Most helmet manufacturers list the head circumference prominently on the helmet packaging. You may see a size designation--small, medium or large--that correlates with a helmet sizing chart that lists head circumference measurements. Try the helmet on.
What size helmet is a 22 inch head?
Standard Size Hat Size Inches
------------- ------------- ---------------
XS 7 22
S 7 1/8 22 3/8
M 7 1/4 - 7 3/8 22 3/4 - 23 1/4
L 7 1/2 23 5/8