Kelly Criterion Calculator
Calculate optimal stake sizes for maximum long-term bankroll growth
Input Parameters
Your assessed probability of winning
1 = Full Kelly, 0.5 = Half Kelly, 0.25 = Quarter Kelly
Results
Enter your parameters and click calculate
Professional Tip
Most professional bettors use fractional Kelly (0.25-0.5) to reduce variance. Full Kelly can be aggressive and lead to significant swings. Only bet when you have genuine positive edge.
How Kelly Criterion Works
The Kelly Criterion is a mathematical formula that calculates the optimal bet size to maximize long-term bankroll growth. It was developed by John Kelly in 1956 at Bell Labs.
f* = (bp - q) / b
Where:
- f* = fraction of bankroll to wager
- b = decimal odds minus 1
- p = probability of winning
- q = probability of losing (1-p)
Important: Kelly Criterion only works when you have a genuine edge. Overestimating your win probability leads to overbetting and potential ruin. Most professionals use fractional Kelly (25%-50% of full Kelly) for more conservative growth.
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How to use this Kelly calculator
An illustrative stake calculation
Enter your available betting bankroll, decimal price and estimated chance of winning. For illustration, choose R1,000, odds of 2.10 and a probability of 50%. These are assumed inputs, not a forecast. Kelly uses f* = (bp - q) / b, where b = decimal odds - 1, p is the win probability and q = 1 - p. Here b is 1.10, so (1.10 × 0.50 - 0.50) / 1.10 gives approximately 0.04545. Full Kelly therefore allocates about R45.45.
Interpreting the fraction
Selecting half Kelly reduces that illustrative stake to about R22.73. A smaller fraction reduces exposure but cannot repair an inaccurate probability estimate. The percentage describes a bankroll allocation, not the chance that your next bet wins. A zero or negative fraction means there is no positive stake under these assumptions.
Avoid overconfidence
Do not enter a percentage merely because a team is popular. Recalculate when the offered price changes, and consider how simultaneous bets on related outcomes could concentrate losses. Keep money for household costs outside the bankroll. Set a rand loss limit before using any calculated amount.
Does full Kelly prevent losing streaks?
No. Even with a sound probability estimate, individual results can produce substantial drawdowns. The formula does not promise a smooth balance or guaranteed growth.
Should I round the stake upwards?
Rounding down keeps exposure closer to your chosen limit. If a bookmaker minimum exceeds the amount you can comfortably risk, skip the wager instead of stretching the budget.