To size a trade from your stop loss, divide your dollar risk by the stop distance in pips multiplied by the pip value per lot. The stop distance decides the lot, so your loss is always the same fixed amount whether the stop is 15 pips or 120 pips wide.
The formula
Lot size = dollar risk / (stop distance in pips x pip value per lot)
Three inputs feed it: how many dollars you are risking (usually one percent of your account), how far your stop sits in pips, and the pip value for that pair and lot.
Worked example
- Account: $10,000, risking 1% = $100.
- Pair: EUR/USD, where a standard lot is $10 per pip.
- Stop: 50 pips from entry.
Lot size = 100 / (50 x 10) = 100 / 500 = 0.20 lots. If EUR/USD hits your stop, you lose exactly $100, which is your planned one percent.
Why the stop must come first
The stop belongs where your trade idea is proven wrong - below the structure, beyond the range, past the level. That distance is dictated by the chart, not by how big a position you want. Once the stop is placed, sizing is pure arithmetic. Read more on placement in where to place a stop loss.
Important: if the math gives you a very small lot, that is the system working. A wide stop on a small account should produce a small position. The fix is a smaller lot or a broker with micro lots - never a wider risk to justify a bigger trade.
Let the tool do the arithmetic
You should understand this formula, but you should not run it by hand on every trade. FxBacktest and most platforms include auto lot-sizing: you drag your stop line to the right place, set your risk, and the correct lot is calculated instantly. When you backtest, this happens on every trade, so you internalize the relationship between stop width and position size without ever touching a calculator.
Lot size from stop loss FAQ
How do you calculate lot size from a stop loss?
Divide dollar risk by (stop distance in pips x pip value per lot). Risking $100 with a 50-pip stop at $10/pip gives 100 / 500 = 0.20 lots.
Should stop loss or lot size come first?
Stop loss first. Place it where the idea is invalidated, then size the position so hitting it costs your fixed risk. Doing it backwards leads to stops that are too tight.
What if the calculated lot size is tiny?
That is the system protecting you from a wide stop on a small account. Trade the small lot or use micro lots. Never widen risk to justify a bigger position.