Position Sizing From First Principles

The one formula that matters more than every indicator combined.

intermediate · 3 min read · 20 XP

This lesson contains the single most useful piece of arithmetic in retail trading. By the end you will be able to size any trade so that being wrong costs exactly what you decided in advance.

Decide the loss first

Almost every new trader thinks in the wrong order: how much can I make? The professional order is the reverse — decide what a loss costs, then work backwards.

The rule: risk a fixed, small percentage of the account on each trade. Commonly 0.5% to 2%. This is risk-per-trade, and it is the input everything else depends on.

Why a percentage rather than a fixed amount? Because it scales automatically. As the account grows, position sizes grow with it; as it shrinks, they shrink — reducing risk exactly when the strategy is struggling, without you having to remember to do it.

The formula

position size = risk amount ÷ (stop distance in pips × pip value per lot)

Worked through:

Risk amount = account × risk %. A $10,000 account at 1% is $100.
Stop distance comes from the chart — the level beyond which the idea is wrong. Say 25 pips.
Pip value per lot is $10 for a standard lot on most USD-quoted pairs.

$100 ÷ (25 × $10) = 0.40 standard lots

If the stop is hit you lose $100. Exactly $100. Not "about" $100.

Notice what determines the size

The stop distance does. A wider stop means a smaller position; a tighter stop means a larger one. The money at risk never changes. This inverts the intuition that a wide stop is "riskier" — it is only riskier if you fail to adjust the size, which is precisely the mistake this formula prevents.

Size this trade

Interactive exercise — enable JavaScript to try it.

Why fixed lot sizes are dangerous

"I always trade 0.1 lots" sounds disciplined and is the opposite.

With a 10-pip stop, 0.1 lots risks $10. With a 100-pip stop, the same 0.1 lots risks $100 — ten times as much, from an identical-looking decision. Your risk is now set by whatever stop the chart happened to require, which is to say: by accident.

Non-USD pairs

The $10-per-pip figure applies when the quote currency is USD. For others:

JPY pairs — a pip is 0.01, and pip value depends on the USD/JPY rate. Roughly $6.70 per standard lot when USD/JPY is 150.
Cross pairs — pip value depends on the quote currency's rate against your account currency.

Most platforms compute this for you. The important thing is to check, not assume, because a wrong pip value silently multiplies your risk.

Rounding

Always round down. Rounding 0.1875 up to 0.19 means you are risking more than your rule allows. It is a tiny amount on one trade and a systematic overexposure across a thousand.

What to remember

Decide what a loss costs before thinking about the profit.
position size = risk amount ÷ (stop distance × pip value per lot).
The stop distance determines the size; the money at risk stays constant.
Fixed lot sizes let the chart set your risk by accident. Always round position size down.

Size every position so that being wrong costs a fixed, small percentage of the account: position size = risk amount ÷ (stop distance × pip value). The stop distance sets the size while the money at risk stays constant — which is why a fixed lot size is dangerous and a fixed percentage is not.

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