This may help you when buying your one of a kind hat!

You'll need either a tape measure or a piece of string. Hold it up against the middle of your forehead (this is the fullest part of your head, and it's also where a hat will sit), and wrap it around your head.  Make sure to hold it firmly, but not too tight.  

Round your measurement to the nearest 1/8", and once you've got a number, you can figure out if you're a small, medium, or large.  Small hats (listed as size 6 3/4 - 6 7/8) are for heads that measure 21 1/8" - 21 1/2" (54 - 55 cm).  Medium hats (listed as size 7 - 7 1/8) are for heads that measure 21 7/8" - 22 1/4" (56 - 57 cm).  Large hats (listed as size 7 1/4 - 7 3/8) are for heads that measure 22 5/8" - 23" (58 - 59 cm).  

If your measurement is bigger than all of these, there are X-large women's hats (listed as size 7 1/2 - 7 5/8) for heads that measure 23 1/2" - 23 7/8" (60 - 61 cm).  There are XX-large women's hats (listed as size 7 3/4 - 7 7/8) for heads that measure 24 1/4" - 24 5/8" (62 - 63 cm).

If your measurement falls in between two sizes?

Go with the bigger one.

If you see a hat that's listed as "One Size Fits Most" typically, only fit heads that measure 21 1/2" - 22 1/4".

FYI:  If you've got very thick hair on the top of your head or a particular style that may interfere with the way the hat sits on your head, be sure to take it into consideration.  You may need to go up a size.

Men can measure their head the same way.  The same measurements for Small, Medium, Large, X-Large, and XX-Large apply.  The main difference is that many men hats are offered in "One Size Fits All", which can be a bit tricky.  Similar to the women's "One Size Fits Most", if your head is very large or very small, it may not fit perfectly.