The three basic measurements of a dog's body
Body Length, Chest and Neck

These are the measurements you will need for most basic garments such as hoodies, t's, coats and dresses, as well as for collars and neckwear.

The Body Length is the length of your dog's back, measured along the top of the spine from the where the neck joins the body, to the base of the tail where the tail joins the body.

The Chest measurement is taken all the way around the biggest part of your dog's ribcage, generally just behind the withers down and around just behind the front legs and back up the other side.

The Neck is measured at the place where your dog's collar would naturally rest* - from about the top of the chest to just above the withers - this measurement is also taken all the way around.

*note: for long necked breeds, the collar is worn much higher, actually at the top of the neck below the head, but in measuring for coats and similar garments, the neck measurement for these dogs is taken lower than the collar, about midway between collar level and withers. For dogs with short thick necks, such as Pugs, and dogs with a very full coat in the neck/chest area like Pomeranians, determining the neck measurement can be a bit tricky. For most apparel, and neckwear accessories like bandannas on this type of dog, you may want to use a neck measurement taken a bit lower than collar level (closer to the withers) for a more flattering fit.