Kelly Criterion Calculator
Calculate the mathematically optimal bet size to maximize long-term bankroll growth. The Kelly Criterion balances potential profit against risk of ruin.
Inputs
Your estimated true probability of winning
Results
Enter valid odds, probability, and bankroll to see results.
Frequently Asked Questions
What is the Kelly Criterion?
The Kelly Criterion is a mathematical formula that determines the optimal bet size to maximize long-term bankroll growth. Developed by John Kelly at Bell Labs in 1956, it calculates the fraction of your bankroll to wager based on your edge and the odds offered.
Why use fractional Kelly?
Full Kelly betting can lead to large swings and is very sensitive to probability estimation errors. Half Kelly reduces variance by ~75% while giving up only ~25% of the growth rate. Most professional bettors use quarter to half Kelly.
When should I not use Kelly?
Kelly assumes you know the true probability accurately, which is rarely the case. If your probability estimates are unreliable, Kelly can lead to over-betting. It also assumes bets are independent — correlated bets require adjusted calculations.