Position Sizing: The Full Formula
The exact calculation that translates your risk rule into a lot size for every trade.
Researched and written by Laurent The Formula
Position sizing connects your risk rule (1-2%) to the specific trade you're about to take. The formula:
Worked Example 1: EUR/USD, USD Account
Risk per trade: 1% = $50
Stop-loss: 30 pips
Pip value for EUR/USD: $10 per pip per standard lot (1.0)
Lot size = $50 / (30 x $10) = $50 / $300 = 0.17 lots (17 micro lots)
Worked Example 2: GBP/JPY, USD Account
Risk per trade: 1.5% = $120
Stop-loss: 45 pips
Pip value for GBP/JPY (varies with USD/JPY rate): approximately $6.67 per pip per standard lot (when USD/JPY = 150)
Lot size = $120 / (45 x $6.67) = $120 / $300 = 0.40 lots
Worked Example 3: EUR/USD, EUR Account
Risk per trade: 2% = 60 EUR
Stop-loss: 25 pips
Pip value for EUR/USD in USD: $10 per standard lot
Convert to EUR: at EUR/USD = 1.08, pip value in EUR = $10 / 1.08 = 9.26 EUR per standard lot
Lot size = 60 EUR / (25 x 9.26 EUR) = 60 / 231.5 = 0.26 lots
Why Stop-Loss Width Determines Lot Size
Notice something important: the wider your stop-loss, the smaller your lot size must be to maintain the same dollar risk. This is not a problem; it's a feature. It means:
- On volatile days (wider ATR, wider stops), you automatically trade smaller
- On calm days (tighter stops), you can trade larger
- Your dollar risk stays constant regardless of market conditions
This is exactly what professional risk management looks like. The lot size adapts to the trade; the risk amount stays fixed.
Never Set the Stop Based on Position Size
A critical mistake: "I want to trade 0.5 lots, so I'll put the stop 20 pips away because that fits my risk." No. The stop-loss goes where the trade idea is invalidated (below support, above resistance, 1.5x ATR). Then you calculate the lot size to fit that stop within your risk percentage.
1. Identify the trade setup
2. Determine where your stop-loss goes (technical level)
3. Calculate lot size based on stop distance and risk %
4. If the resulting lot size is too small to be meaningful, skip the trade
Never: decide lot size first and then fit the stop to it.
Key Takeaways
- • Lot size = (Account balance x Risk %) / (Stop-loss in pips x Pip value per lot).
- • Always calculate position size BEFORE entering the trade, not after.
- • Wider stops require smaller lot sizes to maintain the same dollar risk.
- • Use the Position Size Calculator for speed, but understand the math behind it.