Measure your pet from the base of his neck to the base of his tail as shown in the illustration.
Always round up to the next larger size if the measurement is near or over the range maximum in the smaller size. This is particularly true for coats and jackets since there is no stretch in those materials.
Also round up to the next larger size if your pet has a thick coat, broader chest, or wider underside.
Measure girth around the widest part of the chest, just behind the front legs.