What this calculator does
The Kelly criterion tells you what share of your bankroll to stake to grow it as fast as possible over time, provided your probability estimate is right. It puts a number on an intuition: the bigger the edge, the bigger the stake can be.
How to use it
- Enter the odds. In the format of your choice.
- Give your estimate. The probability, as a percentage, that you assign to the outcome.
- Enter your bankroll and unit. To get the stake as an amount and in units.
- Choose a fraction. ¼ Kelly by default, the most cautious setting.
The formula
- Kelly fraction = (odds × probability − 1) / (odds − 1).
- Stake = bankroll × Kelly fraction × chosen fraction.
- Stake in units = stake / unit value.
- Estimated edge = odds × probability − 1; break-even = 1 / odds.
Worked example
Odds of 2.00, estimated probability 55%, bankroll 1,000, unit 10, ¼ Kelly.
| Step | Calculation | Result |
|---|---|---|
| Estimated edge | 2.00 × 0.55 − 1 | +10% |
| Full Kelly | (2.00 × 0.55 − 1) / (2.00 − 1) | 10% |
| ¼ Kelly | 10% × 0.25 | 2.5% |
| Stake | 1,000 × 2.5% | 25 |
| In units | 25 / 10 | 2.5 u |
Why a fraction of Kelly
Full Kelly assumes an exact estimate. Overrate a probability by just a few points and you stake far too much. Staking ¼ or ½ Kelly sharply reduces bankroll swings for barely lower growth: that is the trade-off most disciplined bettors make.
If your estimate does not clear the break-even point, the suggested stake is zero. The calculator does not look for a bet where there is no edge.
Limits
The result is only as good as your estimate. For several bets placed at the same time, or bets that are linked, the simple formula overstates the stake: lower the fraction.