Bankroll math · bet sizing
Kelly Criterion Calculator
Find the mathematically optimal fraction of your bankroll to risk on any +EV bet.
How Kelly Criterion works
The formula is f* = (b×p − q) / b where b = decimal odds − 1, p = your win probability, q = 1 − p. It maximizes expected log bankroll growth without risking ruin.
Full Kelly produces severe drawdowns even with a real edge. Most serious bettors use half-Kelly: you give up about 14% of optimal growth in exchange for roughly half the variance. Use the no-vig calculator to get a market baseline for your probability estimate.
FAQ
What is the Kelly Criterion?
A bet-sizing formula that maximizes long-run bankroll growth, developed by J.L. Kelly Jr. at Bell Labs in 1956. It gives zero weight to negative-EV bets.
Why does Kelly output 0% for negative EV bets?
If your estimated win probability doesn't exceed the breakeven implied by the odds, the formula returns zero or negative — meaning don't bet. Negative EV shrinks your bankroll no matter how you size it.
Should I always use half-Kelly?
Half-Kelly is the common practical choice. Quarter-Kelly is for when you're less confident in your probability estimate, since Kelly sizing is very sensitive to overestimated edge.
Can Kelly be used for parlays?
Yes — use the combined parlay decimal odds and your estimated probability that all legs win. Individual leg probabilities are only multiplicative if the legs are independent (not SGP legs).
More: Calculator hub · Odds Converter · Parlay · Hedge · No-Vig
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