The central idea
Expected value is a probability-weighted average. Actual profit is the realised outcome from a particular bet or sample.
EV per unit = p × decimal odds − 1
Worked example
At odds of 2.10 and an estimated 52% chance, EV is +0.092 per unit, or +9.20 units per 100 staked. The next result is still a win or loss—not a guaranteed 9.20-unit profit.
Limits of the calculation
Positive EV depends on the probability estimate being accurate. Variance can produce losses across short and substantial samples even when the model is sound.