Back: Measure length of pet’s back from the base of the neck to the base of the tail. See below for estimating by breed.
Neck : Measure the circumference of pet’s neck for a snug fit. No need to add inches as you would with a collar.
Chest/Girth : Measure girth around the broadest part of the chest and add 2”.
Note :  If the dog has a stout build, select one size larger.

Recommended sizing by breed

XX-Small: Teacup and dogs under 5 lbs
X-Small: Australian Terrier, Chihuahua, Pomeranian, Toy Poodle, Yorkshire Terrier
Small: Boston Terrier, Jack Russell Terrier, Maltese, Pug, Silky Terrier
Medium: Bichon Frise, Beagle, Cocker Spaniel, Corgi, Scottish Terrier
Large: Brittany Spaniel, Bulldog, Collie, Schnauzer, Springer Spaniel
X-Large: Doberman Pinscher, German Shepherd, Golden Retriever, Labrador Retriever, Rottweiler