Kelly Criterion Calculator: How Much Should You Bet?

Finding an edge is half the job. The other half is betting the right amount: enough to grow your bankroll, but not so much that a bad run wipes it out. The Kelly criterion, published by Bell Labs scientist John L. Kelly Jr. in 1956, gives the stake that maximizes long-run growth. Enter your bankroll, the odds and your win probability below.

The Kelly formula explained

f = (bp − q) ÷ b

• f is the fraction of your bankroll to stake
• b is the net odds: decimal odds minus 1 (so +150 gives b = 1.5)
• p is your estimated probability of winning
• q is the probability of losing, 1 − p

The top line, bp − q, is your expected profit per $1 staked. Kelly divides that edge by the odds, so you bet more when the edge is big and less when you need a long shot to come in.

Worked example: a 55% edge at -110

You have a $1,000 bankroll. Your model gives a team a 55% chance to cover, and the price is -110.

• Decimal odds: 1.909, so b = 0.909
• p = 0.55, q = 0.45
• f = (0.909 × 0.55 − 0.45) ÷ 0.909 = (0.500 − 0.45) ÷ 0.909 = 0.055

Full Kelly says stake 5.5% of your bankroll, or $55. Half Kelly is $27.50 and quarter Kelly $13.75.

Now suppose your estimate is wrong and the true chance is 52%. At -110, that bet has negative expected value, and full Kelly would have you betting $55 into it. That is the case for fractional Kelly.

Why most bettors use fractional Kelly

• Estimates are noisy. A model that says 55% might really mean anything from 51% to 59%.
• Swings are brutal. Full Kelly can halve a bankroll during an ordinary losing streak.
• Growth barely suffers. Half Kelly delivers about three-quarters of full Kelly’s growth rate with far less volatility.

Leans.ai, for example, sizes its published picks in units using what it calls a modified Kelly approach. Our AI betting tools guide covers how other tools handle stake sizing.

When to use a Kelly calculator

• When you have a probability estimate you trust, from your own model or a tested tool. Check the market’s view first with our implied probability calculator.
• After confirming the bet has positive value in our expected value calculator. If EV is negative, Kelly returns zero or less, so don’t bet.
• For prediction market positions, where the contract price is your implied probability. A 40-cent contract is b = 1.5 before fees. See our prediction markets guide.
• When you want a staking rule that scales with your bankroll rather than a fixed amount.

Common Kelly mistakes

• Using the sportsbook’s price as p. If your probability equals the implied probability, the edge is negative after vig and Kelly returns nothing. You need an independent estimate.
• Betting several games at full Kelly at once. Simultaneous bets compound risk. Scale each stake down when you have many open positions.
• Not updating the bankroll. Kelly is a percentage of what you have now, so stakes shrink after losses and grow after wins. That is what protects you from ruin.

See all our free betting calculators.

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