What the overround actually is
Every price a bookmaker offers carries an implied probability: a runner at 3.0 (2/1) implies a 1-in-3, or 33.3%, chance of winning. Add up the implied probabilities of every runner in a race and a fair book would total exactly 100%. A real bookmaker's prices always total more than 100%, and the amount over 100% is the overround, the margin built into the prices. On UK horse racing it is typically 10-30%, bigger in large-field handicaps. This calculator does the sum for you: enter the prices, read the overround, and see the fair odds you would get if the margin were stripped out. Those fair odds are longer than the prices on offer, which is exactly why the margin makes a race so hard to beat.
From The AI Lab
We measured the margin across thousands of real races
The overround is not an abstraction. It is the house edge, charged on every bet. In The AI Lab we measured it on real data and tested whether anything beats it.
Enter every runner's price
Type the bookmaker's price for each runner in the race, in fractional (11/4, evens) or decimal (3.75) format. The calculator adds up the implied probabilities and shows the overround: the margin the bookmaker has built into the prices.
Enter at least two runners with valid prices. The more of the field you include, the more accurate the overround: a full card is the true picture of the bookmaker's margin on that race.
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OpenFrequently Asked Questions
Common questions
- The overround is the bookmaker's margin, expressed as a percentage. Add up the implied probability of every runner in a race (1 ÷ decimal odds for each) and you get the book percentage. A perfectly fair book totals 100%; a real bookmaker's prices total more than 100%, and the amount over 100% is the overround. For example, if a race's prices sum to 118%, the overround is 18%.
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