Kelly Criterion Calculator
The Kelly criterion gives the mathematically optimal stake size to maximize long-term bankroll growth without risking ruin. Enter the odds, your edge estimate, and bankroll.
What is the Kelly Criterion?
The Kelly criterion (developed by John L. Kelly Jr. at Bell Labs in 1956) is the bet size that maximizes the expected logarithm of wealth. In English: it grows your bankroll fastest over the long run without risking ruin.
Full Kelly is mathematically optimal but volatile — losing streaks can draw down 50%+ of bankroll. Most sharp bettors use Half Kelly as a compromise between growth and variance.
The Formula
Kelly fraction = (b × p − q) / b
- b = decimal odds − 1 (the "to win" portion)
- p = your estimated win probability
- q = 1 − p (loss probability)
Example: +110 odds (b = 1.1) with 55% estimated win probability. Kelly = (1.1 × 0.55 − 0.45) / 1.1 = 14.1% of bankroll.
Half Kelly on this same bet: 7.05% of bankroll.
When NOT to Use Kelly
- You can\'t estimate true probability accurately. Kelly assumes you know your edge. Overestimating edge leads to ruinous overstaking. Stick to flat units (1-3% of bankroll) unless you have rigorous prop modeling.
- You\'re betting low-EV markets. Kelly only applies when you have a true edge over the sportsbook\'s posted price.
- You can\'t handle volatility. Full Kelly causes 50%+ drawdowns regularly. Half or Quarter Kelly reduces emotional pressure.
How to Estimate Win Probability
Most bettors overestimate their edge. Realistic approaches:
- Closing line value (CLV) — if you consistently beat the closing line, your win probability is ~52-54% (vs the 50% implied by -110/-110).
- Per-market models — build a simple power-rating model for spreads, or per-minute usage projection for NBA props.
- Fair odds from no-vig — compare your model to the no-vig fair price from the no-vig calculator.
Related Tools
- No-Vig Calculator — find the fair price.
- Bankroll Tracker — log wagers and measure ROI.
- Parlay Calculator — multi-leg payouts.