How to Use the Kelly Criterion for Sports Betting

The Kelly Criterion is a mathematical formula developed by John L. Kelly Jr. in 1956 that determines the optimal size of a bet based on your perceived edge over the bookmaker. Unlike flat staking where you bet the same amount regardless of confidence, Kelly dynamically adjusts your stake — betting more when your edge is larger and less when it is marginal.

The formula is: Kelly % = (bp − q) / b, where b is the net decimal odds (decimal odds minus 1), p is your estimated probability of winning, and q is the probability of losing (1 − p). If the result is negative, the Kelly Criterion is telling you there is no edge and you should not bet.

Full Kelly vs Half Kelly vs Quarter Kelly

In practice, most serious punters use Half Kelly or Quarter Kelly rather than Full Kelly. Full Kelly maximises long-term growth but produces significant bankroll swings that most people find uncomfortable. Half Kelly delivers roughly 75% of the long-term growth rate with substantially reduced variance, making it the most common choice among professional bettors.

Practical Example

Say you believe a team has a 55% chance of winning and the bookmaker offers 2.10. Enter 55% as your win probability, 2.10 as the odds, and your bankroll amount. The calculator shows your edge (the gap between your estimated probability and the implied probability) and the recommended stake. If the edge is negative, the calculator tells you not to bet — the bookmaker has the advantage.