Position sizing is the process of linking a planned invalidation point to a tolerable loss amount. It is more useful than choosing trade size from conviction alone.
The underlying mechanism
A basic model divides the cash amount at risk by the monetary loss per unit if the stop is reached. The denominator should include instrument-specific pip value, price movement, contract multiplier and estimated costs where appropriate.
Position sizing translates a risk budget into a quantity. The budget should be expressed in money first, with instrument-specific pip or price-step value and a realistic exit distance. Margin availability is a separate constraint, not evidence that the suggested size is prudent.
How to evaluate it in practice
Define risk as a percentage or fixed amount of account equity before deciding on the number of units. Convert currencies carefully and allow for slippage, gaps, and correlated positions. Check minimum contract sizes at the provider.
For a hypothetical USD 10,000 account risking 1%—USD 100—at a 25-pip planned distance and a pip value of USD 10 per standard lot, the approximate size is 0.4 standard lots (100 ÷ (25 × 10)). It is an illustration, not a live recommendation. Our worksheet exposes these inputs.
Common mistakes and limits
A calculated position size is not a guarantee of a capped loss. Stop-market orders can slip and some products have discontinuous price behavior; correlated exposures can exceed intended portfolio limits.
Stops and realized fills can diverge. The SEC stop-order guidance makes clear that a trigger is not guaranteed execution; trading costs and weekend gaps require extra allowance.
An exercise for careful readers
Use the position-size tool to compare two stop distances at the same fixed cash-risk budget, using a documented hypothetical pip value.
Recalculate the hypothetical size at several wider stop distances, then add a fixed cash cushion for spread, commissions and potential slippage. If your contractual margin requirements or concentration limits disallow it, reduce or avoid the position.
Continue your research
Explore the related topic hub, the risk warning and our editorial standards for additional context. External primary-source references appear below this article.
