Kelly criterion calculator
Optimal stake size for bankroll growth
Calculate the mathematically optimal stake for any value bet based on your bankroll, the odds offered, and your estimated probability.
What is the Kelly criterion?
The Kelly criterion is a formula developed by John L. Kelly Jr. at Bell Labs in 1956. It tells you what fraction of your bankroll to bet to maximise long-term growth — assuming you have an edge over the bookmaker. To know if you have edge, use our Value Bet calculator first; to find the true probabilities behind market odds, use No-vig fair odds. Bet too little and you grow your bankroll slowly. Bet too much and you risk going broke. Kelly finds the precise spot in between.
Hedge funds use variations of Kelly to size positions. Professional sports bettors use it for every bet. Card counters in blackjack use it. It's the closest thing to a mathematical law of money management.
See also: Odds converter, Arbitrage calculator, Value bet finder, Bankroll growth simulator
How to use this calculator
- 1Enter your total bankroll (the money you can afford to lose at betting).
- 2Enter the bookmaker's decimal odds for the bet you're considering.
- 3Enter YOUR estimated probability — this is the hardest part. It must be your honest assessment, not the bookmaker's odds working backwards.
- 4Choose your Kelly fraction. We strongly recommend Half Kelly (50%) for most users — it captures 75% of full Kelly's growth with much less variance.
Example: a value bet on Real Madrid
Your bankroll is €1,000. The bookmaker offers Real Madrid at 2.20. You've analysed the match and believe Real Madrid's true probability of winning is 55% (the bookmaker's implied probability is only 45.5%, so you've found an edge). Full Kelly = (1.20 × 0.55 − 0.45) / 1.20 = 17.5% of bankroll = €175. Half Kelly = €87.50. We'd recommend Half Kelly — same direction, much lower risk of ruin from probability estimation errors.
The formula
Kelly% = (bp − q) / b, where:
- b = decimal odds − 1 (your net winnings per unit staked)
- p = your estimated probability of winning (as a decimal: 0.55 for 55%)
- q = 1 − p (probability of losing)
Multiply Kelly% by your bankroll to get the stake.
The Kelly trap
Kelly is brutally sensitive to your probability estimate. If you think a bet has a 55% chance but it's really 50%, full Kelly will gradually drain your bankroll. This is why pros use Half Kelly or even Quarter Kelly — they protect against overconfidence in your own analysis. Never bet full Kelly unless you're a professional with verified long-term edge.
When to use Kelly
- Any time you genuinely believe a bet has positive expected value
- When you need a consistent staking method instead of guessing
- Long-term bankroll growth strategies
- Never on entertainment bets where you don't have a real edge
Frequently asked questions
What is the Kelly criterion calculator?
The Kelly criterion is a formula developed by John L. Kelly Jr.
How does the Kelly criterion calculator work in practice?
Your bankroll is €1,000. The bookmaker offers Real Madrid at 2.20. You've analysed the match and believe Real Madrid's true probability of winning is 55% (the bookmaker's implied probability is only 45.5%, so you've found an edge). Full Kelly = (1.20 × 0.55 − 0.45) / 1.20 = 17.5% of bankroll = €175. Half Kelly =…
What is the most common mistake with the Kelly criterion calculator?
Kelly is brutally sensitive to your probability estimate. If you think a bet has a 55% chance but it's really 50%, full Kelly will gradually drain your bankroll. This is why pros use Half Kelly or even Quarter Kelly — they protect against overconfidence in your own analysis. Never bet full Kelly unless you're a…
When should I use the Kelly criterion calculator?
Any time you genuinely believe a bet has positive expected value. When you need a consistent staking method instead of guessing. Long-term bankroll growth strategies.