Implied probability, bookmaker margin and fair odds calculator

Every price is a probability. Decimal odds of 2.00 mean the bookmaker is pricing the outcome at 50% (1 ÷ 2.00). Add up the implied probabilities of every outcome in a market and the total comes to more than 100%: the excess is the bookmaker’s margin, or overround. Take the margin out and what is left is the fair price. Enter the prices for one market — decimal (2.10) or fractional (11/10); for a two-way market such as over/under 2.5 goals or both teams to score, enter two and leave the third box empty.

How is implied probability calculated?

Implied probability = 1 ÷ decimal odds. For fractional odds a/b, the decimal price is 1 + a/b, so 11/10 is 2.10 and implies 47.6%. Dividing each implied probability by the market’s total removes the margin and gives the bookmaker’s underlying view, summing to 100%.

How are fair odds calculated?

Fair odds = 1 ÷ the probability without the margin. At 2.10, 3.40 and 3.60 the prices add up to 104.8%, so the home win’s 47.6% becomes 45.4% without the margin and its fair odds are 2.20. This calculator spreads the margin across the outcomes in proportion to their implied probabilities, the simplest method; other methods put more of it on the outsiders, so read a long shot’s fair price as a guide.

TACTIX compares these market probabilities with its own model’s: see the model vs bookmaker leaderboard and how accurate TACTIX is. For stakes and returns, use the betting calculator; for how odds work, football odds explained.

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18+ · Predictions are probabilities, not certainties, and nothing here is betting advice. Please gamble responsibly — BeGambleAware.org.