Kelly Criterion for Sports Betting – Optimal Bet Sizing | BookieBrawler

How the Kelly Criterion formula works for sports betting, how to calculate optimal bet size, and why fractional Kelly is preferred.

The Kelly Criterion Formula

Kelly % = (bp − q) / b Where: b = decimal odds − 1, p = win probability, q = loss probability (1 − p)

Kelly Criterion Example

You find a bet at +150 (decimal: 2.50) and estimate a 45% win probability.

The Kelly Criterion says to bet 8.3% of your bankroll. However, most professional bettors use <strong>fractional Kelly</strong> (typically ¼ to ½ Kelly) to reduce variance.

• b = 2.50 − 1 = 1.50
• p = 0.45, q = 0.55
• Kelly = (1.50 × 0.45 − 0.55) / 1.50 = (0.675 − 0.55) / 1.50 = <strong>8.3%</strong>

Full Kelly vs. Fractional Kelly

• <strong>Full Kelly:</strong> Maximum growth rate but extremely volatile - large drawdowns are common
• <strong>Half Kelly:</strong> 75% of the growth rate, significantly less variance - the sweet spot for most bettors
• <strong>Quarter Kelly:</strong> Very conservative, smooth equity growth, minimal drawdowns