Place a measuring tape around your nose and over your ears, wrapping around to the base of your skull. If your measurement falls in between sizes, go with a smaller size.

 Size  cm  in
 Toddler  <47  <18.5
 Small  47.5-53  18.75-21
 Medium  53.5-59  21.25-23.25
 Large  59>  23.5>