Implied probability and bookmaker margin calculator
Go from odds to probability and from probability to odds. Enter a full market to see how much margin the bookmaker keeps and what the fair odds would be.
What implied probability is
Every price contains a probability: the one the outcome would need for the bet neither to make nor lose money in the long run. Odds of 2.50 imply 40%. If you think the outcome happens more often than that, the price looks high to you; if you think it happens less often, it looks low.
It is the basic tool for comparing your opinion with the price. Saying “this team will win” does not help you decide; saying “I think they win 45% of the time and the odds imply 40%” is something you can measure later.
The formulas
Implied probability = 1 ÷ decimal odds × 100
Odds = 100 ÷ probability in %
Bookmaker margin = sum of the implied probabilities of all outcomes − 100
In a market with no margin, the probabilities would add up to exactly 100%. Bookmakers offer slightly lower odds so that they add up to more. That excess is their margin, also known as the overround.
1X2 example
A match with odds of 2.10 / 3.40 / 3.60:
- 1: 1 ÷ 2.10 = 47.62%
- X: 1 ÷ 3.40 = 29.41%
- 2: 1 ÷ 3.60 = 27.78%
- Sum: 104.81%, that is, a margin of 4.81%
To estimate the probability without the margin, divide each one by the sum: 45.43%, 28.06% and 26.50%. The equivalent fair odds would be 2.20, 3.56 and 3.77. This proportional split is the simplest method, though not the only one: in practice, bookmakers do not always spread the margin evenly between favourites and outsiders.
2-way example
In a two-outcome market at 1.90 / 1.90, each side implies 52.63%. The sum is 105.26%, a margin of 5.26%, and the fair price would be 2.00. So when betting at 1.90 you need to win more than 52.63% of the time just to break even.
Common mistakes
- Confusing implied with true probability. The implied figure includes the margin, so it overstates the probability the bookmaker assigns to the outcome.
- Comparing without looking at the full market. A single price does not tell you the margin. You need every outcome in the market.
- Forgetting that the margin compounds. On a parlay it multiplies, as explained in the parlay calculator.
- Assuming it sums to 100. If a market across two different bookmakers sums to less than 100%, you may have a surebet.
Do you have odds in another format? Run them through the odds converter first. Implied probability is also the basis for working out a Kelly stake and for understanding CLV.
Frequently asked questions
How do I convert odds to a percentage?
Divide 100 by the decimal odds. Odds of 1.50 imply 100 ÷ 1.50 = 66.67%.
What margin is normal for a bookmaker?
It varies widely by bookmaker, sport and market. Main markets in the big leagues tend to carry less margin than secondary markets or small leagues. Check by entering their odds into the calculator.
Why do probabilities add up to more than 100%?
Because the bookmaker builds its margin into the odds. That difference above 100% is what it expects to keep on average.
Are fair odds the true probability?
Not necessarily. They are an estimate that removes the market’s margin. It is still the market’s opinion, not a certainty about what will happen.