The Kelly Criterion tells you the bet size that maximizes long-run bankroll growth given a real edge. Enter your bankroll, your honest estimated win probability, and the offered American odds. Most pros use half or quarter Kelly to soften variance. Last updated: May 2026.
Formula: f = (b × p − q) ÷ b, where b = decimal odds − 1, p = your win probability, and q = 1 − p. If f comes out negative, you have no edge and the right bet size is zero.
A formula developed by John Kelly in 1956 that gives the bet fraction maximizing long-run logarithmic growth of a bankroll. It assumes you have a known edge and can rebet repeatedly.
Full Kelly is mathematically optimal but extremely volatile — a string of losses can cut your bankroll in half. Half Kelly captures roughly 75% of the growth with much lower variance, which is why most professional bettors use it.
Kelly is brutally sensitive to bad inputs. Overestimating your edge by even a small amount can flip the recommendation from "bet" to "ruin." When in doubt, use a smaller fraction of Kelly or skip the bet.
It means the bet is -EV — your estimated win probability is lower than the implied probability of the odds. Kelly's correct recommendation is zero. Don't bet.
The basic single-bet Kelly formula assumes one wager at a time. For simultaneous bets, you need fractional Kelly across the slate or a more advanced "Kelly portfolio" model.