Buy–sell arithmetic: a price difference is not net proceeds
A unit price can rise while the amount left after fees is smaller than the price difference suggests. The useful question is about cash flows: what you paid and what you receive after the costs included in your calculation.
Draw two money paths
Purchase cost combines the value of the units and purchase fees. Net sale proceeds subtract sale fees from the sale value. The difference between proceeds and cost is the calculated result, not merely the difference between unit prices.
Investor.gov highlights purchase, sale and ongoing costs. This simple model cannot include a charge you omit, and does not automatically calculate tax, currency movements or execution slippage.
References: Investor.gov — Understanding fees
Separate a transaction fee from a per-unit fee
A fixed fee for one transaction is not multiplied by the number of units. A percentage charge needs a clear base: the purchase or sale value. Keep the fee statement so that assumption can be checked.
Compare amounts in one currency. If the records use different currencies, establish a dated conversion before combining them; otherwise exchange-rate effects can be mistaken for price performance.
Read a percentage with its denominator
The calculated return here divides the net difference by total acquisition cost. It is not an annualized rate or a performance comparison across unequal time periods.
The tool only calculates from your inputs. It does not execute transactions, select assets or predict prices. A hypothetical result is not a profit promise or a final settlement statement.
Two small fees change the result
Buying 10 units at 50 with a fixed fee of 5 costs 505. Selling them at 55 with a fixed fee of 5 leaves 545. The difference is 40, not 50.
Relative to cost, 40 ÷ 505 × 100 ≈7.92%. All amounts use one fictional currency and only the listed fees; the example includes no tax, ongoing charges or recommendation to sell.