Since people have different shaped heads, helmet size (the size of the top of your head) is not always proportional to the chinstrap size (the size of your face). The Ops-Core retention system provides a significant performance advantage over traditional systems, but has less overall strap adjustment range. This is why we offer two retention system sizes instead of one to comfortably fit the full spectrum of users.
To ensure that you order the right size chinstrap, we recommend that you check your 'face size' with the following measurement technique. This is more accurate than relying on helmet size alone, and can save you the aggravation of buying the wrong size system.