The Kelly Criterion Explained (and Why Full Kelly Is Dangerous)

The short answer

The Kelly criterion is a formula for the bet size that maximises long-run growth given your edge and reward-to-risk. The catch: "full Kelly" is brutally aggressive — a small error in your win-rate estimate can wreck the account. So traders use a fraction (half Kelly or less) and cap it.

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What Kelly does

Bet too little and you grow painfully slowly; bet too much and volatility eventually ruins you. Kelly finds the mathematically optimal middle — the bet size that compounds fastest over the long run, given your win probability and payoff. It's the bridge between "having an edge" and "growing the account."

Why full Kelly is dangerous

Growth peaks at the optimal size, then falls off a cliff — over-betting leads to ruin.

Full Kelly assumes you know your true win probability exactly. You don't — you're estimating it. Overestimate your edge by even a little and full Kelly tips you onto the right side of that curve, where drawdowns are savage. The cost of betting too big is far worse than betting too small.

The practical version

Real traders use half Kelly (or less) and put a hard cap on it — often no more than ~2–3% of the account per trade no matter what the formula says. You sacrifice a little theoretical growth for a massive reduction in risk of ruin. Smooth and alive beats fast and broke.

Hamster's note: Full Kelly is like flooring the gas because the math said the road is straight. The math didn't account for me misjudging the road. Half Kelly keeps both hamster and account in one piece.

Quick check — the Kelly formula says bet 18% of your account. What do you actually do?

Cut it down — use half (≈9%) at most, and probably cap it far lower (2–3%). Full Kelly assumes a perfectly known edge; since you're estimating, betting the full amount risks deep, hard-to-recover drawdowns.

Key takeaways

Kelly = the growth-optimal bet size for your edge.
Full Kelly is too aggressive — small estimation errors cause ruin.
Use half Kelly or less, with a hard per-trade cap.

Glossary: Kelly criterion Position sizing Risk management Expected value The Money

Hamster keeps it real: Kelly assumes you actually know your edge. You don't — you estimate it, usually too high. That's why even the math says bet less than you think. Confidence is the most overpriced input in trading.

FAQ

What is the Kelly criterion?

The Kelly criterion is a formula that calculates the position size which maximises the long-run growth rate of your account, given your edge (win probability) and reward-to-risk. It balances growing fast against avoiding ruin.

Why is full Kelly dangerous?

Full Kelly assumes you know your win probability exactly, but in reality you estimate it. Overestimating your edge pushes you past the optimal bet size into a region of severe drawdowns. The penalty for betting too big is far worse than for betting too small.

What is half Kelly?

Half Kelly means betting half of what the Kelly formula suggests. It sacrifices a small amount of theoretical growth for a large reduction in volatility and risk of ruin, which is why most practitioners use half Kelly or less, plus a hard cap.

How does Kelly relate to the 1–2% rule?

The common 1–2% risk-per-trade rule is effectively a conservative, simplified cap in the spirit of fractional Kelly — small enough that estimation errors and losing streaks cannot ruin the account. Most traders favour such a cap over computing full Kelly each trade.