What Is Bookmaker Overround?

What it means

Overround is the amount by which all runners' implied probabilities add up to more than 100%.

Overround is the total of all runners’ implied probabilities minus 100 percentage points.

A worked example

If three runners are each priced at decimal 2.8, their combined implied probability is about 107.14%, an overround of 7.14%.

If a two-outcome market offers 1.80 on both sides, each implies 55.56%, totalling 111.11%; the overround is 11.11 percentage points.

How to check the numbers

Convert every quote into decimal odds and an implied probability before comparing prices. Use the price available when the bet could actually be placed, not a later historical quote.

Common mistakes and limitations

Overround is a market-wide measure; it does not tell you which individual runner is value.

A mathematical edge depends on the quality of the probability estimate; small errors in estimated chance can reverse an apparent advantage.

A three-runner overround calculation

Imagine a three-runner market priced at decimal odds of 2.0, 3.0 and 4.0. The implied percentages are 50%, 33.33% and 25%. Together they total 108.33%, so the overround is 8.33 percentage points.

Practical checks and limitations

Overround is not a guarantee of the bookmaker’s actual profit. Liabilities, promotions, changes in odds and the eventual result all matter. To compare margins, use prices captured at the same moment.