Formula to determine lot size from stop loss width: calculate USDJPY, EURUSD, and GOLD using the yen account and the dollar account

“Limit a single loss to 1% of the account and back-calculate the lot size,” yet as soon as you switch the instrument or the account, the calculation may not add up. The formula remains one and only part that changes is one. In this article, we consolidate the formula into one line and calculate the changing part across six scenarios: three instruments and two account currencies.

Conclusion first

• Lot size = allowable loss ÷ (stop loss width × profit or loss per price movement). Profit or loss per price movement refers to the profit or loss when the price moves by one unit in one lot.
• What changes by instrument or account is only “profit or loss per price movement.” It is obtained by contract size × conversion rate. Conversion is only necessary when the currency in which profit or loss is calculated differs from the account currency.
• The calculation result is truncated to the nearest lot increment (e.g., 0.01). Rounding up would exceed the allowable loss.

Assembling the formula

The profit or loss when the price moves by one unit in one lot is hereafter called the “profit or loss per price movement.” The "1 unit" of price is 1 yen for USD/JPY, 1.0000 for EUR/USD, and 1 dollar for GOLD.

Profit or loss per price movement = contract size × conversion rate

Amount lost if stopped out = lot size × stop loss width × profit or loss per price movement

Solving for the lot size yields the following formula.

Lot size = allowable loss ÷ (stop loss width × profit or loss per price movement)

Stop loss width is entered as a price difference, not in pips. A 30-pip move in USD/JPY is 0.30 yen, a 30-pip move in EUR/USD is 0.0030, and for GOLD you use the price difference in dollars as shown on the chart.

Whether conversion is needed depends on the currency in which the instrument’s profit or loss is expressed.

• USD/JPY: profit or loss is in yen. If the account is in yen, no conversion is needed; if the account is in dollars, convert yen to dollars (yen ÷ USD/JPY rate).
• EUR/USD: profit or loss is in dollars. If the account is in dollars, no conversion is needed; if the account is in yen, convert dollars to yen (dollars × USD/JPY rate).
• GOLD: profit or loss is in dollars. The same as EUR/USD.

Assumptions used in the calculation

• Contract size: USD/JPY and EUR/USD are 100,000 units, GOLD is 100 ounces. Since this can vary by account or instrument, verify in the instrument’s specification (right-click the instrument in the quote board → “Specification”).
• Conversion rate: 1 dollar = 158 yen. This is a provisional value for explanation.
• Allowable loss: yen accounts = 10,000 yen (1% of 1,000,000 balance), dollar accounts = 100 dollars (1% of 10,000 dollars).
• Stop loss widths: USD/JPY 30 pips (0.30 yen), EUR/USD 30 pips (0.0030), GOLD 15 dollars. These numbers are for demonstration and not recommendations. GOLD is set to 15 dollars because the hourly price range average over 13 years is about 5.5 dollars, while in the last 12 months (Oct 2025–Sept 2026) it has been around 21–24 dollars (measured from MetaQuotes-Demo 1-minute data).
• Lot increment: 0.01.
• Spreads, slippage, and fees are not included.

Six calculations

USD/JPY (profit or loss expressed in yen)

• Yen account: profit or loss per price movement is 100,000 yen. If stopped out by 1 lot, loss is 100,000 × 0.30 = 30,000 yen. Lot size = 10,000 ÷ 30,000 = 0.3333 → 0.33. Actual loss is 0.33 × 30,000 = 9,900 yen.
• Dollar account: convert the loss of 30,000 yen to dollars by dividing by 158, giving 189.87 dollars. Lot size = 100 ÷ 189.87 = 0.5267 → 0.52. Actual loss is 0.52 × 189.87 = 98.73 dollars.

EUR/USD (profit or loss expressed in dollars)

• Dollar account: profit or loss per price movement is 100,000 dollars. Loss for 1 lot is 100,000 × 0.0030 = 300 dollars. Lot size = 100 ÷ 300 = 0.3333 → 0.33. Actual loss is 99.00 dollars.
• Yen account: multiply 300 dollars by 158 to get 47,400 yen. Lot size = 10,000 ÷ 47,400 = 0.2110 → 0.21. Actual loss is 0.21 × 47,400 = 9,954 yen.

GOLD (profit or loss expressed in dollars)

• Dollar account: profit or loss per price movement is 100 ounces × 1 dollar = 100 dollars. Loss for 1 lot is 100 × 15 = 1,500 dollars. Lot size = 100 ÷ 1,500 = 0.0667 → 0.06. Actual loss is 0.06 × 1,500 = 90.00 dollars.
• Yen account: 1,500 dollars × 158 = 237,000 yen. Lot size = 10,000 ÷ 237,000 = 0.0422 → 0.04. Actual loss is 0.04 × 237,000 = 9,480 yen.

If you line them up in yen accounts, the loss when stopped out for 1 lot is 30,000 yen for USD/JPY, 47,400 yen for EUR/USD, and 237,000 yen for GOLD. The lot sizes to keep within 10,000 yen are smaller: 0.33, 0.21, and 0.04 respectively. Even with similar stop widths like “30 pips” or “15 dollars,” the lots differ by about eightfold between USD/JPY and GOLD.

Also note what happens if you round up. In a yen account for USD/JPY, increasing 0.33 to 0.34 results in a loss of 10,200 yen, exceeding the allowable loss by more than 200 yen. A small difference, but it will consistently push in the same direction.