Position size calculator
This position size calculator answers the question most traders ask too late: how big should this trade be? You supply your account balance, the percentage you are willing to lose if the trade fails, and the distance to your stop-loss. It returns the lot size that makes those three numbers consistent.
The formula
Position size (lots) = money at risk ÷ (stop distance in pips × pip value per lot)
Pip value per lot is $10 for pairs quoted in US dollars. For other pairs the calculator converts it using the current price, exactly as the pip calculator does.
Worked example
Illustrative numbers. Balance $8,000, risk 1.5% per trade, stop-loss 30 pips away on EUR/USD.
- Money at risk: 8,000 × 1.5% = $120.
- Loss per lot if the stop is hit: 30 pips × $10 = $300.
- Position size: $120 ÷ $300 = 0.4 lots (40,000 units).
Same trade on USD/JPY at 150.00: pip value per lot is about $6.67, so a 30-pip stop costs about $200 per lot and the size becomes $120 ÷ $200 = 0.6 lots. Same risk, different size, because pips are worth different amounts.
Reading the result
The answer is a maximum, not a target. Round it down to the nearest size your provider allows. Keep in mind three things the formula cannot see:
- Gaps and slippage. In fast markets or over a weekend, price can jump past your stop, and the loss can exceed the plan.
- Costs. The spread and any overnight swap are extra; see trading fees explained.
- Margin. A correctly sized trade still needs enough margin; check it in the margin calculator.
Many traders keep risk per trade small precisely so that a string of losses, which happens to everyone, leaves the account able to continue.