Position Sizing: What the 1 Percent Rule Actually Constrains

The formula, and the mistake it prevents

Three inputs decide the size: your account, the percentage you accept losing, and the distance from entry to stop. The distance is what most people leave out. Two trades with identical position sizes can carry wildly different risk if one has a stop 1 percent away and the other 10 percent, which is why sizing by amount invested rather than by distance to the stop produces a portfolio whose risk nobody can state.

Set the stop from the chart, where the idea would be proved wrong, and let the position size fall out of the arithmetic. Doing it the other way round — choosing a size you like and then placing the stop where it fits — turns the rule into decoration.

Why drawdowns are asymmetric

Losing 50 percent requires a 100 percent gain to get back, because the gain is calculated on a smaller base. The table above shows how quickly this bites: at 5 percent risk per trade, ten losses cost 40 percent of the account and need 67 percent to recover; at 10 percent risk the same streak costs 65 percent and needs 186 percent. Ten losses in a row is not exotic — a strategy winning 60 percent of the time still produces one about once every ten thousand trades, and most strategies win far less often than that.

That asymmetry is the whole argument for small position sizes. It is not caution for its own sake: it is the recognition that a strategy with a genuine edge still needs to be alive when the edge shows up, and a 65 percent drawdown makes that mathematically much harder.

Leverage, correlation and the two ways the rule leaks

The rule survives leverage as long as the stop is honoured: leverage changes the capital required, not the loss, because the loss is set by the stop distance. It breaks if the market gaps past the stop, which is exactly what happens overnight, at weekends and on news — a stop is a request, not a guarantee.

The second leak is correlation. Five separate 1 percent positions in assets that move together are one 5 percent position wearing five names. Count your risk by what actually moves independently, not by how many tickets you opened, and cap total open risk as well as risk per trade.

Frequently asked questions

Is 1 percent the right number for everyone?

It is a common starting point, not a law. Traders with long track records often use less, and the right figure depends on how often your strategy loses in a row and how much drawdown you can sit through without abandoning the method. If you do not know your own worst streak, start lower rather than higher.

Do I recalculate the size after every trade?

Yes if you size on the current balance, which is the usual approach: it shrinks positions automatically during a losing run and grows them as the account recovers. Sizing on a fixed starting balance keeps the risk constant in currency terms and therefore rising in percentage terms as you lose, which is the opposite of what you want.

How does this relate to the Kelly criterion?

Kelly computes the mathematically growth-optimal fraction from your win rate and payoff ratio, and it usually returns a much larger number than 1 percent. Practitioners typically bet a half or a quarter of it, because full Kelly assumes you know your edge exactly, and an overestimated edge makes full Kelly ruinous rather than optimal.