Betting With The Kelly Criterion

- June 17, 2026

The Kelly Criterion is a mathematical formula used to determine the optimal size of a wager when an advantage exists. Originally developed by John L. Kelly Jr. in 1956, the method has become popular among professional gamblers, investors, and bankroll managers because it seeks to maximize long-term growth while minimizing the risk of ruin.

The basic principle behind the Kelly Criterion is simple: bet more when you have a larger edge and less when your advantage is small. The formula calculates the percentage of your bankroll that should be wagered based on the probability of winning and the odds being offered. By adjusting bet sizes according to both edge and bankroll size, the Kelly approach avoids the common mistakes of overbetting and underbetting.

For example, if a bettor estimates that a coin will land heads 55% of the time and receives even-money odds, the Kelly formula recommends wagering 10% of the bankroll. If the bankroll grows, future wagers increase proportionally; if losses occur, bet sizes automatically decrease.

One of the primary advantages of the Kelly Criterion is its focus on long-term bankroll growth. However, full Kelly betting can produce significant volatility and large drawdowns. As a result, many experienced bettors use a "fractional Kelly" strategy, such as half-Kelly or quarter-Kelly, which reduces risk while still capturing much of the growth potential.

Although no betting system can eliminate uncertainty, the Kelly Criterion provides a disciplined, mathematically sound framework for managing wagers. For bettors with accurate probability estimates, it remains one of the most respected bankroll management methods available.