How to Calculate Forex Lot Size

Ask ten traders what the most important calculation in forex is and most will not say profit — they will say position size. Size is what turns a normal losing trade into an account-damaging one. Here is the full calculation, step by step.

The four inputs

Account balance — the money in the account this trade draws from.
Risk percentage — how much of the balance you are willing to lose if the stop loss is hit. Common choices are 1–2%.
Stop loss distance — how far away, in pips, the trade is "wrong".
Pip value per lot — what one pip is worth per standard lot, in your account currency.

The formula

Lots = (Balance × Risk%) ÷ (Stop pips × Pip value per lot)

Worked example

Balance: $1,000
Risk: 2% → $20 maximum loss
Stop: 50 pips on EUR/USD
Pip value per standard lot on EUR/USD with a USD account: $10

Lots = (1,000 × 0.02) ÷ (50 × 10) = 20 ÷ 500 = 0.04

So the position is 0.04 lots — 4,000 units of EUR. If the stop is hit, the loss is about $20, exactly the amount you agreed to risk.

Always round down

Suppose the formula gives 0.047 lots. Lots trade in 0.01 steps, and the instinct to round to 0.05 quietly raises your real risk above the limit you set. Round down, always. The difference looks trivial on one trade; repeated over hundreds of trades, rounding up is how small over-risks compound.

Where the numbers come from

Pip value per lot depends on the pair and your account currency. On EUR/USD with a USD account it is exactly $10; on pairs like USD/JPY it floats with the exchange rate.
If your account currency is not the quote currency of the pair, the pip value needs a currency conversion — that is where most manual calculations go wrong.

Doing it without the arithmetic

The lot size calculator runs this whole process in one form: balance, risk %, stop pips, pair, account currency, optional conversion rate. For a version that also checks whether your take profit makes the trade worth taking, use the position size calculator, which adds entry/stop/stop-distance and reward-to-risk into the same view.

Estimates only, not financial advice — real fills, spreads and swaps vary by broker.