Kelly Calculator
Optimal stake size from your bankroll, decimal odds and true win probability. Quarter, half and full Kelly with edge % and expected log growth.
How the Kelly criterion works
Kelly maximises long-run bankroll growth by sizing every bet to your edge. Bigger edge = bigger stake; no edge = no bet. Over thousands of wagers, Kelly compounds faster than any flat-staking plan.
Formula: f = (o × p − 1) / (o − 1), where o is decimal odds and p is true win probability.
Quarter Kelly is the practical standard
Real-world probability estimates are noisy. Full Kelly assumes you know p exactly — get it wrong and your variance explodes. Quarter Kelly keeps ~95% of the growth with a quarter of the bankroll volatility. Use it.
When Kelly says zero
If the calculator returns 0% or a negative full Kelly figure, the book odds imply a higher win rate than you do. There is no edge — staking anything has negative expected value. Skip it.
FAQ
- What is the Kelly criterion?
Kelly is a formula that calculates the stake size which maximises long-run logarithmic bankroll growth. Given an edge, it tells you exactly how much of your bankroll to risk to compound fastest without going broke.
- Why use quarter Kelly instead of full Kelly?
Full Kelly assumes your probability estimate is exact. In reality estimates are noisy, so full Kelly produces wild swings and risk of ruin. Quarter Kelly delivers ~95% of the long-run growth at a fraction of the variance — it is the practical standard for sharp punters.
- What if Kelly returns a negative number?
A negative Kelly fraction means the bookmaker odds imply a higher win probability than you do — there is no edge, so the optimal stake is zero. Skip the bet.
- What is the Kelly formula?
For decimal odds o and true win probability p, Kelly fraction f = (o × p − 1) / (o − 1). Multiply by your bankroll for the stake.
- Does Kelly work for parlays and props?
Yes — Kelly works for any single bet with binary outcome and known odds + true probability. For multi-leg parlays, multiply the leg probabilities and use the parlay decimal price as o.
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