How Bookmaker Margin Works
Last reviewed 24 August 2026 · Written and checked by the bet.ing editorial team
Bookmaker margin is the amount by which a market's implied probabilities exceed 100%. A market totalling 105% carries 5% overround, which is charged on every bet placed into it. Lower margin means better prices, and it compounds across every bet you make.
How to measure it
Convert each outcome to implied probability and add them up. In a two-way market at 1.91 / 1.91 the total is 52.36% + 52.36% = 104.72%. The overround is 4.72%; expressed as a share of the whole book, the margin is 4.51%.
The distinction matters when you compare markets with different numbers of outcomes. Margin-as-a-share is the more honest figure because it answers "what fraction of money staked does the book keep?"
| Margin | Expected return per 100 staked | After 500 bets of 10 |
|---|---|---|
| 2.0% | 98.00 | −100.00 |
| 4.5% | 95.50 | −225.00 |
| 7.0% | 93.00 | −350.00 |
| 12.0% | 88.00 | −600.00 |
| 20.0% | 80.00 | −1,000.00 |
Why margin varies by market
Bookmakers charge more where they know less and where customers care least about price. A Premier League match-winner market might run at 4%; a player-props market or a 30-runner outright can run above 20%. The headline prices look exciting precisely because the margin is enormous.
Margin is also loaded unevenly. Longshots typically carry more of it than favourites, which is the mathematical face of the well-documented favourite–longshot bias.
Comparing bookmakers properly
- Compare the same market at the same moment, not prices from different times.
- Compare full markets, not single selections — one generous price can hide a fat book.
- Use the margin calculator and compare the total percentage, not individual odds.
What it means for you
Margin is the reason breaking even requires being right more often than the price implies. It is charged silently and continuously. Reducing the margin you pay — by betting lower-margin markets and shopping prices — is the only edge available to a bettor with no forecasting advantage at all.