Position Size Calculator

Position size

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Capital at risk

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Notional exposure

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Enter an entry and a stop to see a size.

How the maths works

Risk in dollars is your equity multiplied by the risk percentage. Risk per unit is the distance between entry and stop, multiplied by the contract multiplier (1 for a share, 100 for a standard equity option, $50 per index point for ES). Divide the first by the second and round down: that is the largest whole position that keeps a stop-out at your limit.

units = floor((equity × risk%) ÷ (|entry − stop| × multiplier))

What the number does not know

• Slippage and gaps. A stop is an instruction, not a guarantee. An overnight gap or a fast tape can fill you worse than the price you typed.
• Commissions and fees. Subtract them from the risk budget before you size, or accept a slightly larger loss than the box shows.
• Margin versus notional. Futures margin is a fraction of notional, so the exposure figure is not the cash your broker will hold.
• Correlation. Three positions sized at 1% each are not 3% of risk if they are the same trade wearing different tickers.
• Your own adherence. The calculation assumes you take the stop. If you widen it, the size was fiction.

This is arithmetic, not advice. It does not know your account, your plan or your tolerance for drawdown, and it is not a recommendation to trade any instrument in the list.

Where this fits the method

Sizing is the part of risk management that can be measured. The risk management guide covers the rest: how much heat to carry, how to treat a losing streak, and why the stop distance should come from the chart rather than from the size you want. The article on sizing by volatility explains the ATR-based alternative to a fixed percentage.