● EXPLAINER · BY MARCDUCK

What is the Kelly Criterion in Sports Betting?

Published May 17, 2026 · Updated October 3, 2026

Kelly criterion is the mathematically optimal stake size given your win probability and odds. Full Kelly maximizes long-run growth but amplifies variance. Here's the formula, when to use fractional Kelly, and why we use 1/8 Kelly.

The Formula

f* = (p × b - q) / b

Where:

f* = fraction of bankroll to stake
p = your probability of winning
q = 1 - p = your probability of losing
b = decimal odds minus 1 (the profit ratio)

If you have 60% prob at -110 (decimal 1.909, b = 0.909):

f* = (0.60 × 0.909 - 0.40) / 0.909 = 0.16, bet 16% of bankroll.

Why Full Kelly is Dangerous

Kelly assumes your probability is exact. It rarely is. Real probabilities have noise. Full Kelly amplifies estimation error catastrophically, if your "60%" is actually 55%, you've wildly overstaked and your bankroll cratters fast.

Real sharps use fractional Kelly: a fraction (1/2, 1/4, 1/8) of the optimal stake. Lower fractions trade some growth rate for much lower variance.

Why We Use 1/8 Kelly

Bookie Bullies uses 1/8 Kelly with variance adjustment. The reasoning:

Our model's probability has real noise (we display the uncertainty band).
Calibration is unproven on a per-bucket basis (we publish the Brier score on the Track Record page, where you can compare it with the 0.25 coin-flip baseline).
1/8 Kelly limits drawdown risk while still sizing meaningfully on high-conviction picks.
If our true edge is +3% but we estimate +5%, full Kelly overbets by about two-thirds and gives up roughly 45% of its growth rate, and betting more than twice the true Kelly stake shrinks the bankroll over time. 1/8 Kelly stays far below that line.

Variance-Adjusted Kelly

For moneyline stakes we measure how much the four blend components (model, market, Statcast ensemble and a line-history price) disagree, as a standard deviation capped at 0.15, and size Kelly on the blended probability minus 0.25 of that spread. Wider uncertainty → smaller stake. Tight consensus → bigger stake. This is "uncertainty-aware Kelly."

Practical Stakes

At 1/8 Kelly, most picks land at 0-3 units (where 1u = 1% of bankroll). The bankroll calculator on the homepage converts units to dollar stakes for your input bankroll. A 2.5u Lock at $1000 bankroll = $25, meaningful but not bankroll-risking.

When to Increase Kelly Fraction

If you've verified your edge over hundreds of picks (positive ROI, positive CLV), 1/4 Kelly becomes reasonable. 1/2 Kelly only after thousands of bets. Full Kelly is for closed-form games (poker hand math, blackjack card counting) where the probability is mathematically exact, never for sports betting where probabilities are estimated.

Apply This Today

Theory is one thing, using the concept on tonight's slate is the value. These hubs are the practical application: