It is generally more accurate to have someone else measure your head, rather than trying to do it by yourself.
If you currently have lots of hair, make sure it is pressed down as flat and as tight as possible with a cap, a hairnet, or bobby pins. This will eliminate bulges that distort head contours.
Use a cloth tape measure to ensure accuracy.
1. Around the Head:
Measure all around the head. Starting at your natural hairline in front, follow your hairline to just above your ear. Measure around the back to the bend in your neck, just above your other ear, then around front to where you began.
Average measurement is 22 inches
2. Front to Back:
Measure from your hairline in front (where the roots of your hair start in the center of your forehead), back over the top of your head, then down to where the roots of your hair stop on the back of your neck (usually about an inch below the indentation where your skull meets the top of your neck).
Average measurement is 14 1/4inches
3. Ear to Ear:
Starting from one side in the crease where the front of your ear attaches to your head, bring your tape measure straight up from that point and directly over the top of your head and then straight down to the same position at your other ear. (Make sure the tape measure is not angled toward the front or back.)