Overround in betting
Overround is the amount by which the implied probabilities of every outcome in a betting market add up to more than 100%. A fair book would total exactly 100%, so the excess is the bookmaker’s built-in edge. You meet it on every price list, from a GAA championship match to a handicap at Leopardstown, even though no bookmaker prints the figure beside the odds.
How do you calculate the overround?
Convert each price to implied probability by dividing 1 by the decimal odds, then add the results for every outcome in the market.
Take a football match priced Home 6/5, Draw 9/4, Away 5/2. In decimal that is 2.20, 3.25 and 3.50, giving 45.45%, 30.77% and 28.57%. The total is 104.79%, so the overround is 4.79%.
Practice varies between operators and markets, and there is no fixed standard: big-match outright prices usually carry a slimmer overround than a crowded race card or a niche market.
Overround versus bookmaker margin
The two terms are often swapped, but they measure the same edge on different bases.
Overround | Bookmaker margin |
|---|---|
| Excess of the book over 100% | Share of total stakes kept |
| 4.79% in our example | 4.57% in our example |
| Book total minus 100% | 1 minus 1 divided by book total |
| Quick to add up by hand | Needs one extra division |
| Ignores how money is spread | Assumes a perfectly balanced book |
Where you meet overround as a punter
Comparing the same market at two bookmakers is the simplest use: the lower total gives you more of your stake back over time, whatever the result.
What I check first is the overround on each-way and accumulator bets, because the edge compounds when selections are multiplied together.
It is not a forecast.
