Lot Size From Risk and Stop: What a Good Calculator Must Handle

[Shadow Trader]

The core formula is short: the amount risked divided by the stop distance expressed in account-currency value per lot. Most errors come from the inputs and the rounding, not from the arithmetic.

Stop distance should be measured from the realistic fill, not from the chart price. For a buy, the entry fills at the ask while a protective stop triggers on the bid, so the spread widens the true distance. Commission per lot adds a fixed cost that is best included in the risk figure rather than ignored.

Conversion matters when the quote currency differs from the account currency, since the value of one pip must be translated at a current rate. Contract size also varies across instruments, so hard-coded constants that work for currency pairs may fail on metals or indices.

Rounding is the quiet trap. Brokers allow volume only in fixed steps, and rounding to the nearest step can push risk above the intended amount. Rounding down keeps the risk at or below the limit. If the minimum lot already exceeds the permitted risk, a calculator should refuse the trade instead of silently increasing size. Showing the final risk as a percentage of equity provides a last sanity check before the order goes out.