Kelly Criterion Calculator
Optimal bet sizing based on edge percentage and bankroll. Includes half-Kelly and quarter-Kelly options.
Kelly Criterion Bankroll Calculator
The Kelly Criterion tells you the mathematically optimal fraction of your bankroll to bet on a positive-EV opportunity to maximise long-run growth without risk of ruin. Used by professional sports bettors, poker pros, and quant funds. Casino slot bets are negative-EV — Kelly says bet zero. Use this for sportsbook, poker, or arbitrage situations where you genuinely have an edge.
Result
Enter your edge to see optimal full / half / quarter Kelly bet sizes.
The formula
f* = (b × p − q) / b, where:
b= decimal odds − 1 (net profit per unit wagered)p= your assessed win probabilityq= 1 − p (loss probability)f*= optimal fraction of bankroll to wager
If f* ≤ 0, you have no edge — bet zero. If f* is positive, full Kelly maximises long-run geometric growth. Most professionals use half-Kelly or quarter-Kelly because real-world probability estimates are noisy: betting full Kelly with overconfident probabilities risks ruin.
When to use Kelly
- Sportsbook bets where you have a documented edge over the closing line (e.g. soft books, market inefficiencies, niche leagues you know well).
- Poker: implied odds calculations, stake selection, bankroll allocation.
- Arbitrage / matched betting: sizing the larger leg of an arb.
- NOT for slots, crash, mines, dice, plinko — all are negative-EV. Kelly says bet zero. Anyone telling you otherwise is selling something.
Practical caveats
Kelly assumes you know your true win probability. In sports betting, your edge is usually 1–5% — turning that into Kelly fractions means tiny bet sizes (1–5% of bankroll). If your "edge" is based on gut feel, your real probability is closer to the implied odds — Kelly says bet ~zero. The hardest part of Kelly is honest self-assessment.