Using a cloth tape measure placed in the center of your forehead, measure all the way around your head.
*In some cases helmet sizes will vary between the Seer S1602IV (Current) Riot Helmet and the Seer S1711 (New) Ballistic Helmet. For example:
To download a copy of the User's Guide for the Seer S1711-CPD Ballistic Helmet, please click this link.