freesewing/markdown/dev/guides/prerequisites/units/en.md

title	order
Units	40

Internally, freesewing uses millimeter. When you write 1, that’s one mm. When you write 7.8, that’s 7.8mm.

While you can use cm or inch on the FreeSewing website, that is merely a layer of abstration on top of the internal units, which are always mm.

So as a pattern designer, you will work with mm.