Forex Position Sizing: Understand the Calculation and the Limits of a Planned Loss

Position sizing connects the amount referenced by a trade with the effect of a price movement. A sizing worksheet can make assumptions visible, but it cannot guarantee an execution price or cap every possible loss. That distinction is crucial when an example uses a stop distance. This educational framework explains the arithmetic without recommending that a reader trade, choose a particular risk percentage or rely on a formula as a safety guarantee.

Begin with a clearly defined scenario

State the account currency, instrument, direction, entry assumption, adverse-price scenario and relevant costs. A vague goal such as risk a little is not enough for a calculation. The same number of units can produce different account-currency effects across pairs. If the quote currency differs from the account currency, conversion is needed. Use the actual product specification. A formula copied from an online calculator can be wrong if its definition of units, lots or price increments does not match the intended contract.

Translate price distance into money per unit

In a simplified EUR/USD example, a 0.0020 dollar-per-euro move on 10,000 euros corresponds to $20 before costs. If the educational scenario allocates $40 to that modeled price movement, the arithmetic points to 20,000 euros under the same assumptions. This is not a recommended position or loss budget. It omits execution differences and other charges. The example shows why doubling units doubles sensitivity; it does not show that a real account's loss would stop at exactly $40.

Add costs rather than hiding them

Spread, commissions, financing and conversion can affect the account result. Their treatment depends on whether entry and exit prices already reflect executable sides. Avoid counting the spread twice or omitting it by using an idealized midpoint. If a fixed commission applies, its relative impact can be larger on a smaller position. A complete worksheet should identify each cost and when it arises. If a cost cannot be known precisely in advance, use a stated assumption and examine how a worse outcome changes the scenario.