Works nicely! What exchange(s) does it use to determine the USD/Bitcoin price?
I made a simple Fiat/BTC/Sats calculator.
Some issues with existing ones include that your data is sent to a backend instead of being converted locally, the inability to easily copy and paste, and the readability issue that arises from not being able to see the exact amount due to the non-display of all eight decimal points.
Link here: https://mempool.space/tools/calculator
Discussion
A combination of the price of all the top exchanges.
You can find the backend source here https://github.com/mempool/mempool/blob/master/backend/src/tasks/price-updater.ts