Mastering Position Sizing: Calculate Your Perfect Trade Size for Success

Learn how to calculate the perfect trade size with our expert guide on position sizing.

Image source: Position Sizing in Trading: How to Calculate & Examples - Britannica

Position sizing is the process of deciding how many shares, contracts, or lots to trade so that your loss stays within a pre-set risk limit. The core formula is: position size = amount you’re willing to risk per trade ÷ risk per unit.¹^{, 2}

Simple formula

Start with two numbers:

Account risk: how much money you will lose if the stop-loss is hit.
Per-unit risk: the distance between your entry price and stop-loss, measured per share, contract, or pip.

Then calculate:

[ \text{Position size} = \frac{\text{Account risk}}{\text{Entry price} - \text{Stop-loss price}} ]

For forex, the same idea is often expressed with account value, risk percent, pips risked, and pip value.²

Example

If you have a $10,000 account and risk 1% per trade, your maximum loss is $100. If you buy a stock at $50 and place your stop at $49.50, your risk is $0.50 per share, so the position size is 200 shares because (100 \div 0.50 = 200).³^{, 4}^{, 5}^{, 1}

Practical steps

Decide your risk per trade, often a fixed percent like 1%.⁵^{, 3}
Set the stop-loss level first, so the trade’s risk is defined.⁴
Divide the dollar risk by the per-unit risk to get the size.¹^{, 2}
Round down if needed so you do not exceed your risk limit.³

Mermaid flow

Diagram

Common mistakes

The biggest mistake is choosing the number of shares first and then forcing the stop-loss to fit the trade. Another mistake is ignoring leverage, pip value, or contract multiplier, which can make the actual risk much larger than expected. A smaller, consistent risk rule usually protects capital better than guessing trade size.⁶^{, 7}^{, 2}^{, 4}^{, 5}