Convert every price in the market to an implied probability and add the results. A total of 106% is a 106% book with a 6% overround. You must include every possible outcome for the result to describe the complete market.
What the margin-free column does
The tool divides each implied probability by the total book percentage. That scales the complete set back to 100% and produces one simple estimate of fair probability and fair odds.
This proportional method assumes the margin is spread evenly in relative terms. Real bookmakers may distribute margin unevenly, especially across favourites and outsiders, so the fair prices are analytical estimates rather than predictions.
| Measure | Meaning | Use |
|---|---|---|
| Book percentage | Sum of every implied probability | Checks the complete market total |
| Overround | Book percentage minus 100% | Shows the headline margin built into the prices |
| Margin-free probability | Implied probability normalised to a 100% total | Creates a comparable benchmark |
Worked football 1X2 example
| Outcome | Decimal odds | Implied probability |
|---|---|---|
| Home | 2.10 | 47.62% |
| Draw | 3.40 | 29.41% |
| Away | 3.60 | 27.78% |
| Total book | 104.81% |
The book percentage is 104.81%, so the overround is 4.81 percentage points. A two-outcome market priced at 1.91 on both sides has implied probabilities of 52.36% and 52.36%, producing a 104.71% book and 4.71% overround.
Overround is not the bookmaker's realised profit
Overround describes the prices before the result and customer stakes are known. Theoretical hold is often estimated by dividing the overround by the book percentage, but actual profit depends on the distribution of stakes, price changes, promotions, liabilities, deductions and the winning outcome.
Bookmaker overround questions
How do you calculate bookmaker overround?
Convert every decimal price to an implied probability using 100 divided by the odds. Add every outcome and subtract 100 percentage points from the total.
What percentage does a bookmaker take?
No single percentage applies to every market or completed book. Overround is a price-based margin measure, not proof of what the operator will earn from the actual stakes placed.