Calculator is missing or uninstalled on my Mate 10. How to get it back?

As far as I know you can only disable calculator, but you can’t uninstall it on Mate 10. If you’ve root access to your phone then it’s a different story. Follow the steps here to check if you’ve accidentally disabled it:

1. From the device’s Settings, tap Apps & notifications > Apps.
2. Check if you can find calculator here and whether it’s disabled.
3. If it’s disabled, you can enable it here.

factory reset you should be able to get it back

Just download one from the google play store. The pre-install calculator is too basic. You should get one that comes with unit conversions.