Mutt Muffs Size Chart

The overall circumference measurement of your dog's noggin at the largest point is the best determination of size, because if you choose a size too large, the muffs won't seal well against his head, and you lose all the sound-reducing properties you were shooting for. 

The circumference is the measurement that would create a circle by going across the top of the dog's head, over the ears, and under the chin.

Your dog's weight is also a good indicator of which size to choose.