Measure your Pet:
Length: Measure from the base of the tail to where the collar typically sits at the base of the neck.
Chest: Measure the entire circumference at the widest point of your pet's chest (typically right behind the front legs).
Neck: Measure the neck's circumference at the point where the collar sits.

X-SMALL: Yorkie, Toy Poodle, Chihuahua, Cats
Back Length: 8"
Chest: 10"-13" (Most Important)
Neck: 7"-9"
Pets Weighing: 3lbs.-6lbs
SMALL: Pomeranian, Bischon, Shih Tzu, Maltese
Back Length: 10.5"
Chest: 13"-16" (Most Important)
Neck: 9"-12"
Pets Weighing: 6lbs.-10lbs.
MEDIUM: Bischon Fris, Fox Terrier, Jack Russell, Scottie
Back Length: 12"
Chest: 16"-19" (Most Important)
Neck: 12"-15"
Pets Weighing: 11lbs.-20lbs.
LARGE: Cocker, Spaniels, Westie, Boston Terrier
Back Length: 15"
Chest: 19"-21" (Most Important)
Neck: 15"-18"
Pets Weighing: 20lbs.-30lbs
X-LARGE: Beagle, Schauzer, English Bull Dog, Border Collie
Back Length: 18"
Chest: 21"-26" (Most Important)
Neck: 18"-21"
Pets Weighing: 30lbs.-40lbs.
XX-LARGE: Labs, Collie, German Shepherd
Back Length: 22"
Chest: 26"-31" (Most Important)
Neck: 21"-23"
Pets Weighing: 40lbs.- 60lbs.