Kelly Criterion

The Kelly Criterion is a mathematical formula that determines the optimal fraction of your bankroll to stake on a bet, maximizing long-term bankroll growth while minimizing ruin risk.

Kelly Formula

f = (bp − q) / b

Where: b = decimal odds − 1 · p = estimated win probability · q = 1 − p

Example: Odds 2.50, estimated win probability 50%. b=1.5, p=0.5, q=0.5. f = (1.5×0.5 − 0.5) / 1.5 = 0.25/1.5 = 16.7% of bankroll.

Fractional Kelly

Most professional bettors use a fraction of full Kelly (e.g., 25–50%) to account for probability estimation error and reduce variance. EVBets defaults to Quarter Kelly in its stake calculator.

Kelly vs. Flat Staking

Flat staking (same amount per bet) is simpler but sub-optimal. Kelly adapts stake size to your edge: bigger stakes on high-edge bets, smaller on low-edge bets. Over the long run, Kelly outperforms flat staking on +EV bets.

Frequently Asked Questions