Kelly criterion calculator
Calculate the mathematically optimal bet size based on your edge and bankroll. Used by professional bettors and investors worldwide.
Your honest assessment of the true probability — not the bookmaker's implied probability.
Kelly stake: 10.00% of bankroll
Recommended bet: £100.00
| Kelly percentage | 10.00% |
| Recommended stake | £100.00 |
| Implied probability (from odds) | 33.3% |
| Your estimated probability | 40.0% |
| Edge | +6.7% |
| Expected value per £1 staked | £0.2000 |
Fractional Kelly options
Many experienced bettors use half or quarter Kelly to reduce variance. Full Kelly is mathematically optimal but produces large bankroll swings.
What is the Kelly Criterion?
The Kelly Criterion is a formula that calculates the optimal percentage of your bankroll to stake on a bet when you believe you have an edge. Developed by John Kelly at Bell Labs in 1956, it maximises long-term bankroll growth while minimising the risk of ruin. The key input is your honest estimate of the true probability — if this is wrong, the Kelly output is wrong too.
Why use half Kelly?
Full Kelly produces the fastest bankroll growth in theory, but it also produces significant swings. A 20% Kelly stake means your bankroll fluctuates dramatically between bets. Half Kelly (staking half the Kelly percentage) sacrifices some growth for much smoother results. Most professional bettors use half or quarter Kelly.
The biggest risk
The Kelly Criterion is only as good as your probability estimate. If you think you have a 50% chance but the true probability is 40%, Kelly will tell you to overbet — and you'll lose money faster than flat staking. Be honest about your edge. If you're not sure you have one, you probably don't.
How the Kelly Criterion works
The formula
Kelly % = ((decimal_odds × probability) − 1) ÷ (decimal_odds − 1)
Where decimal_odds is the odds offered and probability is your estimated true probability of winning (as a decimal). If the result is zero or negative, you have no edge and should not bet.
Edge and expected value
Your edge is the difference between your estimated probability and the implied probability from the odds. Expected value per £1 = (probability × profit) − (1 − probability). A positive EV means the bet is profitable in the long run — but only if your probability estimate is accurate.
Why fractional Kelly?
Full Kelly maximises the geometric growth rate of your bankroll, but the variance is extreme. A few losses at full Kelly stakes can cut your bankroll in half. Half Kelly achieves approximately 75% of the growth rate with significantly less volatility. Quarter Kelly is even smoother. Most professionals land somewhere between half and quarter Kelly.