Different manufactures, and different styles of dog apparel sometimes require different measurements so please read the sizing guidelines included with each style of clothing you are considering.
As a general rule, measuring should be done along the dog's backbone from the base of the head to the base of the tail, which is referred to as the "top line measurement". This is from the base of where the collar rests to where the tail is attached. The measurements you see on all our apparel indicate this top line measurement. However, it's important that the dog's girth be taken into account as well as the top line measurement.
For chest measurement, measure the chest at its widest point, which is typically right behind the front legs. If the dog has a large girth, going up a size in the top line measurement is always a smart choice, since some additional fabric will be used up.
Note that there are different sizing ranges for different types of garments, due to how binding or flexible the material used is. Coats are full length, whereas jackets are shorter.
