Position Sizing and Risk Per Trade

Size is the variable that sets risk

Entry price, stop level and position size are the three inputs that decide how much money is at stake in a trade, and size is the one under complete control at the moment of entry. Two traders can take an identical setup on an identical pair and expose entirely different amounts of capital, purely because one sized the position to the account and the other accepted the platform's default.

The arithmetic

Position sizing works backwards from the loss being accepted. Three inputs are needed: the cash amount at risk, the distance from entry to the stop level in pips, and the value of one pip for that instrument at one unit of size. Size is then the risk amount divided by the stop distance multiplied by the pip value. Everything else — conviction, the look of the chart, how the previous trade went — sits outside the calculation.

A worked example

On a 5,000-unit account, a trader accepting 1% of equity is risking 50 units of currency on the trade. With a stop 25 pips from entry, on a pair where one pip on a standard lot is worth 10 units, the arithmetic gives 50 divided by 25 times 10, or 0.2 lots. Widening the stop to 50 pips with the same 50 units at risk halves the size to 0.1 lots. The relationship is mechanical: a wider stop is not riskier, it is simply a smaller position for the same money at risk.

Fixed-fractional sizing and a losing run

Expressing risk as a percentage of current equity rather than as a fixed cash figure makes position size shrink automatically during a drawdown and grow again during a recovery. The arithmetic of drawdowns is asymmetric — a 50% loss requires a 100% gain to return to the starting point — which is why percentage-based sizing is the common convention, rather than a fixed amount that becomes a larger share of a shrinking account.

Correlated positions behave as one position

Risk per trade only describes the whole account while the trades are independent. Currency pairs that share a currency, or that respond to the same interest-rate expectations, tend to move together — so three positions sized at 1% each can behave like a single 3% position for as long as the correlation holds. Correlations also change over time, and they tend to be strongest during shocks and risk-off sessions, precisely when a portfolio would benefit from them being weak.

What position sizing does not do

Sizing controls exposure, not outcome. It does not improve an entry, does not make a strategy profitable, and does not guarantee that the loss stops where the stop level sits: gaps, slippage and weekend openings can carry price straight past it. What it does is turn the size of a single bad trade into a known quantity decided in advance, instead of something discovered afterwards.