Bankroll
The Kelly Criterion for Sports Betting, Explained
7 min read
You can be a brilliant handicapper and still go broke if you size your bets wrong. The Kelly criterion solves the other half of betting: not which bet to make, but how much to put on it. It's the math behind the "stake" number on every DeadMoneyVault pick.
The core idea
Kelly ties your bet size to your edge. A small edge gets a small bet; a big edge gets a bigger bet. Crucially, because the recommended bet is always a fraction of your bankroll, you can never be wiped out by a single loss. It's designed to maximize long-term growth, not to win any one night.
The formula
For a straight bet, the Kelly fraction is:
If f comes out zero or negative, the bet has no edge — Kelly is telling you to pass. That's a feature, not a bug.
A worked strikeout-prop example
Suppose the model says a pitcher has a 58% chance to go over his strikeout line, and the over is priced at +100 (decimal odds 2.00, so b = 1).
- p = 0.58, q = 0.42, b = 1
- f = (1 × 0.58 − 0.42) / 1 = 0.16
Full Kelly says bet 16% of your bankroll. That's aggressive — and it assumes your 58% estimate is exactly right. Which brings us to the most important practical adjustment.
Use fractional Kelly
Full Kelly is only optimal if your probabilities are perfect. In the real world they're estimates, and overestimating your edge with full Kelly leads to brutal swings. The fix is fractional Kelly — betting a consistent fraction of the recommendation:
- Half Kelly in our example → bet 8% instead of 16%. You keep about three-quarters of the growth with far less volatility.
- Quarter Kelly → bet 4%. Even smoother, popular with bettors who want to survive long losing stretches without stress.
Where the edge estimate comes from
Kelly is only as good as the win probability you feed it. Garbage in, garbage out. That probability has to come from an honest model and be validated against the market — which loops back to closing line value. If you're beating the close, your probabilities are trustworthy enough to size with confidence.
DeadMoneyVault computes a fractional-Kelly stake for every VIP pick from the model's edge, so you don't have to run the math at the rail. See today's edges and stakes, or start with strikeout props 101.
Frequently asked
- What is the Kelly criterion?
- The Kelly criterion is a formula that determines the optimal fraction of your bankroll to wager based on your edge and the odds. It maximizes long-term bankroll growth while mathematically guaranteeing you never bet your entire bankroll on a single play.
- What is the Kelly criterion formula?
- The formula is f = (bp − q) / b, where f is the fraction of bankroll to bet, b is the decimal odds minus 1 (your profit per unit staked), p is your estimated probability of winning, and q is 1 − p. If the result is zero or negative, the bet has no edge and you shouldn't make it.
- Why do bettors use fractional Kelly?
- Full Kelly is optimal only if your win-probability estimate is perfect, which it never is. Fractional Kelly — betting one-half or one-quarter of the recommended amount — sharply reduces bankroll swings and the damage from overestimating an edge, at a small cost to theoretical growth. Most disciplined bettors use a fraction.