How Do I Measure for a Hat?

So this is probably a “duh, I knew that…really” question but at the moment, I’m clueless. Hats I’ve knit in the past didn’t have a “size” listed, I just knit the pattern. Well, I’ve got one now that has “sizes” and I’m CLUELESS as to how to determine which size to knit.

small 18", medium 19". or large 20". Is this the size of the finished hat circumference? My head is just bigger around than that but I know they stretch, especially, apparently, this one I’m gonna knit.

How do you figure sizes on a hat?

I usually knit a hat about 1" smaller around than the circumference of the intended head. The sizes are usually the finished size, unless otherwise noted.