Free calculator
Size a stake with the Kelly criterion.
Enter the odds, your probability estimate, and your bankroll. The Kelly fraction tells you how much of the bankroll one bet can risk — and why betting the full amount is rarely sensible.
Kelly fraction
Kelly assumes your probability estimate is right — which is why fractional Kelly exists. A sizing framework, not a guarantee.
How to read the result
The Kelly fraction is (odds × probability − 1) ÷ (odds − 1) — the share of your bankroll that maximises long-run growth if your probability estimate is correct.
The fraction assumes your estimate is right. Full Kelly maximises long-run bankroll growth only if the probability you supplied is accurate. Real estimates are not — a three-point miss can turn a full-Kelly stake into a losing bet.
That is why fractional Kelly exists. Half Kelly stakes half the full-Kelly amount, quarter Kelly a quarter. Lower fractions trade theoretical growth for protection against estimation error and correlated bets.
No edge, no stake. When your estimate does not clear the price, the fraction is zero or negative — the calculator tells you to sit out rather than force a stake.
For example: decimal odds of 2.50, a 55% probability estimate, and a €1,000 bankroll produce a full-Kelly stake of €250 (25% of bankroll) — half Kelly would stake €125.
Also see
De-vig / fair odds
Strip the bookmaker margin out of two- or three-way decimal prices and read the de-vigged fair odds and overround.
Open calculator →
Expected value
Compare your probability estimate against a price and see whether the bet has an edge — or a negative expectation.
Open calculator →
Arbitrage stake split
Split a stake across outcomes for an equal return, and check whether the prices are an arbitrage at all before costs.
Open calculator →