Analyse a market
Two outcomes minimum. Add as many rows as the market has.
Market result
- Book percentage
- 104.81%
- Overround
- +4.81%
- Theoretical payout
- 95.41%
- Theoretical margin
- 4.59%
Warning:
Margin removed by proportional normalisation. The overround measures the excess built into the prices, not the bookmaker's profit.
Add as many outcomes as the market has, enter their decimal odds and read the book percentage, the overround, the theoretical payout and the estimated margin-free odds of every outcome straight away. The margin removal applied is proportional normalisation.
How does this calculator work?
The tool converts every entered price into an implied probability, adds those probabilities up to obtain the market's book percentage, then redistributes the excess to estimate margin-free odds per outcome. Everything is calculated in the browser, on every keystroke.
The number of outcomes is free: two for a binary market, three for a 1X2, more for a market with multiple outcomes. The preloaded templates are a shortcut, they do not restrict anything.
The formulas applied
Four formulas are enough to produce every result shown.
implied probability = 1 ÷ decimal odds
book percentage = sum of the implied probabilities
overround = book percentage − 100%
normalised probability = implied probability ÷ book percentage
The estimated margin-free odds of an outcome are the inverse of its normalised probability. The market's theoretical payout is 1 ÷ book percentage, and the theoretical margin 1 − payout.
A complete example
On a 1X2 market priced 2.10 / 3.40 / 3.60, the implied probabilities are 47.62%, 29.41% and 27.78%. Their sum reaches 104.81%: the book percentage. The overround is therefore 4.81 points.
The theoretical payout comes to 95.41%, and the theoretical margin to 4.59%. After normalisation, the probabilities become 45.43%, 28.06% and 26.50%, that is estimated margin-free odds of 2.20, 3.56 and 3.77.
Those are the figures the tool above produces with the preloaded 1X2 template: they act as a sanity check if a displayed result looks suspicious to you.
Overround and theoretical margin are not the same measure
The overround is expressed in points above 100%, the theoretical margin as a percentage of stakes. On a book at 104.81%, the overround is 4.81 points, but the theoretical margin 4.59%.
The gap comes from the denominator: the overround relates the excess to 100%, the theoretical margin relates it to the full book percentage. Both are correct, they simply do not answer the same question. In everyday language they are often used for one another — which does not matter for an order of magnitude, but does as soon as markets are compared with each other.
Neither measure is a profit. An operator keeps the full overround only if stakes are spread exactly along the implied probabilities, which never happens. That distinction is set out in detail on the bookmaker margin page.
The margin removal method applied
This calculator applies proportional normalisation: every implied probability is divided by the market total.
It is the easiest method to verify by hand, and it is explicitly identified in the code rather than hidden inside an opaque calculation. It assumes the margin is spread evenly across outcomes — a convenient assumption, not a demonstrated one.
Other methods — power, logarithmic, Shin, favourite-longshot bias correction — redistribute the excess differently and produce different margin-free odds from the same starting prices. The gap stays small on a balanced market and becomes significant as soon as a heavy favourite faces a heavy underdog. The comparison of methods and their limits are detailed on margin-free odds.
What the result does not say
Three limits to keep in mind before reading anything into a figure shown here.
A low margin does not indicate a good bet. It indicates prices that are tighter overall at that operator, on that market, at that moment. It says nothing about the accuracy of its sporting estimate.
Margin-free odds are not a real probability. They are an estimate derived from observed prices, dependent on the redistribution method. The real probability of a sporting event is observable by nobody.
An isolated calculation only describes a moment. Odds move: the margin of a market three days out and that of the same market an hour before kick-off often differ, as the guide on odds movements shows.
To convert a single price into another format, the companion tool is the odds converter.
Frequently asked questions
Is this tool free and account-free?
Yes. The calculation runs in the browser, with no account, no data sent and no record of the odds you enter.
Which margin removal method does the calculator use?
Proportional normalisation: every implied probability is divided by the market total. Other methods exist and give different results on the same odds.
How is this different from the bookmaker margin page?
This page calculates, the other one explains. The bookmaker margin page sets out the formulas, the definitions and the examples; this tool applies the calculation to your own odds.
Does the overround equal the bookmaker's profit?
No. The overround measures the excess built into the prices. Real profit depends on the stakes received, how they are spread, the results and the operator's costs.
Can I enter odds with a comma?
Yes. Comma and dot are accepted interchangeably: 2,10 as well as 2.10.
Sources & method
This calculator applies the standard decimal odds conversion formulas and the proportional normalisation of probabilities, a method documented on the "margin-free odds" page.
- Implied probability of an outcome = 1 ÷ decimal odds.
- Book percentage = sum of the market's implied probabilities; overround = book percentage − 100%.
- Theoretical payout = 1 ÷ book percentage; theoretical margin = 1 − payout.
- Normalised probability = implied probability ÷ book percentage; margin-free odds = 1 ÷ normalised probability.