Kelly Criterion Calculator
Enter your odds, the true probability, your bankroll, and choose a Kelly fraction to calculate the optimal stake size.
What is the Kelly Criterion?
The Kelly Criterion is a formula developed by John Kelly at Bell Labs in 1956. It calculates the mathematically optimal percentage of your bankroll to stake on a bet, given the odds and your estimated edge. It maximises long-term growth while minimising the risk of ruin.
Full Kelly vs Fractional Kelly
Full Kelly betting is aggressive — it maximises theoretical growth but comes with brutal variance. A single bad streak can wipe out a huge chunk of your bankroll. That's why most professional bettors use fractional Kelly (typically quarter Kelly or half Kelly). You give up a small amount of expected growth in exchange for dramatically smoother results.
The Formula
Kelly % = (bp − q) / b, where b = decimal odds − 1, p = true probability of winning, and q = 1 − p. If the result is negative, you shouldn't bet at all — the odds don't offer value.
Important Caveats
The Kelly Criterion assumes you know the true probability exactly — which you never do. Any overestimation of your edge leads to over-betting. That's another reason to use fractional Kelly. It also assumes you can bet fractions of a unit, and that each bet is independent. Use it as a guide, not gospel.