- Bookie Ledger
- Betting terms and conditions explained
- Overround explained: the margin built into a set of odds
Term
Overround explained: the margin built into a set of odds
Overround is the amount by which the implied probabilities of every outcome in a market add up to more than 100%. The excess is the margin built into the odds. In the example here, odds of 2.50, 3.20 and 3.10 imply 103.51% in total, so the overround is 3.51%.
What is the overround?
Every price implies a probability: 1 divided by the decimal odds. For a market with several outcomes, add the implied probabilities together.
A fair market would add to 100%. The excess over 100% is the overround, also called the margin. It is built into the prices, and you do not pay it as a separate charge.
Bottom line: overround = the sum of implied probabilities − 100%.
How do you work it out?
Take the example odds 2.50, 3.20 and 3.10.
| Outcome | Decimal odds | Implied probability (1 ÷ odds) | Share after removing the margin |
|---|---|---|---|
| Outcome 1 | 2.50 | 40.00% | 38.64% |
| Outcome 2 | 3.20 | 31.25% | 30.19% |
| Outcome 3 | 3.10 | 32.26% | 31.16% |
| Total | 103.51% | 100.00% (rounded) |
The overround is 3.51%. A two-outcome example at 1.91 and 1.91 gives 52.36% twice, a total of 104.71%, so 4.71%. Numbers are examples, not prices.
Bottom line: add the implied probabilities and subtract 100%.
What are the limits of the method?
Dividing each implied probability by the total is the proportional method. It is one of several ways to remove a margin, and others spread it differently across outcomes.
The share left is an estimate, not the true chance of an outcome. The calculator takes your own estimate as an input.
An exchange charges a commission on winnings. That is a different charge from a margin built into prices, and it is covered on the Betfair commission page.
Bottom line: the result depends on the method chosen and is not a forecast.
Where to go next
- Run the numbers in the expected value calculator.
- Read how to compare odds fairly.
- Learn the formats in odds formats.
Bottom line: each link stays inside this site.
What changed?
No changes recorded since first publication on 7 November 2026.
Bottom line: this is the first version of the page.