The Kelly criterion tells you what fraction of your bankroll to stake on a bet, given the odds and your estimated chance of winning. It maximises long-term bankroll growth while mathematically protecting you from ruin. Enter three numbers — decimal odds, your win probability, and your bankroll — and this calculator returns the exact stake, plus the full-Kelly percentage and your edge.
How the Kelly criterion works
Kelly answers one question: how much should you risk when you have an edge? Bet too little and your bankroll grows slowly; bet too much and variance can wipe you out even when every bet is +EV.
The formula is f = (bp − q) / b, where b is the decimal odds minus 1 (your net winnings per unit staked), p is your win probability, and q is 1 − p. The result f is the fraction of your bankroll to stake. If it comes out zero or negative, you have no edge and Kelly says do not bet.
Because full Kelly is aggressive and your probability estimates are never perfect, most disciplined bettors use a fraction of it — half or quarter Kelly. That cuts your bankroll volatility sharply while keeping most of the growth, which is why this calculator lets you toggle between full, half, and quarter.
Worked example
Say you find a bet at decimal odds of 2.00 (even money) and you estimate it wins 55% of the time. Here b = 1, p = 0.55, q = 0.45.
Full Kelly = (1 × 0.55 − 0.45) / 1 = 0.10, or 10% of your bankroll. On a $1,000 bankroll that is a $100 full-Kelly stake. Your edge is (2.00 × 0.55 − 1) = +10%.
Most bettors would not stake the full 10%. Half Kelly drops it to 5% — a $50 stake — for far smoother swings with almost the same long-run growth. That is exactly what the calculator shows with its default inputs.