Back Length Measurement
To measure your dog's back, place one end of fabric tape at the base of the tail and measure to the nape of the neck.
Chest Measurement
Measure the fullest part of your dog's chest, keeping the tape inline around the body, allow two fingers between your dog's chest and measuring tape for comfort.
Collar Size
If you have an existing collar that fits correctly, lay the collar out flat and measure from where the fabric/leather is attached to the buckle to the hole/base of the clip used to secure the collar.
Alternatively, using a fabric tape measure, measure around your dog's neck where the collar is most likely to rest. Allow two fingers between your dog's neck and tape to ensure a comfortable fit.