Overround calculator
Paste a race field or any market to see what the prices add up to, how much margin is built in, and each runner’s fair odds with the margin taken out.
This 8-runner market adds up to a book of 116.4%: a margin of 16.4% over the true 100%.
Every price scaled by the same factor. The simplest and most common.
Fair odds by runner
| Runner | Price | Implied | Fair chance | Fair odds (dec) |
|---|---|---|---|---|
| Runner A | 5/2 | 28.6% | 24.5% | 4.07 |
| Runner B | 3/1 | 25.0% | 21.5% | 4.66 |
| Runner C | 4/1 | 20.0% | 17.2% | 5.82 |
| Runner D | 6/1 | 14.3% | 12.3% | 8.15 |
| Runner E | 8/1 | 11.1% | 9.5% | 10.48 |
| Runner F | 12/1 | 7.7% | 6.6% | 15.13 |
| Runner G | 16/1 | 5.9% | 5.1% | 19.79 |
| Runner H | 25/1 | 3.8% | 3.3% | 30.26 |
This market
- Margin per runner (average)
- 2.05 pts
- Shin z
- 0.024
Fair odds are an estimate of the price with the margin removed. They are not a prediction, and different methods give different answers.
How the overround is worked out
Every price implies a probability: divide 1 by the decimal odds. Add those up across the whole market and a fair book would come to exactly 100%. Bookmaker prices come to more, and the excess is the margin.
- Implied probability = 1 ÷ decimal odds (5/2 = 3.50, so 28.6%)
- Book = the implied probabilities added together
- Margin = book minus 100%
Worked example: an 8-runner race
The example field is priced 5/2, 3/1, 4/1, 6/1, 8/1, 12/1, 16/1 and 25/1. The implied probabilities are 28.6%, 25.0%, 20.0%, 14.3%, 11.1%, 7.7%, 5.9% and 3.8%, which add up to 116.4%: a margin of 16.4%.
Taking the margin out proportionally, the 5/2 favourite’s fair odds are 4.07 (about 3/1) and the 25/1 outsider’s are 30.26. Shin’s method puts more of the margin on the outsiders, so it gives 3.91 for the favourite and 38.78 for the outsider. For the background on why prices carry a margin at all, see the UK guide to bookmaker margin and overround.
Related tools and guides
- Odds converter, any price in decimal, fractional or American, with its implied probability.
- Each way calculator, what the win and place parts return for any place terms.
- Rule 4 deduction calculator, when a runner is withdrawn after you bet.
- Horse racing betting, racing tools and guides in one place.