Most new traders decide what to trade carefully and how much to trade almost at random. Position sizing fixes the second part: you choose how much money you are willing to lose if your stop loss is hit, and the position size follows from that.
The formula
Lots = (Account Balance × Risk %) ÷ (Stop Loss in Pips × Pip Value per Lot)
Example: $10,000 account, 1% risk, 50-pip stop on EUR/USD (pip value $10 per standard lot)
- Amount at risk:
10,000 × 1% = $100 - Position size:
100 ÷ (50 × 10) = 0.20 lots
If the stop loss is hit, you lose $100 — no more (excluding slippage and costs).
Why 1–2%?
Many educators suggest risking about 1–2% of your account per trade. This is a common rule of thumb, not a regulatory rule or a recommendation, and the right level depends on your own circumstances. The maths below shows why smaller risk per trade is often preferred. After ten losing trades in a row:
| Risk per trade | Account left after 10 losses |
|---|---|
| 1% | 90.4% |
| 2% | 81.7% |
| 5% | 59.9% |
| 10% | 34.9% |
And losses are harder to recover than they look: after a 50% drawdown you need a 100% gain just to get back to where you started.
Common mistakes
- Choosing the lot size first. Set your stop where the trade idea is proven wrong, then size the position to fit — never move the stop to fit a bigger position.
- Ignoring pip value differences. 50 pips on GBP/JPY is not worth the same as 50 pips on EUR/USD. Always calculate.
- Forgetting costs. Spread and commission make the real loss slightly bigger than the stop distance suggests.
- Adding to losing positions. Averaging down silently multiplies your risk beyond what you planned.
Key takeaway
You cannot control whether a trade wins, but you can always control how much a losing trade costs. That is the single most important habit in trading.