To determine the proper mask size use a cloth tape to measure the horse's muzzle as shown.

The measurement is taken as a circumference around the muzzle and chin.

Minimum Maximum
Small Mask 12 in. (30cm.) 17 in. (43 cm.)
Medium Mask 17 in. (43 cm.) 20 in. (51 cm.)
Large Mask 20 in. (51 cm.) 26 in. (66 cm.)