Overround & Bookmaker Margin Calculator

What is overround in betting?

Overround is the total of every outcome's implied probability (1 ÷ decimal odds) in a market. A football match priced 2.10, 3.40, 3.60 adds up to 104.8%, so the margin is 4.8%. Fair odds remove it: divide each implied probability by the total, then take 1 ÷ the result.

Book percentage

104.8%

Overround (margin)

4.8%

Outcome 1: 2.10 → fair 2.20 (45.4%)
Outcome 2: 3.40 → fair 3.56 (28.1%)
Outcome 3: 3.60 → fair 3.77 (26.5%)

How it works

Implied probability of each outcome = 1 ÷ decimal odds.
Book percentage = sum of implied probabilities. Anything over 100% is the overround; the margin is roughly book − 100%.
Fair (no-margin) odds = book percentage ÷ implied probability of that outcome, which spreads the margin proportionally.

Worked examples

Football 2.10 / 3.40 / 3.60

Book 104.8% (4.8% overround). Fair odds 2.20 / 3.56 / 3.77.

Tennis 1.83 / 1.83

Book 109.3%, a 9.3% overround: typical of a poor price on a coin-flip.

Exchange 2.02 / 2.00

Book 99.5%: under 100% before commission, which is why exchanges suit value bettors.

Frequently asked questions

It's the amount by which a market's implied probabilities add up to more than 100%. The extra is the bookmaker's built-in margin, which is why you lose over time if you bet randomly.

Convert each price to implied probability (1 ÷ decimal odds), add them up and subtract 100%. Prices of 1.91 and 1.91 give a book of 104.7%, so a 4.7% margin.

Lower is better. Around 2–5% on major football match results is competitive; 10–30% is common on racing and niche markets. Exchanges are often close to 100% before commission.

Every tip is priced by a bookmaker with a margin. A tipster has to beat that margin before showing a profit, and taking the best available odds shrinks it.

Overround & Bookmaker Margin Calculator

How it works

Worked examples

Frequently asked questions

Further reading

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Overround & Bookmaker Margin Calculator

How it works

Worked examples

Frequently asked questions

What is overround in betting?

How do I calculate the bookmaker's margin?

What is a good overround?

Why does overround matter when following a tipster?

Further reading

Never subscribe to a dud tipster again