This is a test of commonsense. Complete the next sentence:

How to fit a riding helmet
Measure your head.
Helmet sizes correspond to the circumference of your head measured at its widest point. To find your size, use a flexible measuring tape and wrap it around your head about 1 inch/2.5 cm above your eyebrows.
Make sure it stays flat to get an accurate measurement. Note the result in both inches and centimeters, and write it down to take to the store with you.