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Definition

Overround

Also known as Over-round, Book Percentage

By Reviewed Editorial policy

In short

Overround is the amount by which a bookmaker's implied probabilities exceed 100%. A book totalling 105% has a 5% overround, which is the bookmaker's built-in profit margin.

Overround is the technical measure of how much margin a bookmaker has built into a market. It is the sum of the implied probabilities of all outcomes, which in a fair market would be exactly 100% and in a real one never is.

Calculating it

Convert each price to implied probability (1 ÷ decimal odds) and add them up.

A football match priced 2.10 / 3.40 / 4.00:

  • Home: 1 ÷ 2.10 = 47.6%
  • Draw: 1 ÷ 3.40 = 29.4%
  • Away: 1 ÷ 4.00 = 25.0%
  • Total: 102.0%

The overround is 2.0%. That is a sharp book. Competitive football markets often run between 2% and 5%.

Overround, margin and vig

Three words for closely related ideas, and the distinction is worth getting right:

  • Overround is the excess over 100%, so 102% total means 2% overround.
  • Margin is usually expressed as the operator’s share of turnover: 2 ÷ 102 = 1.96%.
  • Vigorish is the American term, typically quoted for two-way markets.

The numbers are close at low margins and diverge as margin rises, so for comparing books it matters which convention a source is using.

Why it varies so much

Overround scales with the number of outcomes and the competitiveness of the market:

Market Typical overround
Two-way, major event 2–4%
Three-way football 2–6%
20-runner handicap race 25–40%
Novelty and outright markets 20–50%

Large fields carry large overrounds because the margin is spread across every runner. This is why each-way and outright betting look generous and frequently are not, and why comparing a single price between bookmakers tells you less than comparing the overround of the whole market.

Using it

Two practical uses. Compare books by overround, not by individual prices, a bookmaker with one standout price and a 108% book is not offering value. And strip it out to recover the true estimate: that process is devigging, and it converts a bookmaker’s prices into its honest probability assessment, which is the benchmark any bet needs to beat.

Common questions

How do I calculate overround?

Convert each price to implied probability by dividing 1 by the decimal odds, then add them up. A market totalling 104% has a 4% overround. It takes about thirty seconds and tells you more about an operator than any review.

What is a good overround?

Two to four percent on a major two-way market is sharp. Three-way football typically runs 2% to 6%. Large fields are much worse: a twenty-runner handicap commonly carries 25% to 40%, because the margin is spread across every runner.

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