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Kelly criterion calculator.
Change the estimate. See the stake change. A formula is only as useful as the assumptions going into it.
Hypothetical inputs · no live prediction
The estimate changes everything.
Explore the maths, then test how much depends on your probability estimate.
2.50% of the hypothetical bankroll, rounded down to cents.
The entered probability is an assumption, not a measured chance.
One loss at this theoretical stake removes $25.00 from the entered bankroll. Using a fraction does not make losses safe or unlikely.
| Probability assumption | Chance | Theoretical stake | Profit if it wins | Loss if it loses |
|---|---|---|---|---|
| Lower estimate | 50.00% | $0.00 | $0.00 | $0.00 |
| Your estimate | 55.00% | $25.00 | $25.00 | -$25.00 |
| Upper estimate | 60.00% | $50.00 | $50.00 | -$50.00 |
Your range crosses the break-even point. A lower estimate produces a zero stake while a higher estimate produces a positive one.
Formula and assumptions
Full Kelly fraction = (decimal odds × estimated win probability − 1) ÷ (decimal odds − 1). Probability is written as a fraction: 55% becomes 0.55. Negative fractions are shown as a zero stake. Half and quarter Kelly divide the positive full fraction by two and four. Stakes round down to cents.
This is a sensitivity example for one binary back bet, not a recommended stake. It excludes fees, commission, voids, dead heats, bookmaker limits, changing prices, correlated bets and existing exposure. The range tests your assumptions; it is not a confidence interval or a forecast.
What does Kelly calculate?
For a simple win-or-lose back bet, full Kelly uses (decimal odds × estimated win probability − 1) ÷ (decimal odds − 1). A negative answer becomes a zero stake in this calculator. Half and quarter Kelly take one half and one quarter of the positive full fraction.
The formula and its idealised assumptions are described in Edward O. Thorp’s The Kelly Criterion in Blackjack, Sports Betting, and the Stock Market, section 2. We checked the source on 18 October 2026.
The calculator does not find a probability, establish an edge or recommend a bet. The sensitivity rows show what happens if your estimate is different. They are hypothetical scenarios, not statistical confidence intervals.
A small assumption can make a large difference.
At hypothetical decimal odds of 2.00 and an estimated 55% chance, full Kelly produces 10% of the entered bankroll. Quarter Kelly produces 2.5%, or $25 from a $1,000 example bankroll. One losing result loses that $25.
If the chance is actually 50%, the same formula produces a zero stake. If it is 60%, quarter Kelly produces 5%, or $50 in that example. A neat stake does not verify any of those probability assumptions. Using a smaller fraction does not correct a wrong estimate.
Stakes round down to cents. Bookmaker minimum stakes, commission, changing odds, correlated bets and existing exposure are excluded. Do not sum these results across several bets and assume the combined exposure is suitable.
Read the bankroll and limits guideResearch comes before the formula.
BetBible Stats offers player and team records through a free account. Use it to research the context behind a claim; it does not validate the probability entered here. Creating an account does not start a trial.
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