Kelly Criterion Bet Size Calculator

About this calculator

The Kelly criterion calculates the fraction of your bankroll to stake on a bet that maximizes the long-run growth rate of your capital, given your true edge. The formula is f* = p − q/b, where p is your win probability, q is the loss probability (1 − p), and b is the net odds received on the bet (payout multiple minus 1). A positive f* means the bet has positive expected value at those odds; a zero or negative result means you should not bet at all.

Sports bettors, poker players and some systematic traders use the Kelly criterion because betting more than the Kelly fraction increases the risk of large drawdowns and ruin even when the bet has a real edge, while betting less than Kelly (often "half Kelly" or "quarter Kelly" in practice) trades some growth for a smoother equity curve. Full Kelly is aggressive and assumes your probability estimate is accurate, which is rarely perfectly true, so many practitioners deliberately stake a fraction of the calculated amount.

This calculator takes your estimated win probability and the payout odds, and returns the Kelly-optimal stake as a percentage of your bankroll.