Weight tells you almost nothing about how a garment will sit. Two dogs at fourteen kilograms can be a completely different shape. Chest is the measurement that decides the size, taken at the widest point just behind the front legs, with the dog standing.
Taking the measurement
Use a soft tape. Keep it snug but not tight — you should be able to slide two fingers underneath. Measure twice; dogs move.
Between two sizes?
Go up. Our pet pieces are cut with a little ease so a harness can go underneath, and a size too small will ride up at the shoulders within an hour of walking.