Kelly Criterion Calculator
Enter your bankroll, your estimated win probability, and the American odds being offered. The calculator shows full Kelly, half Kelly, and quarter Kelly stake sizes.
⚠ Your edge is negative; Kelly recommends not betting this line. Find a better price or pass.
What is the Kelly Criterion?
The Kelly Criterion is a formula for optimal bet sizing developed by John Kelly at Bell Labs in 1956. It tells you what fraction of your bankroll to wager based on:
- Your edge; how much better the true probability is than the sportsbook's implied probability.
- The odds offered; bigger payouts allow larger Kelly stakes.
The Kelly formula
f* = (bp − q) / b
f*= fraction of bankroll to bet.b= decimal odds − 1 (the net payout per dollar staked).p= your estimated probability of winning.q= 1 − p (probability of losing).
Why use fractional Kelly?
Full Kelly maximizes the long-term growth rate of your bankroll, but the short-term variance is brutal; a 100-bet bad run can shrink your bankroll dramatically. Half Kelly sacrifices about 25% of long-term growth in exchange for roughly half the variance, making it the standard recommendation for serious bettors.
Critical caveat
Kelly only works if your estimated win probability is accurate. If you systematically overestimate your edge, Kelly will over-bet and erode your bankroll faster than flat staking. Use Kelly only after you have a tested, calibrated probability model; or use a small fractional Kelly (quarter or eighth) to buffer against estimation error.
Negative edge = no bet
If your probability is lower than the sportsbook's implied probability, Kelly returns a negative number. Never bet a negative edge. The math is telling you the line isn't profitable for you.