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.
