Gambling terms, explained straight

In general mathematics, overround means the amount by which a sum exceeds 1 or 100%; in betting, it specifically refers to the bookmaker's built-in margin.24

Sports Betting

Overround percentage

The overround percentage is the built-in profit margin a bookmaker adds to a betting market, measured as how far the total implied probability of all possible outcomes exceeds 100%.

Formula
For k outcomes, overround = (∑ 1/Oi) − 1, where Oi are decimal odds.4
Typical range
Historically 10% to 15%; some markets now as low as 2%.6
Example
An overround of 1.045 corresponds to a margin of 4.5%.5

Key points

  • A bookmaker builds the margin into quoted odds so summed implied probabilities exceed 100%, making prices worse than fair odds.27
  • If the book is balanced, the overround is the bookmaker's expected profit; in an unbalanced book, actual profit can differ.13
  • Overround is related to but not identical to vigorish; conversion formulas exist in the literature.2

Where this term is used

Not the same as

  • Overround meaning — the general mathematical concept of an excess over 1 or 100%, whereas overround percentage is the specific betting margin
Sports betting Parlay A single bet linking two or more selections, where the combined odds reflect the overround of each leg. Sports betting Moneyline A bet on which side wins a game outright, with odds that include the bookmaker's overround. Sports betting Spread betting A form of betting where the payout depends on the margin of victory, with the spread set to balance action and incorporate margin. Sports betting Moneyline betting Betting on the outright winner of a contest, with odds that embed the overround percentage. Sports betting Point spread A handicap used to level a contest, with odds that include the bookmaker's margin. Sports betting Vig meaning The bookmaker's commission on a bet, closely related to overround percentage but not identical.

Sources

  1. Mathematics of bookmaking - Wikipedia en.wikipedia.org Defines the overround as the bookmaker's margin and explains how it works in balanced and unbalanced books.
  2. Vigorish - Wikipedia en.wikipedia.org Distinguishes overround from vigorish and provides the general mathematical meaning.
  3. Fixed-odds betting - Wikipedia en.wikipedia.org Explains how overround applies to fixed-odds betting and how actual profit can differ.
  4. Edward Wheatcroft* eprints.lse.ac.uk Provides the formula for overround and defines it as the excess of implied probabilities over 100%.
  5. Estimating expected loss rates in betting markets: theory and evidence tandfonline.com Gives a concrete example of an overround of 1.045 corresponding to a 4.5% margin.
  6. Extending Kelly Staking Strategies to Peer-to-Peer Betting Exchanges doc.ic.ac.uk States historical overround levels of 10% to 15% and current lows of 2%.
  7. Profiting from arbitrage and odds biases of the European ... qmro.qmul.ac.uk Explains that bookmakers build margin into odds so implied probabilities exceed 100%.

Sources are drawn from regulators, universities and published research, and each one is labelled with what it actually is — a preprint is not called a paper. Bookmaker and affiliate pages are never cited here, because a page that sells betting is not a neutral authority on it.

Written and maintained by Ellis Warrender, writer and maintainer.

Last updated 28 July 2026. Spotted an error? Tell me and it gets fixed and logged on the corrections page.