Hat sizes might seem confusing, especially if you never look at the size and just try several hats on your head. This cannot be done when you are ordering off the Internet or through a catalog. The first step is to measure your head, then take that measurement and convert it into metric, and then convert this number to a hat size.
Things You'll Need
Wrap the string around your head where your hat will sit. This will be about 1/8" above your ears. Hold the string where one end meets the rest of the string.
Lay the string along a ruler, still holding the spot where the string overlapped. Look at the measurement in inches. Most people will have a measurement between 21 1/8 inches to 24 5/8 inches.
Convert this number into centimeters by multiplying it by 2.54. For instance, if your head is 23 inches around, that is the same as 59 cm (see reference).
Divide the number in step 3 by 8 to get your hat size. This will likely be a number ranging from 6 3/4 to 7 7/8.
Convert your hat size into a general size. Refer to a sizing chart or remember that every 1/4 inch is a different size (see resource). Those with sizes less than 7 are small; 7 - 7 1/8 are medium; 7 1/4 - 7 3/8 are large; 7 1/2 - 7 5/8 are extra large; and those with larger sizes are 2X large.