Tennis odds and implied probability: why price matters
Convert decimal, fractional and American odds into implied probability, and understand why win rate alone is insufficient.
The same winner can be a different decision
A match forecast and a betting decision answer different questions. A forecast estimates who is more likely to win. A betting decision also considers the price, uncertainty and costs. A player can be the more likely winner and still offer an unattractive price. None of the examples below is an official selection.
Convert the price
For decimal odds d, raw implied probability is 1 ÷ d. At 1.80 it is 55.56%; at 2.00 it is 50%; at 2.20 it is 45.45%. Fractional odds 4/5 correspond to decimal 1.80. American odds +120 correspond to decimal 2.20; −125 correspond to 1.80. These are mathematical equivalents, not currently available bookmaker quotations.
Account for the bookmaker margin
Suppose both players are offered at 1.91. Each price implies 52.36%, so the total is 104.71%. The excess over 100% is the overround. Dividing each raw probability by their total produces one simple proportional margin adjustment. That adjustment is a convention, not a uniquely correct estimate of the true probabilities.
A conditional expected-value example
If a model estimates a 60% win probability and decimal odds are 1.80, expected net return before costs is (0.60 × 1.80) − 1 = 0.08, or 8% per unit staked. This expectation depends on the model estimate being accurate. It is not a prediction that a particular bet or a short series will return 8%.
The practical implication
Record the actual price and time. Reassess a decision when the price moves: at 1.60, the same hypothetical 60% forecast produces −4% expected return before costs. Commission, taxes where applicable, liquidity and membership fees can change the economics. Do not increase stakes to make a fee feel worthwhile. A useful process can conclude that no bet qualifies.
Read the free execution guide · All research notes
18+ and subject to local requirements. Betting involves risk of loss. Past performance does not guarantee future results. Examples are hypothetical and exclude costs unless stated. No investment or asset-outperformance claim is made.