What size hat do I wear?

I 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 π; viola you have the diameter!
"Viola", you say, π ?...
Mr Hat, I don't speak French, I just want to know what size hat I wear!
Ah indeed, π is pi, its Greek by the way, it stands for 3.14 or 3.14159 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 size hat, so get out your measuring tape...