The following charts are intended as a guide only. Individual dogs within a breed can crossover a range of sizes depending on physique, and thus care should be taken to measure your animal accurately. 

Dog Clothing:Measure from the base of the neck to the base of the tail

•   8"/20cm - Small breed puppies; small Chihuahua; small Yorkshire Terrier
• 10"/25cm - Chihuahua; Chinese Crested; Yorkshire Terrier
• 12"/30cm - Maltese; Min Pinscher; Pomeranian; Toy Poodle
• 14"/36cm - Shih Tzu; Mini Poodle; Jack Russell; Pekingnese; Pug; Cavalier King Charles
• 16"/41cm - Cocker Spaniel; Beagle; Cairn Terrier; Westie; Bichon Frise; Fox Terrier; Lhasa Apso; Scottish Terrier; Welsh Corgi; Mini Schnauzer;
• 18"/46cm - Border Collie; Whippet; English Bulldog; Dachshund;French Bulldog
• 20"/51cm - Staff Bull Terrier; Chow; Standard Schnauzer;
• 22"/56cm - Bassett Hound; Pointer; Airedale; Boxer; Standard Poodle; Samoyed; Shar-Pei;Bull Terrier
• 24"/61cm - Dalmatian; Setter; Retriever; Greyhound; Labrador; Rottweiler; Bull Mastiff
• 26"/66cm - Afghan; Collie; Doberman; Irish Red Setter; Pointer; German Shepherd; Rhodesian Ridgeback
• 28"/71cm - Great Dane; Old English Sheepdog; St Bernard; Weimeraner

While most pet accessory manufacturers size their clothing in inches, some use the following classification:

Dog Collars:

Measure circumference of neck and add 2"/5cm (to ensure comfort)

•   8"/20cm - Small Breed Puppies; Small Chihuahua
• 10"/25cm - Chihuahua; Small Yorkie
• 12"/30cm - Toy Poodle; Yorkie; Min Pinscher
• 14"/36cm - Cavalier King Charles; Cairn Terrier; Dachshund; Lhasa Apso;Pomeranian; Shih-Tzu; Westie;
• 16"/41cm - Beagle; Mini Poodle; Scottish Terrier, Welsh Corgi; Min Schnauzer
• 18"/46cm - Cocker Spaniel; Standard Poodle; Pug; Standard Schnauzer; Springer Spaniel; Fox Terrier
• 20"/51cm - Afghan; Airedale; Border Collie; Boxer; Dalmatian; Staffie; Vizla
• 22"/56cm - Bassett Hound; Bulldog; Chow; Doberman; Retriever; Irish Red Setter; Pitbull; Pointer; Shar-Pei;Weimeraner;
• 24"/61cm - German Shepherd; Labrador; Old English Sheepdog; Rottweiler; Ridgeback
• 26"/66cm - Bloodhound; Great Dane; Bull Mastiff

Harnesses:

Harnesses are sized by chest circumference, measured just behind the front legs.
Apply this measurement to the size guide contained in the product description for each harness.