While the standard Calculator Android app from Google is usually included with any Android device, it has never been released in the Google Play Store. That has now changed as the Calculator is now available in the store as a standalone app.
This development means that the Calculator app can be updated with more features on a faster basis rather than having to wait for an Android firmware update. While the app is basically the same one that's already on smartphones and tablets, the new 6.0 version does include support for Android Wear. This means smartwatches with the OS can perform simple calculations with the app from now on.
You can download the standalone version of the Google Calculator in the Google Play Store.
Thanks to Matthew for the tip!