I personally use a spreadsheet but I'm sure there are better way to do it.
Do you know any good websites/apps to do it?
There's an app to calculate your wins/loss, I've installed this app, called 'coin portfolio'.
You can check the thread here
https://bitcointalk.org/index.php?topic=2060538.0But, current Os that supported is Android and iOS version will be created as well.
Well, it's better than just write it on spreadsheet as there are several coins listed in the app.