Overround is one of the fundamentals of bookmaking, and is applied to every event by all UK betting sites. It is how bookies factor in a profit to every market they run and is a vitally important concept to help understand how bookies work.

In this guide, The Standard have looked at how the overround works and how bettors can work out what the overround is on a particular market.

Overround is the sports betting term for the built-in margin a bookmaker adds to a betting market. It is the amount by which the implied probabilities of all possible outcomes add up to more than 100 per cent.

This margin gives bookmakers an edge regardless of which selection wins. Rather than needing to correctly predict every result, the bookmaker prices the market so the total implied probability exceeds 100 per cent, creating room for a profit over time.

To calculate implied probability from decimal odds, divide 100 by the odds. The formula is: implied probability (%) = 100 divided by decimal odds.

For example, imagine a four-runner market with odds of 2.50, 3.20, 4.00 and 5.00. The first selection has an implied probability of 40%, while the second is 31.25%. The other two are 25% and 20%, respectively.

Adding those figures gives a total implied probability of 116.25%. The overround is therefore 16.25%, representing the bookmaker's built-in margin in that market. A market priced at exactly 100% would have no overround, although bookmakers generally build a margin into their prices.

A lower overround generally means the bookmaker has taken a smaller margin, which can result in better value odds for bettors. A higher overround means a larger margin has been built into the prices, leaving less value in the market if all other factors are equal.

Overround also varies between markets. It tends to be tighter where there is more liquidity, such as major sporting events, markets close to the start time and highly traded competitions. Smaller or more niche markets can carry wider margins because there may be less betting activity and fewer competing prices.

Comparing the overround for the same market across different betting sites can therefore provide a quick sense check on how competitively the betting odds are priced. Bettors can make the same comparison when using betting apps, although the margin remains only one factor to consider.

Overround applies across sports betting, but is particularly prevalent on greyhound and horse racing betting sites. It is particularly visible in horse racing markets with large fields, where bookmakers must price more possible winners.

Bettors looking at horse racing markets can use the same calculation to understand the margin built into a market, while the principle also applies to football, tennis, boxing and other sports.

Overround is the amount by which the implied probabilities of all outcomes in a betting market exceed 100%. It represents the bookmaker's built-in margin and is calculated by adding the implied probabilities of every possible outcome.

Overround is baked into most betting markets. The size of a bookmaker’s margin varies depending on the sport, market and liquidity, so it can differ between straightforward match bets and more specialised markets.

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