Bookmaker Margin (Overround) Explained: How to Calculate It
Updated 2026-10-025 min read
The bookmaker margin, or overround, is the built-in fee that makes sure the probabilities implied by the odds add up to more than 100%. It is the main reason most bets have negative expected value, and measuring it is the first step toward comparing prices intelligently.
The formula
Convert each decimal odd to an implied probability (1 / odds), then add them all up. The amount above 100% is the margin.
Margin = (1/odds₁ + 1/odds₂ + … + 1/oddsₙ) − 1
A two-way example
A tennis match priced 1.90 / 1.90. Each side implies 1 / 1.90 = 52.63%. The sum is 105.26%, so the margin is 5.26%. The fair price of a 50/50 event is 2.00, so each side is shortened from 2.00 to 1.90.
Compare a sharper price of 1.97 / 1.97: 2 × (1 / 1.97) = 101.52%, a margin of only 1.52%. On the same 50/50 event, a bettor who always takes the first price loses about 5% of every stake on average, against about 1.5% on the second. Over hundreds of bets, this difference decides who can win at all.
A three-way example (1X2)
Odds of 2.10 / 3.40 / 3.60 give 47.6% + 29.4% + 27.8% = 104.8%, a margin of 4.8%. To get the fair probabilities, divide each implied probability by the total: 45.4%, 28.1% and 26.5%.
This is the proportional method, the simplest way to remove the margin. Bookmakers often load more margin on outsiders and draws than on favourites, so the proportional method is an approximation, not a perfect answer.
Why it matters
- Lower-margin markets and bookmakers give you a better starting position: you need a smaller edge to be profitable.
- Fair probabilities (margin removed) are the right benchmark to compare against your own model.
- Margins differ across markets: main leagues are usually cheaper than minor leagues, and 1X2 is usually more expensive than Asian handicap lines.
Frequently asked questions
What is the bookmaker margin?
It is the amount by which the implied probabilities of all outcomes exceed 100%. It is the bookmaker's built-in profit margin.
How do you calculate the overround?
Add up 1 divided by the decimal odds for every outcome in the market, then subtract 1. For odds of 1.90 / 1.90 the result is 5.26%.
What is a good bookmaker margin?
Lower is better. Around 2% or less is considered sharp for a two-way market, while 5% or more is typical of recreational bookmakers.
More guides
AlphaBetAI scans the markets with an ensemble of models and publishes a verifiable track record of every graded result.
Explore AlphaBetAI