When selecting a size in the drop-down, we include the traditional fitted hat/cap size for your reference.
The way to measure your hat size is to use a tape measure (or a length of string that you will have to hold up to a tape measure) to determine the length. Place the string or tape around your head about 1/8" above your ear, across the mid-forehead, completely around your head. Hold the tape firmly, but not too tightly. By doing this, you will be measuring your head exactly where the hat will sit, which will usually be the largest point. Take your measurement number and divide by PI or 3.14.
For example, if your measurement is 23", if you divide by PI you'll get 7.32 or 7 1/4 to 7 1/2 sizing.
If your measurement falls between sizes, choose the next largest size.