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Kelly Criterion Calculator
Calculate optimal bet sizing to maximize long-term growth. Half and quarter Kelly reduce variance while maintaining edge.
What is the Kelly Criterion?
The Kelly Criterion is a mathematical formula that calculates the optimal percentage of your bankroll to wager on a bet with a positive expected value. It was developed by John Kelly at Bell Labs in 1956 and is widely used by professional bettors and investors.
Why Use Half or Quarter Kelly?
Full Kelly is mathematically optimal for maximizing long-term growth, but it comes with extreme variance — your bankroll can swing wildly between bets. Most professionals use Half Kelly (50% of the recommended stake) as a practical compromise: you capture roughly 75% of the growth rate with significantly less variance. Quarter Kelly is even more conservative, ideal for beginners or when your probability estimates are uncertain.
Important: Honest Probability Estimates
Kelly only works if your win probability estimate is accurate. Overestimating your edge leads to overbetting, which can destroy your bankroll. If you're not confident in your estimate, use Quarter Kelly or less. It's better to bet too little than too much — you can always increase your stakes as your confidence grows.
When Kelly Says "No Bet"
A negative Kelly percentage means the bet has negative expected value — you shouldn't place it at any stake. This is valuable information: it confirms that the odds don't justify a bet even if you think the fighter will win.