Hi @wagner4599 and thank you for your question.
For most accounts, the accounting method is the FIFO (First-In First Out) rule. This would mean, as you stated, the first Bitcoin purchased would be the the first Bitcoin sold.
Depending on the exchange, sometimes, you are able to select the a different methods such as LIFO (Last-In First-Out) or HIFO (Highest-In-First-Out).
According to IRS guidance, you can use a specific identification method like LIFO or HIFO if you have records containing the following information:
- The date and time each unit was acquired.
- Your basis and the fair market value of each unit at the time it was acquired.
- The date and time each unit was sold or disposed of.
- The fair market value of each unit when sold or disposed of.
I hope this helps!