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.
Frequently asked questions
What is the Kelly criterion formula for betting?
The Kelly stake as a fraction of your bankroll is f = (b × p − (1 − p)) / b, where p is your estimated win probability and b is the net decimal payout (decimal odds minus 1). If the result is negative there is no edge and Kelly recommends no bet.
Why do bettors use fractional Kelly?
Full Kelly maximizes long-run growth but is extremely volatile and assumes your win probability is exact — which it never is. Because your probability estimate is uncertain, most sharp bettors use a fraction such as one-quarter or one-half Kelly, which sharply reduces drawdowns and the damage from overestimating your edge, at a small cost to growth.
What win probability should I enter?
Use your best estimate of the true probability that the bet wins — not the probability the book's price implies. Most +EV bettors derive it by removing the vig from a sharp line such as Pinnacle. If you only enter the book's own implied probability, Kelly will always return zero, because there is no edge over the price you are taking.
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