Let's say that I got 0.01 BTC for $500 in 2021 and I'm sure that I sold it later during the year, but don't know when or for how much. How do I calculate the gain?
You are more specific about buy date that mean you know the platform or the place you made the purchase from, and thus you can get the Bitcoin address.
All bitcoin data is open source and can be traced by everyone so it will be easy to know the date, signature and selling price.
go to
https://blockchair.com/pdf/statement:
- Paste your Bitcoin/Altcoin address
- Select the reporting period
- Download your WALLET STATEMENT