Kelly criterion calculator
Updated August 2026
How much of your bankroll to stake when you think you have an edge — and when to stake nothing.
10.0%
full Kelly stake
The idea in one breath
Kelly sizes your stake to your advantage: fraction = edge ÷ odds. Think you win 55% at even money? Your edge is 10 points, even money pays 1-to-1, so stake 10% of your bankroll. The deep result: this sizing grows a bankroll faster, long-run, than any other, including braver ones.
The catch, in the next breath
The formula trusts your probability completely. Feed it overconfidence and it will size your overconfidence optimally. Since humans run hot, calibration is the real prerequisite, which is exactly the skill a play-money market lets you measure before any bankroll is on the line.
Quick answers
What is the Kelly criterion?+
A formula for bet sizing that maximizes long-run bankroll growth: stake the fraction of your roll equal to your edge divided by the odds. Bigger edge, bigger stake; no edge, no bet.
Why do people bet half Kelly?+
Full Kelly assumes your probability estimate is exactly right, and it never is. Half Kelly keeps most of the growth with far less violence, which is why most professionals treat full Kelly as a ceiling, not a target.
What if the calculator says 0%?+
It means your own estimate gives you no edge at this price, the odds already charge more than your chance is worth. Kelly's most valuable output is the word no.