The best way to measure your dog is using a soft tape measure. If you do not have one you can use a piece or string (or shoe lace) around the dog's neck and head and then measure the length of the string with a ruler.
Measurement A: Around the widest part of the head. This is essential for house collars and martingale collars as these will slip over the dogs head and do not open
Measurement B: Behind the ears around the thinnest part of the neck.
Measurement C: Around the base of the neck at the widest point where the collar will sit. Use this measurement when choosing a braided tag collar
Collars are adjustable so when ordering choose the size that fits best with all these measurements. If you require smaller or larger collars then you can request these when ordering via the Create your Own Collar listings by selecting 'other neck size' and specifying your requirements