How to Calculate Position Size

Position size is the number of lots you trade, and it should be calculated so that if your stop loss is hit, the loss equals a predetermined percentage of your account. The formula is: position size (lots) = (account balance × risk %) ÷ (stop-loss pips × pip value per lot). Every input is known before you place the trade, which means the size is never a guess. This guide explains each component, works through examples on different pairs, and shows the errors that most often break the calculation.

Why size is derived, not chosen

Many new traders pick a lot size first and then see how the trade goes. That approach makes the loss on any given trade a function of where the stop happened to be, rather than a deliberate decision. Reversing the order, deciding the risk first and deriving the size from it, is the single most important habit in position sizing, because it makes your losses consistent and your account survivable through a run of losing trades.

Consider two trades with the same 1% risk. One has a 20-pip stop and the other a 100-pip stop. Sizing them identically would make the second trade five times riskier. Sizing them by the formula makes both cost exactly 1% if stopped out.

The four inputs

Account balance. Use the current balance or equity, in your account currency. Some traders use a fixed notional figure to avoid sizing up too quickly after wins; either approach works as long as it is consistent.

Risk percentage. The share of the account you are prepared to lose on this one trade. Common figures are 0.5% to 2%. At 1%, twenty consecutive losses would reduce the account by roughly 18%; at 5%, the same run would cost about 64%.

Stop-loss distance in pips. The difference between your entry and your stop-loss level, determined by the chart, not by the size you want to trade. See how to set stop loss and take profit.

Pip value per lot. The cash value of a one-pip move on one standard lot, in your account currency. For pairs quoted in USD (EUR/USD, GBP/USD, AUD/USD) on a USD account, this is 10 USD. For other pairs it depends on the exchange rate. The pip calculator returns this figure for any instrument.

The formula

Position size (lots) = (account balance × risk %) ÷ (stop-loss pips × pip value per lot)

The numerator is the cash you are willing to lose. The denominator is the cash lost per lot if the stop is hit. Dividing one by the other gives the number of lots that makes the two equal.

Worked examples

EUR/USD

Account balance 10,000 USD, risk 1%, stop loss 25 pips, pip value 10 USD per lot.

• Cash risk = 10,000 × 0.01 = 100 USD
• Loss per lot at stop = 25 × 10 = 250 USD
• Position size = 100 ÷ 250 = 0.40 lots

If the trade is stopped out, the loss is 0.40 × 25 × 10 = 100 USD, exactly 1% of the account.

How stop distance changes the size

Holding the cash risk at 100 USD and the pip value at 10 USD, the size adjusts inversely to the stop:

A wider stop does not mean more risk; it means a smaller position. This is the mechanism that lets you place the stop where the chart says it belongs.

A pair with a non-USD quote currency

For USD/JPY, the quote currency is JPY, so the pip value in USD depends on the rate. With USD/JPY at 147.00, one pip (0.01) on one standard lot is 1,000 JPY, which converts to 1,000 ÷ 147.00 = 6.80 USD.

Account balance 10,000 USD, risk 1%, stop loss 30 pips.

• Cash risk = 100 USD
• Loss per lot at stop = 30 × 6.80 = 204 USD
• Position size = 100 ÷ 204 = 0.49 lots (rounded down from 0.4902)

The actual loss if stopped is 0.49 × 30 × 6.80 = 99.96 USD. Rounding down keeps the risk at or below the target; rounding up would exceed it.

Rounding to the volume step

Brokers specify a minimum volume and step, commonly 0.01 lots. Always round down to the nearest permitted step.

Account balance 10,000 USD, risk 1%, GBP/USD, stop loss 80 pips, pip value 10 USD.

• Cash risk = 100 USD
• Loss per lot at stop = 80 × 10 = 800 USD
• Position size = 100 ÷ 800 = 0.125 lots, rounded down to 0.12 lots
• Actual risk = 0.12 × 80 × 10 = 96 USD

If the formula produces a figure below the minimum volume, the trade is too large for your account at that stop distance. The correct response is to skip it or find a setup with a tighter, structurally valid stop, not to trade the minimum and accept more risk than planned.

Accounting for the spread and commission

The formula measures risk from entry to stop. Costs add to it. If the spread is 1 pip and you are risking 25 pips, the effective risk is closer to 26 pips, and commission adds a fixed cash amount on top. For most trades with sensible stop distances the difference is small, but on very tight stops it becomes material. You can either include the spread in the stop-loss pip count, or model the net loss at the stop including all costs before placing the trade.

Position size versus margin

Position size and margin are different calculations answering different questions. Position size answers "how much should I trade to risk X?". Margin answers "how much of my balance will be set aside to hold that size?" and is calculated as (lots × contract size × price) ÷ leverage. A 0.40-lot EUR/USD position at 1.1000 with 1:100 leverage requires (0.40 × 100,000 × 1.1000) ÷ 100 = 440 USD of margin. That is a constraint to check, not an input to the sizing decision. See how to calculate margin for the full method.

Leverage does not change the correct position size. Higher leverage lowers the margin requirement for the same size, which can tempt traders into larger positions, but the risk-based size is the same regardless of the leverage setting.

Using the calculator

The position size calculator performs this calculation for any instrument and account currency. Enter balance, risk percentage, stop-loss pips and the pair, and it returns the lot size and the cash risk. Use it for every trade until the arithmetic is automatic, and continue to use it afterwards for pairs where the pip value is not a round number.

Key takeaways

• Position size = (balance × risk %) ÷ (stop-loss pips × pip value per lot); it is derived from risk, not chosen first.
• Fix the cash risk per trade, typically 0.5–2% of the account, and let the stop distance determine the lot size.
• Wider stops produce smaller positions; the cash risk stays constant.
• Pip value depends on the pair and account currency; use the pip calculator for non-USD-quoted pairs.
• Round down to the broker's volume step, and skip trades where the required size is below the minimum.
• Margin and leverage constrain what you can hold; they do not decide what you should trade.