Position size calculator
Most traders choose a trade size first and only later discover what a stop-loss would cost. A position size calculator reverses the order. You decide the most you are willing to lose on one idea, measure how far away your stop is, and the tool gives the size that keeps the loss at that level if the stop is filled at its price.
The formula
Position size (lots) = risk amount ÷ (stop distance in pips × pip value per lot)
Pip value per lot is $10 for pairs quoted in US dollars, and is worked out from the exchange rate for other pairs, exactly as in our pip calculator.
Worked example
The figures below are illustrative.
- Account balance $10,000, risk 1% per trade: the risk amount is $100.
- EUR/USD with a stop 25 pips from the entry: each lot moves $10 per pip, so a 25-pip stop costs $250 per lot.
- Position size = $100 ÷ $250 = 0.4 lots (40,000 units).
A second example: risking 2% of the same $10,000 ($200) on USD/JPY at 150.00 with a 40-pip stop. One lot is worth about $6.67 per pip, so the stop costs about $266.67 per lot, and the size is $200 ÷ $266.67 = 0.75 lots.
How to read the result
The answer is the largest size that keeps the planned loss at your chosen percentage. Round down to the nearest size your provider allows, never up. The calculation assumes the stop is filled at its level; in fast markets, over weekends or around news, prices can jump past a stop, so the real loss can be larger.
A wider stop gives a smaller position and a tighter stop a larger one, for the same money at risk. That is the point: the stop should be set by what the chart or your reasoning says, and the size adjusted to fit, not the other way round. Check the margin this size needs with the margin calculator, and remember that risk percentages add up across several open positions.