Free Kelly Criterion Calculator: Optimal Bet Sizing

How It Works

Enter the American odds you are getting, your honest estimate of the win probability, and your current bankroll. Pick a Kelly fraction, and the calculator shows you:

• Recommended bet in dollars

• Percent of bankroll to wager

• Full Kelly % for reference

• Your edge and the break-even win rate the odds require

If your win probability does not beat the break-even, the calculator tells you to bet nothing. That is Kelly working correctly, not a glitch.

What the Kelly Criterion Actually Does

The Kelly Criterion is a formula for sizing bets in proportion to your edge. Bet too little and you leave growth on the table. Bet too much and a normal losing streak can wipe you out. Kelly finds the stake that maximizes long-term bankroll growth. The formula is edge / odds: your expected profit divided by the net decimal odds. The bigger your edge and the longer the odds, the more it tells you to bet, but always as a percentage of your current bankroll, so your bet sizes shrink automatically when you are losing.

Why Most Bettors Use Half or Quarter Kelly

Full Kelly maximizes growth on paper, but it assumes your win probability is exactly right, and it swings hard. A single bad run at Full Kelly can cut a bankroll in half. Because nobody estimates their true edge perfectly, most disciplined bettors run Half or Quarter Kelly. You give up a little theoretical growth for far smaller drawdowns and a much smoother ride. Getting the win probability honest matters more than the fraction, so lean conservative when you are unsure. Pair this with sound bankroll management and it becomes a real long-term edge.