How is my cost basis calculated?

The cost basis is what your securities actually cost you. It is what gets subtracted from the sale price to give your gain or loss.

In France, when you have bought the same security several times at different prices, the law requires a weighted average: everything you spent is added up and divided by the number of securities held.

Example: you buy 100 shares at €80, then 200 shares at €110. You spent €8,000 + €22,000 = €30,000 for 300 shares, that is €100 per share on average. If you sell 150 shares, their cost basis is 150 × €100 = €15,000, regardless of when you had bought them.

Fees are handled automatically: purchase fees are added to your cost basis, sale fees are deducted from your sale proceeds. They therefore reduce your taxable amount, provided you entered them.

Other countries use other calculation methods. Earnnest applies the one for the country you selected.