Calculate the mathematically optimal bet size for any edge with the PromoGrind Kelly Criterion Calculator — maximize long-term bankroll growth without overbetting.
Use the calculator →The Kelly Criterion is a bankroll management formula developed by John L. Kelly Jr. in 1956. It calculates the optimal fraction of your bankroll to stake on a bet with a known edge, maximizing the long-term geometric growth rate of your bankroll while avoiding the risk of ruin that comes with overbetting.
Where f* is the fraction of bankroll to bet, b is the decimal odds minus 1 (the net odds received), p is your estimated probability of winning, and q is the probability of losing (1 − p). A positive result means you have an edge and should bet; a negative result means the bet has negative expected value and should be avoided.
Full Kelly is theoretically optimal but produces large variance in practice. Even a small overestimate of your edge can lead to aggressive bet sizes and sharp bankroll drawdowns. Most professional bettors use 25–50% of the full Kelly recommendation — known as quarter or half Kelly — which reduces variance dramatically while preserving the majority of the long-term growth advantage.
Kelly sizes bets proportionally to your edge. A 2% edge on even-money odds calls for a 2% bankroll bet. A 5% edge calls for a 5% bet. The stronger your edge, the larger the recommended stake — but the formula is quick to punish overconfident probability estimates, which is why honest, calibrated win probability inputs are essential.