Betting Math

Overround

The total implied probability exceeding 100% across all outcomes in a market — the bookmaker's total margin.

Also known as: bookmaker margin, hold percentage, book percentage

Definition

Overround (also called the book percentage or hold) measures the total bookmaker margin in a market. It's calculated by summing the implied probabilities of all outcomes.

In a fair market, this sum equals exactly 100%. In practice, it's always higher. A -110/-110 market has a 104.76% overround (4.76% margin). Some markets run tighter (102-103%) while props can reach 110%+.

Lower overround means better value for bettors. Sharp books like Pinnacle run tighter margins than recreational books.

Formula

overround = Σ(1/decimalOdds_i) for all outcomes i
margin = overround - 1

Worked Example

Three-way soccer market: Home 2.10, Draw 3.40, Away 3.50. Overround = 1/2.10 + 1/3.40 + 1/3.50 = 0.476 + 0.294 + 0.286 = 1.056 (5.6% margin).

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Related Terms

Vig

The bookmaker's built-in commission. The difference between true odds and what you're offered.

Implied Probability

The probability of an outcome implied by the odds. Includes the bookmaker's margin (vig).

True Probability

The actual estimated likelihood of an outcome, with the bookmaker's margin removed.