Kelly Criterion Calculator
Determine optimal bet sizing based on your edge and bankroll.
What is the Kelly Criterion?
The Kelly Criterion is a mathematical formula that calculates the optimal percentage of your bankroll to wager on a bet with positive expected value. It maximizes long-term growth while minimizing the risk of ruin. Most sharp bettors use a fractional Kelly (typically quarter or half) because the full Kelly amount can lead to large swings. The formula requires knowing your true probability of winning, which is the hardest part since any overestimation of your edge leads to overbetting. If the Kelly output is zero or negative, the bet has no edge and should be skipped.
Frequently Asked Questions
What's a safe Kelly fraction?
Most sharp bettors use quarter Kelly (0.25x) or half Kelly (0.5x). Full Kelly maximizes long-term growth but produces large bankroll swings; fractional Kelly trades some growth for much lower variance.
Why is my Kelly result negative?
Negative Kelly means the bet is -EV - your true probability doesn't beat the implied odds. Kelly recommends not betting; betting anyway is a guaranteed long-term loser.
What if I'm wrong about my edge?
Kelly is highly sensitive to edge estimation. Overestimate by 1% and you'll overbet. Use quarter Kelly as protection against edge-estimation error, and re-estimate after every 100 bets.
Can I use Kelly for parlays?
No, not directly. Kelly assumes independent outcomes and a single bet. For correlated parlays, use a smaller fixed fraction or skip the parlay entirely.