Wrap a soft tape measure around your head, just above the eyebrows, keeping the tape level from front to back
If you don't have a soft tape measure, use a piece of string and then lay the string out on a table. Use a ruler to measure the total length
If the helmet is a gift, and you don't want to (or can't) measure the head, then measure the inside of his/her current helmet or a baseball hat they wear frequently
