Uhlmann 2018 1600N FIE Foil Mask
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.
If you measure in at the following you need this size mask:
|XS||Small (0)||Medium (1)||Large (2)||XL (3)|
|Under 21"||21"-23"||24"-25"||26"-28"||Over 28"|