Calculator
Kelly Criterion Calculator — Optimal Stake Sizing
The Kelly Criterion is a formula that tells you the exact fraction of your bankroll to risk on a bet given your edge and the odds. Bet more than Kelly and you risk ruin; bet less and you grow slower. Enter your true probability, the offered odds, and your bankroll.
Full Kelly is the mathematically optimal stake, but it's aggressive. Most bettors use Quarter Kelly (25%) to reduce variance while keeping most of the growth.
Find +EV bets to size with Kelly →Your estimated real probability — not the book's implied odds
The price you can get at the book right now
Your total sports-betting bankroll
How this works
Formula
kelly_fraction = (decimal × p_true - 1) / (decimal - 1)
Worked example
True prob 55%, price +110 (2.10 decimal): (2.10 × 0.55 - 1) / 1.10 = 14.1% of bankroll — or 3.5% at quarter-Kelly.
FAQ
Why use quarter or half Kelly?
Full Kelly maximizes log-bankroll growth but with brutal variance and high ruin-risk if your edge is mis-estimated. Quarter-Kelly is the practical sweet spot.
What's a typical Kelly fraction for +EV bets?
On a 2% edge at -110 odds, full Kelly is about 4% of bankroll, quarter-Kelly is 1%.
When does Kelly recommend zero stake?
When the price is -EV. If your true probability doesn't beat the implied probability, Kelly returns 0 or negative — meaning don't bet.
Can I use Kelly for parlays?
Yes — treat the parlay as one bet with combined decimal odds and joint probability. Be conservative; parlay edges are easy to over-estimate.
Does Kelly account for correlated bets?
Standard Kelly doesn't. For correlated plays — same-game parlays, multiple legs on the same team — downsize manually. The parlay calculator computes straight-line payout for now; a correlation-aware optimizer is on the roadmap, not shipped.