Kelly Criterion Calculator
Compute the Kelly-optimal stake as a fraction of your bankroll from your win probability and odds, with the safer half-Kelly amount.
How it works
b = decimal odds − 1 · q = 1 − p · f* = (b·p − q) ÷ b · Stake = f* · bankroll
The Kelly criterion is the bet size that maximises the long-run growth rate of a bankroll. It rises with your edge and falls as the odds shorten, and it turns negative — meaning do not bet — the moment your win probability drops below the break-even rate. Full Kelly is aggressive and swingy, so most practitioners bet half-Kelly or less, which keeps most of the growth while roughly halving the volatility and the risk of a deep drawdown.
Worked example
Bankroll $10,000, win probability 55%, decimal odds 2.00 (even money, b = 1). q = 0.45. f* = (1 × 0.55 − 0.45) ÷ 1 = 0.10, so Kelly says stake 10% = $1,000. Half-Kelly would be $500 — the same expected edge with much gentler swings.
Sources: Kelly criterion — Wikipedia
Frequently asked questions
Why would I bet less than full Kelly?
Full Kelly maximises growth but produces large bankroll swings and assumes your probability estimate is exact. Since estimates are never perfect, half-Kelly (or quarter-Kelly) sacrifices a little growth for far smaller drawdowns and more forgiveness if your edge is overstated.
What does a negative Kelly fraction mean?
It means the bet has no edge — your win probability is below the break-even rate the odds imply. The optimal stake is zero: any wager erodes your bankroll over time, so the calculator recommends not betting.
Can I use Kelly for investing?
Yes, in principle — Kelly applies to any repeated favourable bet, including position sizing in trading. But market returns are continuous and estimates of edge are noisy, so investors typically use a heavily fractional Kelly and combine it with diversification.
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Educational tool only — not gambling or investment advice. Bet sizing cannot remove the risk of loss.