Overround

In a perfectly fair betting market, the implied probabilities of all possible outcomes would sum to exactly 100%. Bookmakers instead price markets so the total comes to somewhat more than that, and the excess, the overround, represents the house's structural edge across the market regardless of which outcome actually occurs.

The overround is calculated as sum of implied probabilities for all outcomes − 100%. It differs from closing line value, which measures a bettor's edge relative to the market's final price, because overround is the bookmaker's own margin, present in the odds from the moment a market opens through to the close, even as line movement shifts individual prices along the way. Converting quoted odds back into true implied probability requires removing this margin first.

Bettors and analysts use the size of the overround to compare how competitive different bookmakers' prices are; a market with a smaller overround gives the bettor better average value on the same bet. The overround is rarely split evenly across outcomes: bookmakers often shade more margin onto the side attracting heavier public betting, so an outcome's true fair probability is not simply its proportional share of the market's total overround.