Free tool
Bookmaker margin calculator.
Every book is built to add up to more than 100% — that gap is the margin you pay. Enter the odds for every outcome and see exactly how much edge is baked in, and what fair odds would look like.
A low margin isn't a good bet — it's a fairer price. This shows how much edge is built into the book, which is a useful way to compare where you're getting value. It says nothing about which outcome will happen; the true probabilities are still yours to judge.
The worked example
A two-way market priced 1.95 and 1.95: implied probabilities 51.3% + 51.3% = a 102.6% book, a 2.6% overround and a 2.5% margin — roughly what a sharp exchange charges. The fair odds are each 2.0. Price the same market 1.83 and 1.83 and the book jumps to 109.3% — a 9.3% overround, high-street territory, and far worse value.
Straight answers
What is the bookmaker's margin?
The margin — also called the overround or vig — is how much the odds add up to over 100%. Add every outcome's implied probability (1 ÷ its decimal odds); the amount above 100% is the bookmaker's built-in edge, the reason the book pays out less than a fair price.
How do you calculate the overround?
Overround = (sum of 1 ÷ odds for every outcome − 1) × 100%. A two-horse market priced 1.95 and 1.95 sums to 102.6%, a 2.6% overround. The higher the number, the worse the value.
What is a good overround?
Lower is better for the bettor. Betting exchanges typically run close to 100–102% because traders compete the margin away; high-street bookmakers often sit at 105–120%, and accumulator or novelty markets far higher. It's one honest way to compare where you're getting value.
What are fair odds?
Fair odds strip the margin out, so all outcomes sum to exactly 100%. They're each outcome's price multiplied by the book percentage — a useful benchmark for how much edge you're conceding, though the true probabilities are still a matter of judgement.
Measuring the margin is arithmetic. Judging the true probabilities is the whole game.
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18+ · Education, not betting advice · No tips, no picks · BeGambleAware.org
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