The Kelly Criterion, Why It's More Dangerous Than It Looks

[Fairman]

The Kelly Criterion, Why It's More Dangerous Than It Looks

The Kelly criterion is a formula that calculates the "optimal" bet size to maximize long-term growth, given your win rate and reward-to-risk. It appears in books and videos as a smart way to size positions.

A simple version: Kelly % = Win rate - (Loss rate / Reward-to-risk ratio).

Example: win rate 50%, reward-to-risk 2:1. Kelly = 0.50 - (0.50 / 2) = 0.25, or 25% of your account per trade.

That number should make you pause. Risking 25% of your account on one trade is extremely aggressive and can produce enormous drawdowns.

Why Kelly is risky in practice:

- It assumes you know your win rate and payoffs exactly, but real estimates are uncertain
- Small errors in estimates can lead to huge overbetting
- It maximizes growth, not comfort, and the swings can be psychologically unbearable
- Real trading involves costs, slippage, and changing conditions

How cautious traders use it:

1. Treat Kelly as an upper limit, not a target
2. Use a fraction of it, such as one-quarter or less, if at all
3. Compare with fixed risk of 0.5% to 1% and see how modest it looks by comparison
4. Remember that your edge estimate may be inflated

For most retail traders, simple fixed fractional risk of 1% or less is safer and easier to manage.

Math can guide you, but survival comes first.

[Fairman]

Here's a worked example of why full Kelly scares me. Say you think your win rate is 55% at 1:1. Kelly says risk around 10% per trade. If your real win rate is actually 50%, the right Kelly number is zero, and you'd be betting 10% of the account on no edge at all. Estimation error does the damage, not the formula.

[Fairman]

Most people who use Kelly in practice use a fraction, like a quarter or a half of the full number. Even then, I'd only consider it once there are a few hundred trades logged from the same strategy. With a small sample, a fixed 0.5 to 1% is a safer bet. The problem is that Kelly is very sensitive to the inputs. If your real win rate is 45 percent instead of the 50 percent you measured on 60 trades, full Kelly overbets badly. With the example in the post, 50 percent and 2 to 1, full Kelly is 25 percent per trade, which nobody should risk.

[Fairman]

Last thought on Kelly. Even if your numbers were perfect, full Kelly produces drawdowns most people can't stomach, deep ones, regularly. Growth-optimal on paper and survivable in your head are two very different things. I'd rather grow slower and still be trading next year. Half Kelly gives about three quarters of the growth rate with much smaller drawdowns, which is why it's popular with people who use the formula seriously. But for most retail traders, even half Kelly comes out far above 2 percent per trade. If your calculation suggests that, take it as a sign that your edge is real, not as permission to bet big.