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Lot Size Calculator

A position size calculator helps limit how much a single idea can cost. Traders use it with stop-losses; investors can use the logic to cap single-stock exposure.

AtlanticQuote editorial team. Updated 26 September 2026. How we work

Position size calculator

Position size
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Price chart with a clearly marked protective floor line under the price

The formula

Amount at risk = account × risk %
Position size = amount at risk ÷ loss per unit if the stop is hit

This calculator is set up for currency pairs, where the stop is measured in pips. The same idea works for shares: with a $20,000 portfolio and a limit of 1% ($200) on a trade, a share bought at $50 with a stop at $45 ($5 risk per share) allows 40 shares.

For long-term investors

Most long-term investors do not use stop-losses, but the principle of deciding in advance how much any single holding can hurt still applies. Many keep individual shares to a small fraction of the portfolio. See portfolio diversification.

Further reading