You will need either an old collar or a flexible tape, and a dog .

If you want to measure from an old collar, just make a note of where it is buckled, take it off the dog and measure from the tip or end of the buckle to the correct hole. This is your dog's neck size. The size of the collar we make will depend on which hole you specify you want to do it up to, and also the fact that a collar measurement "flat" is not the same as a dog neck "around."
To measure the dog, place your tape loosely (like two fingers under) around the base of the neck where the collar will sit. This is the neck size you will need to quote when you order.
For an idea of sizing across a few breeds here is a start: (This is a rough guide only)
x small 34-36 cm:
small 44-46 cm: Beagle; Bichon Frise; Cavalier King Charles Spaniel; Cocker Spaniel; Dashund; Pug; Welsh Corgi or Miniature Poodle.
medium 51-55 cm: German Shorthaired Pointer; Boxer; Collie; Bull Dog; Short Haired Retriever;
large 60-65 cm: Bull Mastif; Great Dane; Rottweiler; Saint Bernard; German Shepherd or
Put a collar on your dog, but put it up firmly around the ears, and The collar should be snug, it needs to be able to pull up with the ends close together. You should If you don't have a buckled collar, use a measuring tape, but do the same thing, snug up around the ears and under the |