If you add up the implied probabilities of every outcome of a match, you should get 100%. In reality, you always get more. That excess is the bookmaker margin (also called the overround or vig).
The calculation
Why it matters
The margin is the bookmaker's commission. It's built into every price. In practice, if you bet at random, you lose roughly the value of the margin over the long run.
Ballpark figures:
- "Sharp" bookmakers (who accept big bettors): 2 to 3% on major markets.
- Mainstream bookmakers: often 5 to 8%.
- Exotic markets (goalscorers, correct scores, accumulators): sometimes 15 to 30%.
The link with value betting
The implied probability includes the margin. The "true" probability estimated by the bookmaker is therefore slightly lower. For a bet to have value, your estimate has to beat the odds despite that margin.
At the intermediate level, we'll see how to remove the margin to recover the "fair" probabilities (this is called devigging).
Interactive tool
Margin (overround)
Educational tool. Results are theoretical estimates, not betting advice.