Free bankroll-sizing model
Kelly criterion calculator for fractional bet size
Enter a bankroll, fixed price and your independently estimated win probability. The calculator shows full Kelly and a smaller fractional stake, or zero when the entered probability does not clear break-even. It does not estimate the chance of winning.
Fractional Kelly result
Model: full Kelly = (p × b − q) ÷ [b × (p + q)], where p is win probability, q is loss probability and b is net decimal odds. The selected result multiplies full Kelly by 0.25. A non-positive numerator produces a zero stake.
The result depends entirely on the probability estimate
The entered 50.00% win estimate is 4.55 percentage points above the 45.45% break-even level. That is an assumption supplied by you, not evidence verified by this page. If it is overstated, the stake is overstated too.
Kelly is a repeated-bet model with strict assumptions
Kelly's original objective is long-run logarithmic capital growth under known probabilities and odds. A real sportsbook user normally has uncertain probabilities, finite opportunities, changing prices, stake limits, correlated positions and personal loss limits. Those differences can dominate the arithmetic. Fractional Kelly reduces the formula result; it does not repair a bad probability estimate or make a bet safe.
- Use one exact market, current price and compatible settlement rules.
- Do not copy the displayed implied probability back into the win-probability field as proof of an edge.
- Treat pushes as stake-return events only when the live rules say so.
- Account separately for taxes, commission, limits, partial acceptance and correlated bets.
- Never increase a pre-set entertainment budget because the formula returns a positive number.
Theory source: J. L. Kelly Jr., “A New Interpretation of Information Rate,” Bell System Technical Journal 35 (1956), 917–926. Practical input and fractional-Kelly reference: CGMBet System User Guide, section 16.7. Drawdown and model-limitation research: Hsieh and Barmish, “On Kelly Betting: Some Limitations”.
Kelly criterion calculator questions
What does the Kelly criterion calculate?
It calculates the bankroll fraction that maximizes expected logarithmic growth under a repeated-bet model when the entered odds and true outcome probabilities are accurate. It is a sizing formula, not a prediction or a guarantee of profit.
Why does this calculator default to quarter Kelly?
Fractional Kelly scales down the full formula result. Quarter Kelly commits 25% of the theoretical full-Kelly stake, making uncertainty and drawdowns more visible without claiming that any fraction is personally suitable.
Why can the recommended stake be zero?
If the entered win probability does not exceed the break-even probability implied by the price after any push probability is considered, the modeled expected value is not positive and Kelly assigns no stake.
Where should the win probability come from?
It must be your independent, evidence-based estimate for the exact market and settlement rules. The calculator does not create a probability, scrape a sportsbook, treat displayed odds as truth, or verify that an apparent edge is real.
Does a Kelly stake prevent a losing run?
No. Even a correctly specified positive-edge model can experience long drawdowns, and an overstated probability can make the result dangerously large. Limits, correlated bets, changing odds and finite betting horizons also sit outside this simple model.