Starting from underneath the chin, go up the side of the head, over one ear, over the top of the head, back down the other ear, and back to under the chin to make a complete circle. (Essentially you're measuring the circumference of the part of your face that the mask will be encircling.) Use a soft ruler or yarn to measure, and make sure to keep it snug or else you'll wind up with a mask that's too large.

Also Note: Your masks are adjustable! If you receive your mask and are unsure if it fits, please ask your coach or an experienced team mate to help you adjust your mask before exchanging it for an alternate size. Most fencers will fit a small or medium mask!