Google seems to publish more and more of the Android system apps to Google Play.
After the Dialer and Contacts app, the calculator is the next app.
With Version 6.0 the app also supports Android Wear (it's pretty cool to have a calculator on your watchâ€¦).
But sadly, the app is only available for Nexus devices and also the APK, which is downloadable on APKMirror, requires Marshmallow or a later version (Android N).
Here you can see some screenshots, how the app looks on watches:
Images for this post from Android Police