M2
 

Helmet Size	XS/S	M/L	XL/XXL
Head Circumference (cm)	52 - 55	56 - 59	60 - 63
Head Circumference (in)	20.5 - 21.75	22 - 23.25	23.75 - 25

M5
 

Helmet Size	S/M	L/XL
Head Circumference (cm)	54 - 58	58 - 62
Head Circumference (in)	21.25 - 23	23 - 24.5

Project 23 Full Face
 

Helmet Size	XS	S	M	XL	XXL
Head Circumference (cm)	53 - 54	55 - 56	57 - 58	61 - 62	63 - 64
Head Circumference (in)	20.75 - 21.25	21.5 - 22	22.5 - 22.75	24 - 24.25	24.8 - 25.2

Note: Measure at a point approximately one inch above the eyebrows in front and at a point on the back of the head that results in the largest measurement.