Measuring your head is a starting point for the entire sizing procedure. Due to varying shaped heads that are apparently the same size when measured by a tape may not necessarily fit the same size.
A cloth tape measure may be used to make your initial measurements. The circumference of your head should be measured at a point approximately one inch above the eyebrows in front and at a point in the back of the head that results in the largest possible measurement. Take several measurements to make sure you have the largest one.
PLEASE NOTE THAT THIS IS A ROUGH GUIDE AND NOT EXACT. THE SHAPE OF YOUR HEAD IS AS IMPORTANT AS THE MEASUREMENT WHICH MAY AFFECT THE FINAL FIT! THE FOLLOWING SIZES ARE AN APPROXIMATION AND ARE BASED ON AVERAGE HEAD SIZES. IN SOME CASES VARIATION MAY OCCUR.