You've found a +EV bet. Great. But how much should you actually wager? Bet too little and you're leaving profit on the table. Bet too much and a bad streak can wipe out your bankroll. The Kelly Criterion is the mathematical answer to this exact problem, and it's built directly into the Bettor Results odds tables with a specific risk management system designed to match the confidence level of each bet.

What is the Kelly Criterion?

Developed by John Kelly in 1956, the Kelly Criterion is a bankroll management formula that mathematically optimizes bet sizing for long-term growth. It was originally designed for information theory but was quickly adopted by professional gamblers and investors because it solves two problems at once: it prevents over-betting (which leads to going broke) and under-betting (which means missing out on growth).

The formula is straightforward: Kelly% = (bp - q) / b, where b is the net decimal odds (your profit per dollar wagered), p is the true probability of winning, and q is the probability of losing (1 - p). The result tells you what percentage of your bankroll to wager for optimal long-term growth.

A Practical Example

Let's say you find a prop bet at +200 odds (decimal odds of 3.0) and the true probability of hitting is 40%. Using Kelly:

b = 3.0 - 1 = 2.0
p = 0.40
q = 0.60
Full Kelly % = (2.0 × 0.40 - 0.60) / 2.0 = (0.80 - 0.60) / 2.0 = 10%

Full Kelly says you should bet 10% of your bankroll. But as we'll explain below, full Kelly is dangerously aggressive in practice. That's why Bettor Results uses fractional Kelly, with the fraction depending on how confident we are in the underlying probability estimate.

Why Full Kelly is Too Aggressive

Full Kelly is mathematically optimal, but only if your edge calculation is 100% accurate. And it never is. In the real world, probability estimates carry uncertainty. Maybe the true win probability isn't exactly 40%; maybe it's 37% or 43%. That estimation error, compounded across hundreds of bets, can turn an optimal strategy into a bankroll-destroying one.

Full Kelly also produces wild variance. You'd experience massive swings in bankroll size even if the math works out over thousands of bets. This is why virtually no professional bettor uses full Kelly. Fractional Kelly (betting a fraction of what the formula recommends) dramatically reduces bankruptcy risk while still capturing most of the long-term growth.

How Bettor Results Implements Kelly: A Three-Tier Confidence System

Not all odds calculations are created equal. The reliability of an EV calculation depends heavily on the data source used to establish the "true" probability. At Bettor Results, we use a three-tier confidence system that adjusts your Kelly fraction based on how much we trust the underlying number:

Tier 1 — Pinnacle Source (Highest Confidence): Quarter Kelly (1/4)

When we can devig odds from Pinnacle (widely considered the sharpest sportsbook in the world, with their correspondingly lowest vig reflecting that) we have a true two-sided market to work with. Both the over and under are priced by sharp bettors, giving us the most reliable probability estimate possible. For these bets, we apply quarter Kelly (1/4 of what full Kelly recommends). Quarter Kelly is the industry standard for serious recreational bettors: it balances bankroll growth with preservation and provides a meaningful cushion against estimation errors.

Using our example above, where full Kelly said 10%, quarter Kelly would recommend 2.5% of your bankroll.

Tier 2 — Median Source (Medium Confidence): Quarter Kelly (1/4)

When Pinnacle odds aren't available for a specific market, we fall back to the median odds across all available sportsbooks as our devig source. The market consensus from eight books is still a strong signal, unbiased source of information. As such, we apply the same quarter Kelly sizing. You're getting solid probability estimates backed by broad market agreement.

Tier 3 — Estimated Source (Lower Confidence): Eighth Kelly (1/8)

Some props, particularly exotic or one-sided markets, don't have a true opposite side available to devig against. In these cases, we use the best available odds as a proxy to estimate the true probability. Because the confidence level is inherently lower when we can't run a proper two-sided devig, we cut the Kelly fraction in half again to one-eighth Kelly (1/8). This accounts for the additional estimation uncertainty and further protects your bankroll on less reliable calculations.

With our 10% full Kelly example, eighth Kelly would recommend just 1.25% of your bankroll — a much more conservative position that reflects the lower confidence.

Why This Approach Works

Our system combines sharp market wisdom (Pinnacle) with broad market consensus (median odds from 8 books) and handles both standard markets (moneyline, spreads, totals) and exotic props that don't have clean two-sided pricing. The conservative fractional Kelly sizing (quarter for high-confidence, eighth for lower-confidence) protects against the inevitable estimation errors that come with any probability model.

The result is a system that naturally scales your bet sizes with your confidence: bigger positions on bets where the probability estimate is most reliable, smaller positions where there's more uncertainty. Over hundreds of bets, this approach captures the mathematical edge while keeping drawdowns manageable.

Practical Application

Visit our various betting data pages and navigate to the Prop Odds or Game Odds tab. Enter your bankroll in the field at the top of the table. The Bet Size column instantly shows you the dollar amount (or percentage without a bankroll value entered) recommended for every bet, automatically adjusted based on the confidence tier of each calculation. Sort by EV% descending to see the highest-edge opportunities first. Click "Bet" to go directly to the sportsbook with your bet slip pre-filled (when available) and enter the recommended dollar amount.

A zero in the Bet Size column means Kelly says don't bet because the edge isn't there. Trust the math and skip it. Over time, disciplined Kelly-based sizing is one of the most powerful tools available to any sports bettor. Continously betting -EV bets leads to gambler's ruin, the inevitable loss of your entire bankroll. It's as inevitable as death, taxes, and Timothee Chalamet's eventual twink death.

Key Principles for Bankroll Management

Kelly works best as part of a broader discipline. Track your results over time. Don't chase losses after a bad day.

The mathematical beauty of Kelly is that it naturally self-corrects: when you're winning, your bankroll grows and so do your bet sizes, letting you compound gains. When you're losing, bets automatically get smaller, protecting what's left. Combined with our three-tier confidence system, it's a robust framework that handles the full spectrum of betting markets you'll encounter on Bettor Results.