Kelly Criterion Calculator

The Kelly criterion is a mathematical formula that determines the optimal percentage of your bankroll to wager on a bet with positive expected value. It maximizes the long-term growth rate of your bankroll while accounting for both edge and odds.

Kelly % = (p × b − q) / b, where p = win probability, q = loss probability (1 − p), and b = net decimal odds (decimal odds minus 1). For example, with 55% win probability and 2.10 odds: Kelly = (0.55 × 1.10 − 0.45) / 1.10 = 14.1%.

Fractional Kelly means betting a fraction (typically 25-50%) of the full Kelly recommendation. Full Kelly maximizes growth but causes large swings. Half Kelly reduces variance by 75% while keeping 75% of the growth rate. Most professionals use quarter to half Kelly.

Don't use Kelly when your probability estimates are unreliable, when you're betting correlated events (like parlays), or when the Kelly percentage exceeds 20-25% of your bankroll (usually means your edge estimate is too aggressive). In these cases, use a conservative fractional Kelly.