1. You can use a tape measure to determine the length in centimeters.
2. Place the string around your head about 1/8" above your ear, across the mid-forehead, completely circling your head. Hold the tape firmly, but not too tightly.
3. Basically, you need to measure your head exactly where the hat will sit. If your measurement falls between sizes, choose the next largest size
4. Always measure more than once to make sure your size is accurate.