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, snug but not tight. 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 XX-LARGE: Irish Wolfhound, Great Dane, Mastiff, Newfoundland, St. Bernard Size Fits Length Fits Necks Fits Chests XX-SMALL: Teacup and dogs under 5 lbs X-SMALL (Length) Up to 8" (Neck)8"-10" (Chest) 10"-13" SMALL (Length) 8"-12" (Neck) 10"-11 1/2" (Chest)13"-15" MEDIUM (Length) 12"-16" (Neck)11 1/2"-14" (Chest)15"-20" LARGE (Length)16"-20" (Necj) 14"-17" (Chest)20"-25" X-LARGE (Length) 20"-24" (Neck) 17"-21"(Chest) 25"-29"