Kelly Criterion Calculator
Enter the price, your own estimated chance, and your bankroll — this works out a stake size on the Kelly formula, with full, half, and quarter options for how aggressive to be.
Key terms, plainly put
Kelly comes from information theory, not betting — originally worked out for signal noise on telephone lines. Applied here, it answers one thing: given your edge and your bankroll, how much should you actually stake to grow it fastest without risking ruin? It's only as good as your probability estimate.
The maths behind it
Kelly % = ((decimal price − 1) × your chance − (1 − your chance)) ÷ (decimal price − 1). A negative result means there's no edge at your numbers, and the sensible stake is zero.
Worked example
At 2.50 with a 45% estimated chance, full Kelly points to roughly 8.3% of the bankroll on that single bet. Many bettors take half of that — the "Half Kelly" option above — landing near 4.2%, which softens the swings a lot if the estimate turns out slightly generous.
Full Kelly stake, by price and edge
Instead of one example, the whole grid — Kelly percentage across a spread of prices and assumed edges (how far above breakeven you reckon the true chance sits). Halve for half Kelly, quarter for quarter Kelly.
| Decimal odds | +5% edge | +10% edge | +15% edge |
|---|---|---|---|
| 1.50 | 15.0% | 30.0% | 45.0% |
| 2.00 | 10.0% | 20.0% | 30.0% |
| 2.50 | 8.3% | 16.7% | 25.0% |
| 3.00 | 7.5% | 15.0% | 22.5% |
| 4.00 | 6.7% | 13.3% | 20.0% |
| 5.00 | 6.3% | 12.5% | 18.8% |
Common questions
Why go with half or quarter Kelly instead of the full number?
Full Kelly assumes your estimate is exactly right. Since real estimates rarely are, taking a fraction trades away some long-run growth for a much smoother ride and far less risk of a bad losing run.
What does a negative Kelly percentage mean?
At the chance you've entered, the price offers no edge at all — Kelly says skip the bet rather than stake a negative amount.
Does Kelly guarantee profit?
No — it's a sizing method, not a way of finding good bets. Feed it a wrong probability and it'll size incorrectly too.