Kelly Criterion Calculator

The Kelly Criterion is a mathematical formula that calculates the optimal percentage of your bankroll to wager on a bet with positive expected value. It maximizes long-term bankroll growth while minimizing the risk of ruin. The formula is: Kelly % = (bp - q) / b, where b is the decimal odds minus 1, p is the win probability, and q is the loss probability.

Full Kelly can lead to large, volatile swings in your bankroll. Most professional bettors use half or quarter Kelly to reduce variance while still capturing most of the long-term growth. Half Kelly achieves about 75% of the growth rate with significantly less volatility. Quarter Kelly is even more conservative and popular among bettors with smaller bankrolls.

A Kelly percentage of zero (or negative) means the bet does not have positive expected value at the given odds and probability. The Kelly Criterion will never recommend betting on a -EV wager. If you see 0%, it means you should not place the bet.

The Kelly Criterion is very sensitive to probability estimates. Even small errors can lead to over-betting. This is another reason fractional Kelly is recommended — it provides a buffer against estimation errors. Always use the best available data (such as devigged sharp lines) for your probability inputs.