Designed for lightweight comfort. Zippered pockets can store supplies and identification. Reflective strip for safety All sizes are based on your dog’s girth. To determine girth, Use a sewing tape or string to measure behind your dog's front legs to get their chest circumference. Measure around your dog's rib cage about an inch or two behind the front legs. Tiny: 13-16 inches (Yorkshire Terrier, Silky Terrier, Papillon, Maltese, Toy Poodle, Miniature Chihuahua) Extra Small: 16--23 inches (Large Chihuahua, Cavalier King Charles, Pomeranian, Shih-Tzu, Schnauzer, Dachshund) Small: 23-29 inches (Beagle, Australian Shepherd, German Shorthair, Weimaraner) Medium: 29-35 inches (American Staffordshire Terrier, Boxer, Golden Retriever, Dalmatian, Pit Bull) Large: 35-39 inches (German Shepard, Labrador, Rottweiler, Doberman Pinscher, Sheepdog) Extra Large: 39-45 inches (Saint Bernard, Great Dane, Bernese Mountain Dog, Mastiff, Irish Wolfhound)