What the Kelly Criterion Actually Does
It tells you how much of your bankroll to stake when you think the odds are in your favor.
Think of it as a scalpel, not a hammer—cutting the fat off reckless bets while keeping the meat.
Step 1: Identify Your Edge
First, you need a true probability estimate for a fight outcome. Look at striking accuracy, takedown defense, recent form—anything that quantifies a fighter’s chance.
Suppose you believe Fighter A has a 58 % chance to win, but the sportsbook offers +120 (implied probability ~45 %). That gap is your edge.
Step 2: Convert Odds to Decimal Form
American +120 becomes 2.20 decimal. If the odds were -150, you’d flip it to 1.67 decimal.
Keep the numbers clean; a messy conversion will ruin the whole calculation.
Step 3: Plug Into the Kelly Formula
Kelly = (p × b − q) / b, where p = your estimated probability (as a decimal), q = 1 − p, and b = decimal odds − 1.
Using the example: p = 0.58, b = 2.20 − 1 = 1.20, q = 0.42. Kelly = (0.58 × 1.20 − 0.42) / 1.20 ≈ 0.083.
That means wager about 8.3 % of your bankroll on Fighter A.
Step 4: Adjust for Variance
The raw Kelly is aggressive. Most pros trim it to half‑Kelly or even a quarter‑Kelly to survive inevitable swings.
So, with a half‑Kelly you’d bet roughly 4 % of your total funds.
Step 5: Manage Your Bankroll
Never dip below a 10‑unit threshold. If you start with £1,000, keep a reserve of at least £100 before each round of betting.
Re‑calculate after every win or loss; the Kelly fraction changes as your bankroll shifts.
Common Pitfalls
Overestimating p is a suicide pact. The fighter’s hype can cloud judgment faster than a flash knockout.
Ignoring commission (the vig) throws the whole equation off. Factor in a 5 % cut on payouts, or your Kelly turns negative.
Using the same p for every fight is like wearing the same gloves for every bout—doesn’t fit.
Why Kelly Beats Flat‑Betting
Flat bets treat every fight identical, regardless of confidence. Kelly scales stakes, rewarding accurate assessments and punishing misreads.
In the long run, that scaling yields superior ROI, provided you stick to the math.
Quick Reference
Edge = Your Probability − Implied Probability.
If Edge > 0, Kelly > 0 → Bet. If Edge ≤ 0, stay out.
Bet Size = Kelly × Current Bankroll × Adjustment Factor (0.5 for half‑Kelly).
Final Action
Grab your latest fight odds, plug them into the formula, trim to half‑Kelly, and place that precise percentage now.
