How to calculate a bookmaker's margin
Published on 29 July 2026
A bookmaker's margin is its commission — but unlike brokerage fees, it is written nowhere: it is hidden inside the odds. The good news is that it can be computed in three operations, with nothing more than a basic calculator.
The three-step method
Step 1 — convert each odd into an implied probability: probability = 1 ÷ decimal odd. Step 2 — add up the implied probabilities of every outcome of the market. Step 3 — derive the RTP and the margin: RTP = 100 ÷ total × 100, and margin = 100 − RTP.
Two-outcome example (tennis)
Player A at 1.80, player B at 1.95.
1÷1.80 = 55.6%; 1÷1.95 = 51.3%. Total: 106.8%.
RTP = 100 ÷ 106.8 × 100 = 93.6% → margin: 6.4%.
Three-outcome example (football, 1X2)
Home 2.10, draw 3.40, away 3.60.
47.6% + 29.4% + 27.8% = 104.8%.
RTP = 100 ÷ 104.8 × 100 = 95.4% → margin: 4.6%.
The method is identical whatever the number of outcomes: a 30-runner outright market is computed exactly the same way, by adding the 30 implied probabilities.
What makes the margin vary
Measure the margin on a handful of markets and patterns quickly emerge. The competition: the Premier League and other top leagues are priced tighter than lower divisions or exotic competitions, where the margin can be several times higher. The market type: in football, for example, the main 1X2 is usually the most competitive market of a match; side markets (goalscorers, corners, correct scores, handicaps) often carry substantially higher margins. Timing: far from kick-off, uncertainty is higher and margins wider; they tighten as the match approaches. The event: on some very big fixtures, operators temporarily compress their margin for commercial reasons. To measure those gaps on the markets you actually follow, the RTP Calculator does the maths in seconds, whatever the number of outcomes.