Kelly Criterion Calculator
Kelly sizes a stake from the edge you believe you have. Enter your bankroll, the price on offer and your own probability estimate — if the estimate does not beat the break-even probability, the correct stake is zero.
2.08% of bankroll
Before the fractional multiplier.
b·p − q. Positive means the price beats your estimate.
Your estimate must exceed this for any stake.
The formula
The multiplier is the fractional-Kelly setting: 1 for full, 0.5 for half, 0.25 for quarter.
Worked example
Odds of 2.50 with a €1,000 bankroll and your estimate of 45%. Here b = 1.50, p = 0.45 and q = 0.55, so f = (1.50 × 0.45 − 0.55) ÷ 1.50 = 8.33%. Full Kelly stakes €83.33; quarter Kelly stakes €20.83. The break-even probability at 2.50 is 40%, so a 45% estimate is a 5-point edge.
| Your estimate | Edge | Full Kelly | Quarter Kelly |
|---|---|---|---|
| 48% | −4% | 0% (no bet) | 0% (no bet) |
| 50% | 0% | 0% (no bet) | 0% (no bet) |
| 52% | +4% | 4.00% | 1.00% |
| 55% | +10% | 10.00% | 2.50% |
| 60% | +20% | 20.00% | 5.00% |
Common mistakes
- Using implied probability as your estimate. That always gives a negative edge, because the price includes margin. Kelly needs an independent view.
- Overestimating your accuracy. Being 5 points wrong turns a growth strategy into a fast drawdown at full Kelly.
- Not recalculating the bankroll. Kelly is a fraction of your current bankroll, so it should shrink after losses, not stay fixed.
- Applying Kelly to correlated bets. Several simultaneous positions on related outcomes stake far more of the bankroll at risk than each figure suggests.
Frequently asked questions
Related calculators
- Implied Probability CalculatorTurn any price into the percentage chance the bookmaker's odds represent.
- Bookmaker Margin CalculatorMeasure the overround baked into a market and see the fair odds without it.
- Hedge CalculatorFind the stake on the opposing outcome that equalises profit or recovers your stake.
- Bet TrackerLog bets in your own browser and see real ROI, win rate and closing line value.
Last reviewed: 24 August 2026. Formulas on this page are checked against the unit test suite that powers every bet.ing calculator.