What the Kelly criterion does
Kelly answers one question: given an edge, what fraction of your bankroll should you stake to grow it fastest over time? Bet too little and you leave growth on the table; bet too much and a losing run wipes you out. The formula balances the two: stake = edge ÷ net odds. The catch — and it's the whole catch — is that it needs your real probability of winning, not your hopeful one. Over-estimate your edge and Kelly tells you to bet far too much. That's why most disciplined bettors use a fraction of full Kelly (half or quarter), and why we stress-tested the idea on thousands of real races (see The Lab).
Size a stake by your edge
The Kelly criterion sizes each bet to maximise long-run bankroll growth — but only if your win-chance estimate genuinely beats the market. Enter your bankroll, the decimal odds, and your honest estimate of the selection's chance of winning.
e.g. 5.0 = 4/1, 3.0 = 2/1
Half/quarter cut the risk of over-betting.
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Hand-picked siblings of this calculator — natural next steps in the bet-type family.
Frequently Asked Questions
Common questions
- It's a staking formula that sizes each bet in proportion to your edge, to maximise the long-run growth rate of your bankroll. For a simple win bet the stake fraction is (p × O − 1) ÷ (O − 1), where p is your probability of winning and O is the decimal odds. If that comes out negative you have no edge, and Kelly says stake nothing.
Gamble Responsibly
Gambling should be entertaining and not seen as a way to make money. Never bet more than you can afford to lose. If you or someone you know has a gambling problem, help and support is available.