Position size is the answer to "how much?". Sizing starts from the loss you accept, not from the profit you hope for: decide the amount to risk, measure the distance to the stop, and divide.
Formula
risk amount = account balance × risk per trade
size = risk amount / (|entry − stop| × value per unit of price)
Examples
Forex. $10,000 account, 1% risk = $100. EUR/USD long at 1.0850, stop 1.0820: 30 pips. One standard lot is worth $10 per pip, so the stop costs $300 per lot.
size = 100 / 300 = 0.33 lots
Futures. $50,000 account, 0.5% risk = $250. NQ long at 18,000, stop at 17,990: 10 points at $20 per point is $200 per contract. Size: 250 / 200 = 1 contract (round down; 1.25 contracts do not exist; MNQ at $2 per point would allow 12).
Crypto. $5,000 account, 1% = $50. BTC long at 60,000, stop at 59,400: $600 per coin. Size: 50 / 600 = 0.083 BTC.
Common mistakes
- Sizing from the account, not from the stop. "I always trade 1 lot" means a 10-pip stop risks $100 and a 60-pip stop risks $600.
- Rounding up. When the result falls between sizes, round down.
- Forgetting costs. Commission and expected slippage make the real loss at the stop a little larger than the plan.
- Moving the stop after sizing. A wider stop on the same size is a larger risk.
In SetupBook
The trade form has a position-size calculator next to the Risk field in step 1: enter the balance and the risk percentage to get the amount, then the entry and the stop to get the size, and both are written into the form. Symbol multipliers (100,000 for EUR/USD, $20 for NQ, 100 for XAUUSD) come with sensible defaults and can be changed in Settings.