First, you will need some measuring tape. At the front of your hairline, hold the beginning of the tape with your thumb. Wrap the tape behind your ear to the nape of your neck. Circle back around to your other ear and meet at the front of your hairline again. 95% of the population will be an Average Cap size. Caps are made of comfortable, elastic cloth and can stretch to fit head sizes from 21.5" through 22.5".
Sizes may vary depending upon brand.
Most cap constructions have adjustable tabs that allow the cap to adjust up to 1" for a comfortable and secure fit.