Place a measuring tape around the circumference of the head, slightly above the ears and just above your eyebrows.
Head Size		Hat Size
Inches	Cm
21	53	6 5/8	XS
21 1/2	54	6 3/4	S
21 5/8	55	6 7/8	S
22 1/8	56	7	M
22 1/2	57	7 1/8	M
23	58	7 1/4	L
23 3/8	59	7 3/8	L
23 3/4	60	7 1/2	XL
24	61	7 5/8	XL
24 1/2	62	7 3/4	XXL
25	63	7 7/8	XXL