Kelly Criterion for Sports Betting: How to Size Your Bets

You found a +EV bet. Great. But how much should you wager? Bet too little and you leave money on the table. Bet too much and a losing streak wipes you out. The Kelly Criterion solves this problem mathematically — telling you the optimal bet size based on your edge and the odds.

The Kelly Formula

Kelly % = (bp - q) / b, where b = the decimal odds minus 1, p = your estimated probability of winning, and q = 1 - p (the probability of losing). The result tells you what percentage of your bankroll to wager.

Example: You find a bet at +150 (decimal 2.50), and you estimate a 45% chance of winning. b = 1.5, p = 0.45, q = 0.55. Kelly % = (1.5 × 0.45 - 0.55) / 1.5 = 0.083, or 8.3% of your bankroll.

Why Full Kelly Is Too Aggressive

Full Kelly maximizes long-term growth but produces brutal drawdowns. A 30-40% drawdown is common even with a real edge. Most professional bettors use fractional Kelly — typically 25-50% of the full Kelly recommendation. This dramatically reduces variance while still capturing most of the growth.

Using Kelly with +EV Betting

When combined with a +EV scanner like Open Edge EV, the Kelly Criterion becomes especially powerful. The scanner identifies the edge; Kelly tells you how much to bet. Open Edge EV calculates Kelly % automatically for every bet, so you don't need to do the math yourself.

Common Mistakes

Never bet more than Kelly suggests — this is mathematically guaranteed to reduce your long-term growth. Also avoid "phantom edge" — if your probability estimate is wrong, Kelly amplifies the error. Using sharp-book devigged odds as your probability benchmark (which is what Open Edge EV does) gives you the most reliable edge estimate possible.