Kelly Criterion Calculator
How To Use This Calculator
The Kelly Criterion helps you determine the optimal bet size to maximize long-term growth while managing risk. Enter your odds, win probability, and bankroll to see exactly how much to wager.
Input the odds from your sportsbook.
Your estimated chance of winning (use a no-vig calculator to find this).
We recommend 0.25-0.5 to reduce variance.
Enter your total betting bankroll.
What is the Kelly Criterion?
The Kelly Criterion is a mathematical formula developed by John L. Kelly Jr. in 1956. It calculates the optimal bet size based on your edge and bankroll to maximize long-term growth.
Unlike flat betting, Kelly sizing adjusts your wager based on your advantage - betting more when you have a bigger edge and less when your edge is smaller. This protects your bankroll while maximizing growth potential.
f* = (bp - q) / bWhy Use the Kelly Criterion?
Optimal long-term compound returns
Never risk too much on a single bet
Bet more when advantage is higher
Step-by-Step Kelly Calculation
You have a $5,000 bankroll. You find a bet at +150 (2.50) odds with a devigged fair probability of 45%. What should you bet?
Common Mistakes to Avoid
Full Kelly assumes your probability estimate is perfectly accurate. Even small errors lead to significant overbetting. Always use fractional Kelly (0.25-0.5) to build in a margin of safety.
Kelly sizing is based on your current bankroll. If you're using yesterday's bankroll figure after a big loss, you're overbetting. Recalculate bankroll regularly — ideally before every bet.
Kelly only works for +EV bets. If your estimated probability is wrong and the bet is actually -EV, Kelly will amplify your losses. Ensure you have a genuine edge before sizing with Kelly.
If you have 10 Kelly-sized bets running at once, you're risking much more than Kelly intended. Reduce your Kelly fraction based on how many open bets you have, or use a simultaneous Kelly formula.
Frequently Asked Questions
What is fractional Kelly betting?
Fractional Kelly means using a percentage of the full Kelly recommendation. Half Kelly (0.5) or Quarter Kelly (0.25) reduces variance while still capturing most of the expected growth. Most professional bettors use fractional Kelly to protect against estimation errors in their edge calculations.
How do I find my win probability?
Use our No-Vig Calculator to remove the bookmaker's margin from sharp book odds. The resulting "fair" probability is your estimated win probability for Kelly calculations.
What if Kelly suggests betting 0%?
If the Kelly formula returns zero or negative, it means the bet has no edge or is negative EV. Don't place the bet - look for better opportunities where you have a genuine advantage.
Why not use full Kelly?
Full Kelly is mathematically optimal but very aggressive. Small errors in probability estimates can lead to overbetting. Fractional Kelly provides nearly the same growth with significantly less variance and lower risk of large drawdowns.
What happens if you overbet using Kelly?
Overbetting (using more than full Kelly) actually decreases long-term growth and dramatically increases risk of ruin. At 2x Kelly, your expected growth drops to zero. Beyond that, you are mathematically expected to go broke over time. This is why fractional Kelly is strongly recommended.
Can I use the Kelly Criterion for parlays?
The standard Kelly formula is for single bets. For parlays, calculate the combined edge and odds of all legs, then apply Kelly to the parlay as a whole. Because parlays have higher variance and compounding estimation errors, use a smaller Kelly fraction (0.1-0.25).
How often should I recalculate my Kelly bet size?
Recalculate before every bet. Since Kelly is based on your current bankroll, your recommended stake changes as your bankroll grows or shrinks. Many bettors update their bankroll figure daily or weekly.
Pro Tip: Start with Quarter Kelly
New to Kelly betting? Start with a 0.25 multiplier. This captures 75% of the expected growth of full Kelly while reducing variance by 50%. As you gain confidence in your edge calculations, you can gradually increase to half Kelly (0.5). Most professionals never go beyond half Kelly.