Wrap the measuring tape around the fullest part of the head. This will be just above the ears and about a half inch above the eyebrows for most, and take the measurement a tthe forehead. Measure it couple timtes to get the most accurante measure.

SizeX-SmallSmallMediumLargeX-LargeXX-LargeXXX-Large
IN20. 7/8 - 21 1/421 5/8 - 2222 1/8 - 23 1/823 1/4 - 23 5/824 - 24 3/824 3/425 1/4
CM53 - 5455 - 5657 - 5859 - 6061 - 626364
SizeINCM
X-Small20. 7/8 - 21 1/453 - 54
Small21 5/8 - 2255 -56
Medium22 1/8 - 23 1/857 - 58
Large23 1/4 - 23 5/859 - 60
X-Large24 - 24 3/861 - 62
XX-Large24 3/463
XXX-Large25 1/464