You'll need to measure your pup's girth to ensure the proper fit of harnesses and doggy attire, such as sweaters, T-shirts and life jackets. While girth technically refers to the circumference around something, that something is usually very specific in the doggy world.
When taking doggy measurements, girth usually refers to the circumference of your pup's chest at its largest point, which is typically an inch or two behind his front legs. Other canine accessories require you to measure the circumference of a body part other than the chest -- muzzles call for measuring your pup's snout circumference, for instance -- but you'll typically find those accessories specifically mention circumference, not girth. Those that ask for your pup's girth almost always refer to his chest size.
To measure your dog's girth, grab a flexible measuring tape. While your pup's on all fours, wrap the measuring tape around his chest at its broadest point and record the measurement. If your pal likes to plop down on his butt when you try to measure him, ask your significant other or a friend to hold your pup's hips gently while he's standing so he won't sit.