What size hat do I wear?

We hear it all the time "What size hat do I wear?
Well, here it is!
Mr Hat's simple guide for determining your proper hat size.

All hats are sized based upon the diameter of your head. If you measure the circumference, and divide by pi; viola you have the diameter! "Viola", you say, "pi"... Mr Hat, I don't speak French, I just want to know what size hat I wear! Ah indeed, pi is Greek by the way, it stands for 3.14 or 3.1417 if you need to be accurate; in fact it carries on forever past the decimal point, but that's not the point is it? We just want you wearing the proper hat size, so get out your measuring tape...