How to use it
- Pick 2 outcomes (tennis or basketball moneyline, handicaps and totals) or 3 (1X2 with the draw in football).
- Enter the reference bookmaker's odds for the same market and line.
- To check whether a bet has value, add the price your bookmaker offers: you'll see its EV.
What are fair odds?
Every price hides an implied probability: 1 / odds. In a market with a margin those probabilities add up to more than 100%, and the excess is the bookmaker's margin (the vig). Fair odds are what's left once that margin is removed: the true probability according to the most efficient market.
Power vs multiplicative: why it matters
The multiplicative method spreads the margin in proportion to each probability. It's simple but wrong on longshots: bookmakers load more margin onto them (the favourite–longshot bias), so a proportional split leaves them with too high a probability.
The power method corrects that bias. Example at odds of 1.25 and 4.20 (3.81% margin): the multiplicative method gives the longshot fair odds of 4.36, the power method 4.685. If your bookmaker pays 4.50, the multiplicative method shows a +3.2% value bet; the power method says −3.9%. No value.
How to calculate the value of a bet
Expected value (EV) = your bookmaker's odds × fair probability − 1. If it's positive, the bet has value. At odds of 2.10 with a fair probability of 50%, EV = 2.10 × 0.50 − 1 = +5%. The fair price (1 / probability) is the minimum price where value starts.
Common mistakes
Using a recreational bookmaker as the reference: its price carries more margin and less information. Mixing different lines (−0.5 against −0.75). Comparing with a reference price that has already moved. Forgetting that value is a long-run concept: value bets lose too.