HSMAdvisor operates both in imperial(in) and metric(mm) system.
Every field on the main Speeds and Feeds tab can be toggled to either in or mm simply by clicking on the in/mm label, located next to the field.
The value will be converted automatically.
For example if you enter 0.5 in diameter, you can convert it to 12.7 by clicking on the in label like so:
It is totally normal to have some fields metric and others imperial.