The Kelly Criterion is a formula that tells you how much of your bankroll to stake on a bet, based on your edge and the odds. Used right, it grows a bankroll faster than flat staking; used recklessly, it blows up accounts. Here is how Kelly works and why most smart bettors use a fraction of it.
What is the Kelly Criterion?
The Kelly Criterion calculates the optimal stake as a percentage of your bankroll that maximises long-term growth without risking ruin. The formula is f = (bp − q) / b, where b is the decimal odds minus one, p is your probability of winning, and q is 1 − p. In plain terms: the bigger your edge, the more you bet; with no edge, Kelly says bet nothing.
A simple Kelly example
Say a bet is priced at 2.00 (even money) and you believe your true win probability is 55%. Kelly gives f = (1×0.55 − 0.45) / 1 = 0.10, so you would stake 10% of your bankroll. Notice the formula only works if your probability estimate is accurate — which is why your edge has to come from a real model, not a hunch.
| Your edge | Full Kelly stake | Half Kelly (safer) |
|---|---|---|
| 55% at 2.00 | 10% of bankroll | 5% |
| 52% at 2.00 | 4% | 2% |
| No edge (50%) | 0% | 0% |
Why use fractional Kelly
Most experienced bettors stake a fraction — typically half or quarter Kelly — because full Kelly is brutally volatile and assumes your probabilities are perfect, which they never are. Half Kelly captures most of the growth with far less swing, and it cushions the inevitable errors in your model. If you are unsure of your edge, smaller is always safer.
Kelly, AI and expected value
Kelly only makes sense once you have a genuine edge, and that edge comes from finding positive expected value — bets where your probability beats the bookmaker’s price. AI tools help estimate those probabilities, but they are not flawless, so pair Kelly with the discipline in our expected value guide and judge any model honestly using our win-rate guide.
Our take
Kelly is the most mathematically sound staking method there is, but its power depends entirely on the accuracy of your probabilities. Use half Kelly, keep records, and treat any AI estimate as an input — not gospel. Vet your tools with our trust checklist before trusting their numbers with real stakes.
Frequently Asked Questions
What is the Kelly Criterion in betting?
The Kelly Criterion is a formula that sets your stake as a percentage of your bankroll based on your edge and the odds, maximising long-term growth while avoiding ruin.
How do you calculate Kelly?
Use f = (bp − q) / b, where b is decimal odds minus one, p is your win probability and q is 1 − p. The result is the fraction of your bankroll to stake.
Should I use full or half Kelly?
Most bettors use half or quarter Kelly because full Kelly is very volatile and assumes perfect probabilities. Fractional Kelly keeps most of the growth with far less risk.
Does Kelly work with AI predictions?
Yes, if the AI’s probabilities are accurate. Kelly only profits when you have a real edge, so it depends on the quality of the model behind your numbers.
Can the Kelly Criterion lose money?
Yes. If your probability estimates are wrong or you use full Kelly, it can cause large drawdowns. Accurate edges and fractional staking reduce that risk.
