Free tool
Kelly criterion bet sizer
Calculate the optimal fraction of your bankroll to wager based on your edge and the odds, for a sportsbook line or a prediction-market price. Full, half, and quarter Kelly, with an overbetting warning.
Kelly criterion bet sizer
Your total betting bankroll
Your estimated chance of winning
The odds you're getting
Kalshi / Polymarket price (overrides odds)
Kelly maximizes long-run growth · assumes accurate probability estimates
How Kelly sizes a bet
Enter the odds, your estimated win probability, and your bankroll. Kelly returns the stake that maximizes long-term growth. The bigger your edge and the longer the price, the more it tells you to bet.
Why most pros bet a fraction of Kelly
Full Kelly is the growth-optimal size, but it's volatile: a bad run can halve your bankroll. Half- and quarter-Kelly give up a little growth for a much smoother ride, which is why most disciplined bettors use them. The tool warns you when a stake gets aggressive.
Frequently asked
What is the Kelly Criterion?
A formula that calculates the optimal fraction of your bankroll to wager given your edge and the odds. It maximises long-term growth while avoiding ruin. Many sharp bettors use half or quarter Kelly to reduce variance while still sizing bets proportional to their edge.
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