Expected value (EV) measures the average amount a bet would return per unit staked if it could be repeated indefinitely under the same conditions. It combines the size of each possible outcome with the probability of that outcome occurring, producing a single figure that summarises whether a wager is mathematically favourable.
The basic formula multiplies the probability of winning by the amount won, then subtracts the probability of losing multiplied by the amount staked: EV = (probability of win × profit if win) − (probability of loss × stake lost).
Suppose you stake 10 on a hypothetical selection at decimal odds of 3.00, giving a profit of 20 if it wins and a loss of 10 if it loses. If you estimate the true probability of winning at 40%, the expected value is (0.40 × 20) − (0.60 × 10) = 8 − 6 = +2. A positive figure of +2 per 10 staked suggests the bet is favourable according to your probability estimate.
The crucial distinction is that expected value is a long-run average, not a prediction of any single result. A bet with positive EV can still lose, and a negative-EV bet can still win. The value only expresses itself across a large number of similar wagers, which is why sample size and consistency matter.
Expected value depends entirely on the accuracy of the probability estimate. Bookmakers price markets with a built-in margin, so the odds imply probabilities that sum to more than 100%. A bettor only achieves positive EV when their own probability assessment is both more accurate than the market and favourable enough to overcome that margin. Because odds and margins differ between operators, the same outcome may be positive-EV at one site and negative-EV at another.