Position Size Calculator: Forex Lot Size Calculator

A position size calculator answers one question: how many units or lots you can trade so that hitting your stop-loss costs a fixed amount of your account. You set the risk first, and the trade size follows from it.

Inputs and outputs

The calculator needs four inputs: account balance, risk per trade (a percentage or fixed amount), stop-loss distance in pips, and the currency pair. If your account currency differs from the pair’s quote currency, it also needs an exchange rate to convert pip value.

It returns three outputs: money at risk, the loss in account currency if the stop is hit; units, the position in base currency units; and position size, the same figure in lots, where one standard lot is 100,000 units.

The formula

1. Money at risk = balance × risk %
2. Pip value per unit = pip size × conversion rate to account currency (pip size is 0.0001 for most pairs and 0.01 for JPY pairs)
3. Units = money at risk ÷ (stop in pips × pip value per unit)
4. Lots = units ÷ 100,000

Hypothetical example: a $10,000 USD account, 1% risk and a 50-pip stop on EUR/USD. Money at risk is $100. Because USD is the quote currency, one pip on one unit is worth $0.0001, so units = 100 ÷ (50 × 0.0001) = 20,000, or 0.20 lots.

Pairs where the account currency differs

On GBP/JPY with a USD account, a pip is worth 0.01 JPY per unit and must be converted to dollars. At an assumed USD/JPY rate of 150, one pip per unit is worth 0.01 ÷ 150 ≈ $0.0000667, so a $100 risk with a 50-pip stop gives roughly 30,000 units. On USD/JPY and USD/CAD, the pip value is in yen or Canadian dollars and is converted using the pair’s own rate.

Gold (XAU/USD) and other CFDs follow the same logic, but the lot size comes from the broker’s contract specification, not the 100,000-unit forex standard, so confirm it before treating a lot figure as final.

What the result leaves out

The result assumes the stop fills exactly at its price. A stop is a trigger, not a guaranteed fill, so gaps and slippage can make the loss larger than the money-at-risk figure. Spread and commission are not included; a round-trip commission adds both legs to the cost of a losing trade.

It also does not check whether you have enough margin; use the margin calculator for that. Your broker may round the size to its minimum lot step, often 0.01 lots, which changes the actual risk slightly.