I use this calculator/converter app for years, it's old and doesn't have update for a long time but still works perfectly. You can save any currency, incéuding BTC, ETH and other shitcoins and quickly convert from any of them to any other by clicking source then destination currency.

You can also convert any units, lenght, weight, energy, power... and of course it's a basic calculator that is working pretty well, you click on previous result to reuse a number.

Probably the most used app on my phone, ... before phone app 😆