Overround is the amount by which the implied probabilities of all outcomes in a market add up to more than one hundred percent, the margin a bookmaker builds into the odds.
Last reviewed 11 August 2025 · Educational, not advice
Overround is a bookmaking idea. Every set of odds can be turned into an implied probability, the chance the odds suggest for that outcome. In a perfectly fair market the implied probabilities of all the possible outcomes would add up to exactly one hundred percent. A bookmaker shades the odds so that the total comes out higher than that, and the slice above one hundred percent is the overround. It is the margin the book holds for itself, and it is also known as the vigorish, the vig, the juice, or simply the book percentage.
Calculating it is straightforward. Convert each outcome into its implied probability, then add the results. If a two outcome market implies fifty five percent for one side and fifty five percent for the other, the total is one hundred and ten percent, so the overround is ten percent. The higher the total, the larger the built in margin, and the more the quoted odds are tilted away from what a fair price would be. This is why a careful person compares the overround across venues, since a smaller margin means the odds are closer to fair and more of any winnings stay with the bettor.
Prediction market exchanges generally work differently. On an exchange there is no house setting odds to guarantee a margin. Participants trade against each other, and the Yes and No prices of a contract tend to add up to about one dollar, give or take a small spread, rather than to a number stacked above fair. The venue usually earns from trading fees instead of from an overround baked into the odds. That is a real structural difference from a traditional book, though it does not mean trading is free, since fees, the spread, and thin liquidity still cost you. You should always check how a specific platform charges, because designs vary and some blend models.
Seen plainly, overround is a cost measure. It tells you how much margin sits inside a set of odds before you ever consider whether an outcome is likely. A smaller overround is friendlier to the bettor, a larger one is harder to overcome over many bets. It does not tell you which outcome will happen, it does not make any single bet a good idea, and we never name a contract or a side to take. It is simply one lens for seeing how the odds in front of you are priced.
Take a two way market where one side is offered at odds implying fifty two percent and the other at odds implying fifty three percent. Added together that is one hundred and five percent, so the overround is five percent, the margin held by the book. On an exchange the same question might show a Yes contract at fifty one cents and a No contract at forty nine cents, adding to about one dollar with no margin stacked on top, the venue instead charging a separate fee. Same question, two pricing structures, and the overround is what reveals the difference.
Illustrative only. Numbers are examples, not a quote or a prediction, and exclude fees.
A small overround does not make a bet a winner, and an exchange charging fees instead of a margin is not free. Costs of every kind work against you over time. Stake only what you can afford to lose, never to chase a loss, and never on borrowed money. If it stops feeling like a free choice, step back. In the United States you can call or text the helpline on 1-800-GAMBLER or visit ncpgambling.org.
It is the amount by which the implied probabilities across all outcomes of a market add up to more than one hundred percent. That extra slice is the margin a bookmaker builds into the odds, also called the vigorish, the vig, or the book percentage.
Convert each set of odds into its implied probability and add them together. If the total is one hundred and five percent, the overround is five percent. A higher total means a larger built in margin and worse value for the person placing the bet.
Usually not in the same way. On an exchange the Yes and No prices tend to add up to about one dollar, because participants trade against each other rather than against a house. The venue typically earns from fees instead of a margin built into the odds, though you should check each platform.
It is a cost. The larger the overround, the more the quoted odds are tilted away from fair, and the harder it is to come out ahead over time. Comparing the overround across venues shows where the built in margin is larger or smaller. It does not tell you any outcome.
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