 1.90 | 52.63% | 50% | 2.00 |
The total is 105.26%, a margin of 5.26%. Without commission, each player would be priced at 2.00: that gap is what the bookmaker keeps.
Limits
The calculator spreads the margin in proportion to each outcome. In practice, bookmakers often load more margin onto outsiders than favourites, so the fair price is an estimate, not the truth. It is still the best basis for comparing two bookmakers or judging whether a price is worth taking.