Measure where your dog's collar naturally sits, with the tape measure close against its neck. Then see where it fits in the size chart.
Note: The collars have allowances for a two finger gap, so they fit comfortably. All you need to worry about is your pals neck measurement.

Good Dog Collar size guide If your dog is on the cusp of two sizes we suggest going up to the larger size. If you have any questions about sizing, we are happy to help.

Small should fit the average: Jack Russell, Bichon, Cavelier King Charles & similarly sized breeds
Medium should fit the average: Beagle, Border Collie, Boston Terrier & similarly sized breeds
Large should fit the average: Labrador, Visla, Boxer & similarly sized breeds