Kelly Criterion Calculator

Enter the odds you're being offered, your estimated true win probability, and your bankroll. This calculator returns the full and fractional Kelly percentages and the exact recommended stake — the bet size that grows a bankroll fastest without over-betting your edge.

How the Kelly formula works

Kelly answers a single question: given an edge, what fraction of your bankroll maximizes long-run growth? Bet too little and you leave money on the table; bet too much and a bad streak wipes you out. Kelly finds the mathematically optimal middle.

Say you take +150 (decimal 2.50, so b = 1.5) on something you believe wins 50% of the time. Full Kelly is (1.5 × 0.5 − 0.5) / 1.5 = 16.7% of your bankroll. On a $1,000 roll at quarter Kelly (0.25) that's a $41.67 stake, and your edge is 25 cents of expected value per dollar risked.

Why almost everyone bets fractional Kelly

Full Kelly is optimal only if your win probability is exactly right. In real betting it never is — you're estimating. Overestimate your edge and full Kelly over-bets brutally, producing gut-churning swings and real risk of ruin. Betting a quarter or half Kelly cuts volatility dramatically while keeping most of the growth, which is why nearly every professional uses a fraction. Read the full reasoning in the Kelly criterion guide.

Garbage in, garbage out

Kelly is only as good as the win probability you feed it. Enter the book's own implied probability and Kelly always returns zero — there's no edge over the price you're taking. The number you want is the true probability, which sharps get by removing the vig from a sharp line. Confirm a bet is +EV first, then let Kelly size it.

Iron Marker is an analytics tool, not a sportsbook, and this calculator is educational — not betting or financial advice. Must be 21+. Problem gambling? Call 1-800-GAMBLER.