Trading costs: calculate what remains after the headline gain
A gross trading gain is the difference between sale proceeds and purchase cost before additional charges. To understand what the account earned, reconcile the actual cash movements and the broker’s fees.
A $20 gain with $8 in commissions
Buy 100 shares at $20 and sell at $20.20. The gross price difference is $20. If purchase and sale commissions total $8, the result becomes $12 before other charges and taxes.
If the stated fill prices already reflect the bid-ask spread, do not subtract the same spread again. A comparison using mid-prices or last prices may instead need an execution-cost adjustment. Define the starting point before adding costs.
Currency adds another layer
An account measured in euros and a stock traded in dollars require a consistent exchange-rate treatment. Currency conversion fees and exchange-rate changes are different effects. Buying and selling at different exchange rates can change the euro result even when the dollar gain is unchanged.
Keep results in both currencies in your journal. Bring that record to an educational 1:1 session if you want to discuss how you review trades. Compare the calculation with the account statement rather than a chart’s percentage label.
Check recurring charges such as market data, financing or custody where applicable. Decide how to allocate them across your performance record and apply the approach consistently.
For practice, reduce the hypothetical price gain to $0.05 per share. Gross profit becomes $5, which the $8 commissions turn into a $3 loss before anything else. The same fee schedule can have very different effects on large and small gross gains.