Measurements are taken around the chest (below the neck and above the front legs) and around the fullest part of the stomach/ribcage.

XS: Chest: 11″-13″ / Stomach: 13″-17.5″

S: Chest: 13.5″-17″ / Stomach: 16.5″-22″

S/M: Chest: 13″-15.5″ / Stomach: 16″-24

M: Chest: 18″-22.5″ / Stomach: 21″-26″

L: Chest: 21″-24.5″ / Stomach: 27.5″-34″