Full Kelly maximizes long-term bankroll growth. So why does almost no professional bettor use it? Because the real world isn't a math textbook. Full Kelly assumes your edge estimate is perfect. In sports betting, it never is.
When the Kelly formula tells you to bet 10% of your bankroll, that's full Kelly. It's the mathematically optimal bet size if your win probability estimate is exactly correct.
But if you're off by even 2% on your edge calculation, full Kelly will have you massively overbetting. And in sports betting, being off by 2% isn't rare. It's normal.
| Strategy | Growth Rate | Volatility | Risk of Ruin | Who Uses It |
|---|---|---|---|---|
| Full Kelly | 100% | Very High | Moderate | Almost Nobody |
| Half Kelly | 75% | Much Lower | Low | Most Pros |
| Quarter Kelly | ~50% | Very Low | Very Low | Conservative Bettors |
Half Kelly gives you 75% of the growth with dramatically less volatility. You're betting half of what full Kelly suggests, which means even if you overestimate your edge by 50%, you're still betting a reasonable amount.
More importantly, you can actually stomach the swings. A 5% bet feels manageable. A 10% bet on a single game feels reckless. If you can't psychologically handle the bet size, you'll abandon the system during a losing streak. Half Kelly keeps you in the game.
If you're new to Kelly, start with quarter Kelly. If your edge estimates are unproven, use quarter Kelly. If you're betting correlated outcomes, use quarter Kelly. If you value sleep over maximum growth, use quarter Kelly.
You'll grow slower, but you'll survive variance that would wipe out a full Kelly bettor who overestimated their edge.
Professional syndicates with sophisticated models might use 50-75% Kelly. Individual bettors should stick to quarter or half Kelly. The math might say full Kelly is optimal, but the math assumes you're never wrong about your edge. You are.
Sports betting isn't blackjack. You can't count cards and know your exact edge. Every bet is an estimate based on incomplete information. Fractional Kelly is your insurance policy against overconfidence.
Use half Kelly if you've got a proven track record. Use quarter Kelly if you're still building one. And never use full Kelly unless you want to learn the hard way why everyone else uses fractional.