Sports Betting
Overround meaning
Overround is the extra percentage a bookmaker builds into the odds so the implied probabilities of all possible outcomes add up to more than 100%.
Key points
- Bookmakers quote odds whose implied probabilities sum to more than 100%; the excess is the overround.1
- If stakes are balanced across outcomes, the overround is the bookmaker's expected margin.3
- Overround is also called bookmaker margin, vig, or book percentage.2
Common questions
What does overround mean in betting?
Overround is the bookmaker's built-in margin. It is the sum of implied probabilities for all outcomes minus 100%. For example, if odds imply 105% total probability, the overround is 5% — that is the bookmaker's expected profit margin.1
Where this term is used
Not the same as
- Overround percentage — the same measure expressed as a percentage rather than a decimal multiplier
Related terms
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Vig meaning
Vig (vigorish) is another name for the bookmaker's commission or margin, essentially the same concept as overround.
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Sources
- Mathematics of bookmaking en.wikipedia.org Defines overround as the bookmaker's margin and explains how implied probabilities exceed 100%.
- Vigorish en.wikipedia.org Confirms that overround is also called vigorish or bookmaker margin.
- Fixed-odds betting en.wikipedia.org Explains that the overround is the expected margin when stakes are balanced.
- Optimal Online Bookmaking for Any Number of Outcomes arxiv.org Provides the formula Γ = sum of inverses of decimal odds and typical ranges for UK football markets.
- Edward Wheatcroft* eprints.lse.ac.uk Gives the alternative expression (∑ 1/Oᵢ) − 1 for overround.
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.