Expected value combines every possible result using its estimated probability. For a £10 bet at decimal odds of 3.00 with a 40% win estimate, the potential profit is £20 and the potential loss is £10.
The probability-weighted calculation is:
The price of 3.00 has a break-even probability of 33.33%. The example is positive only because the entered estimate of 40% is higher. That does not prove the estimate is accurate or predict the result of one bet.
Small changes to an estimated probability can reverse the result. Record how the estimate was made, compare it with later evidence and use prices that were genuinely available.
Use the odds converter if the available price is fractional, or the bookmaker margin calculator to inspect a complete market before estimating fair probabilities.