Overround
The bookmaker's built-in margin, visible when the odds across all outcomes of a market imply probabilities that add up to more than 100%.
Definition
Any set of betting odds can be converted into an implied probability — for decimal odds, 1 divided by the odds. In a perfectly fair market these implied probabilities would sum to exactly 100%, but a bookmaker shortens the prices so that the total exceeds 100%, and that excess is the overround. It represents the operator's theoretical edge: the share of stakes they expect to keep on average if money is spread across outcomes roughly in line with the true probabilities, making it the sports-betting analogue of a casino game's house edge. A related but technically distinct figure — the margin or "hold" — restates that edge as a percentage of turnover (1 − 100% ÷ book total) rather than as the raw amount by which the probabilities overshoot 100%.
Worked example
Consider an evenly matched tennis final where each player's true chance is about 50%. Fair odds of 2.00 would reflect that exactly, since 1 ÷ 2.00 = 50% and 50% + 50% = 100%. A bookmaker instead prices both sides at 1.91: the implied probability of each is 1 ÷ 1.91 = 52.4%, and the two sum to 104.7% — an overround of about 4.7 percentage points. Expressed as a margin (hold), the operator expects to retain 1 − 100% ÷ 104.7% ≈ 4.5% of the money staked, whichever player wins. The same logic extends to three-way markets: a football match priced Home 2.00 (50.0%), Draw 3.50 (28.6%), Away 4.00 (25.0%) sums to 103.6%, an overround of roughly 3.6%.
Why it matters
Overround is the mechanism by which sportsbooks make money, so understanding it is essential on both sides of the counter. For a bettor it quantifies the built-in cost of wagering and explains why backing every outcome of a market cannot guarantee a profit; comparing the overround on the same event across operators is a direct way to gauge which offers better value, since a lower book percentage means more of the fair price is passed back to the customer. For operators and analysts it is the core lever behind theoretical gross gaming revenue and pricing competitiveness, linking the concepts of odds, implied probability and expected hold into one number.
Related
Note: The underlying concept is stable and mathematical, but terminology is used loosely: "overround", "vig", "juice", "margin", "book percentage" and "over-round" are often treated as interchangeable even though there is a real distinction between the overround (sum of implied probabilities above 100%) and the realised or theoretical margin/hold (edge as a percentage of turnover). Actual margin levels are not standardised and vary widely by sport, market type, operator and competition — from roughly 2% on high-liquidity major-league football lines to 20%+ on some outright, specials or long-shot markets — and can shift as odds move. Regulators generally do not fix or cap overround, so figures should be verified per operator and market.