Kelly criterion

Example

“The Kelly criterion suggested betting 25% of capital on an investment with 60% win probability — but many practitioners use half-Kelly for safety.”

Memory Tip

KELLY CRITERION = the math of optimal bet sizing. Bet too much = ruin. Too little = slow growth.

Why It Matters

The Kelly criterion helps investors and traders determine how much of their capital to risk on each opportunity, which is crucial for long-term wealth building. By balancing aggressive growth with protection against catastrophic losses, it provides a mathematical framework that prevents people from betting too much on any single investment and losing their entire portfolio.

Common Misconception

Many people believe the Kelly criterion tells you which investments to make, but it actually only determines how much money to allocate once you have already identified a favorable opportunity. Another mistake is assuming the formula produces a precise answer that works perfectly in real markets, when in reality it requires accurate probability estimates that are difficult to obtain.

In Practice

Suppose you have a trading strategy with a 55 percent win rate earning two dollars per win and losing one dollar per loss. The Kelly criterion formula calculates you should risk 10 percent of your bankroll on each trade, meaning if you have ten thousand dollars, you would place one thousand dollar bets. This approach theoretically maximizes growth while keeping your account from being completely depleted by a string of losses.

Etymology

Developed by John L. Kelly Jr. at Bell Labs in 1956 for information theory, later applied to gambling and investing.

Common Misspellings

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