The Kelly criterion converts an edge into a stake size: bet the fraction of your bankroll that maximises long-run growth, and nothing when there is no edge. It is the answer to the second question of betting — not what to bet, but how much.
Enter your bankroll, the decimal odds, and your win-probability estimate. The calculator returns the edge, the full-Kelly percentage, and the recommended stake at your chosen fraction. Most disciplined bettors size at a quarter Kelly — it keeps ~88% of the growth at a fraction of the drawdowns.
Edge
10.00%
Full-Kelly %
10.00%
Recommended stake
€25.00
How to use the Kelly Calculator
1
Enter your current bankroll in EUR — the total amount you have set aside for betting.
2
Enter the decimal odds on offer and your estimated win probability in percent (a de-vigged fair probability is the usual source).
3
Pick a Kelly fraction — quarter Kelly is the standard for most bettors — and read the recommended stake. 'No bet' means the edge is non-positive.
Worked examples
Example 1 — the classic 55% at 2.00
Bankroll €1,000, odds 2.00, win probability 55%. Edge = 2.00 × 0.55 − 1 = +10%. Full Kelly = (1 × 0.55 − 0.45) / 1 = 10% of bankroll → €100. At quarter Kelly the recommended stake is €25.
Example 2 — no edge, no bet
Odds 1.80 at a 50% win probability: edge = 1.80 × 0.50 − 1 = −10%. Kelly returns a stake of €0 — there is no stake size that turns a negative-EV bet positive. This is Kelly's most underrated feature: it tells you when not to bet.
FAQ
Kelly Criterion FAQ
What is the Kelly criterion?
The Kelly criterion is a staking formula that maximises the long-run growth rate of a bankroll: f* = (b·p − q) / b, where b is the decimal odds minus 1, p is your win probability, and q = 1 − p. It answers 'how much should I bet?' given an edge — bet too little and you leave growth on the table, bet too much and drawdowns destroy the bankroll even with a real edge.
Why do most bettors use fractional Kelly (half or quarter)?
Full Kelly assumes your probability estimate is exactly right. In practice it never is — and overestimating your edge with full-Kelly stakes is ruinous, because betting more than 2× the true Kelly fraction has a negative expected growth rate. Quarter Kelly captures roughly 88% of full Kelly's growth rate while cutting expected drawdowns by about 75%. That trade is why most disciplined bettors size at a quarter to a half Kelly.
What does it mean when the calculator says 'No bet'?
If your estimated win probability times the decimal odds is below 1, the bet has negative expected value — there is no stake size that makes a losing proposition profitable. Kelly's answer to a negative edge is always zero.
Where do I get the win probability from?
That is the hard part — Kelly only sizes a bet, it doesn't find one. A common reference is the no-vig fair probability derived from a sharp book's odds (de-vig Pinnacle's line and use the fair percentage). Our no-vig calculator does exactly that conversion. Whatever source you use, be conservative: Kelly punishes overconfidence far more than caution.
Does Kelly guarantee I won't go broke?
Kelly staking with accurate probabilities never bets the full bankroll, so formally it cannot hit zero — but real bettors misestimate probabilities, and books limit accounts. Expect deep drawdowns even when sized correctly: full Kelly has a 1-in-3 chance of halving your bankroll at some point. Variance is the price of growth; fractional Kelly reduces the price.
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