Obtaining the perfect collar and harness fit is important for your dog’s wellness, safety, and movement. 

First, heed the the age and breed of your dog:

Keep in mind the rapid growth most puppies undergo. Choose a collar with an adjustable size range to accommodate your pup's current size as well as room for growth.

Does your dog pack on extra fur in cold weather? Some need another collar size to account for changes in fur density in specific seasons. Such breeds might include but are not limited to Pyrenees; Huskies; etc.; and dogs that experience fluctuations in their fur appearance such as by way of regular grooming.

How to Measure Your Dog for a Collar

Does your dog pack on extra fur in cold weather? Some need another collar size to account for changes in fur density in specific seasons. Such breeds might include but are not limited to Pyrenees; Huskies; etc.; and dogs that experience fluctuations in their fur appearance such as by way of regular grooming.

How to Measure Your Dog for a Harness

Obtain the 'chest girth' first:

Measure from behind the front legs to atop the spine, making a full circle. You are measuring the widest part of your dog's chest, behind the front legs. Properly sized, you should fit two fingers between the harness and your dog's body.

Obtain the 'neck girth' too:

Measure the widest part of your dog's neck, from the shoulder blade to above the front legs, where the harness will sit. Note that this neck measurement is different from where a collar sits.

FYI:

Dog collars and even harnesses are often adjustable several inches, a major perk for your pet's potential varying weight and coat if in doubt, choose one where your pet's neck measurement is in the middle of the range.

If a collar was the exact size of your pet's neck, it would be too tight: Allow room to slip two fingers between the collar and the dog's neck or add two inches to the neck measurement.

Note: Even leashes, collars, harnesses, and such identifying, control, and safety tools pose a risk: As with any and every tool, it can be used in a positive or a negative way. Please remain vigilant and informed with your pet's whereabouts, safety, and gear.