Kelly Criterion Calculator

The Kelly Criterion is a mathematical formula that calculates the optimal percentage of your bankroll to stake on a bet to maximise long-run growth. It was developed by John L. Kelly Jr. in 1956. The formula is: Kelly % = (bp - q) / b, where b is net odds (decimal - 1), p is your estimated win probability, and q is 1 - p.

Full Kelly is mathematically optimal only if your probability estimates are perfectly accurate, which is rarely the case. Fractional Kelly (typically half or quarter) reduces volatility significantly while sacrificing only a small amount of expected growth. Most professional bettors use quarter-Kelly to smooth out variance and protect against estimation errors.

A negative Kelly fraction means there is no mathematical edge in this bet at the given odds and probability. The expected value is negative, and Kelly is telling you not to bet at all. This often happens when bookmaker odds imply a lower probability than your own estimate is giving you, or when you have overestimated your win probability.

Kelly works best for bettors with reliable probability models and large sample sizes. Recreational bettors often find full Kelly too aggressive because errors in probability estimates lead to overbetting. Beginners are typically better served by fixed-unit betting until they have enough data to estimate probabilities with confidence.

Even in markets with apparently large edges, staking more than 25% of your bankroll on a single bet exposes you to catastrophic loss from a single incorrect probability estimate. The 25% cap is a widely used safety ceiling in professional sports betting. Full Kelly can theoretically recommend 50%+ stakes in extreme cases, which carries unacceptable ruin risk in practice.