"I stand fearlessly for small dogs, the American Flag, motherhood and the Bible. That's why people love me" - Art Linkletter

Measuring for a collar

When purchasing a collar MM=Thickness in millimeters and IN=Length in centimeters and inches when the collar is laid open. In general, lighter ( smaller MM diameter) collars are more responsive and heavier ( larger MM ) collars are suited to larger dogs with thicker coats.

Herm Sprenger's collars come in MM sizes.  For understandability in the US market they are translated into inch measurements.  It isn't an exact measurement when we state it in inches.  It's the closest that we can come to it. The differences in the actual size of the collars are different with each collar type since the link sizes and type of metals actually affect the outcome.  

Note: When sizing a prong collar, measure your dog just behind the ears and under the chin. Prong collars are meant to fit snugly and not to go over the dog.  Measure and then add about 2 inches.  The collar should be able to be put on and taken off.  You can add or reduce the size by adding and removing the individual prongs. 

Prong Collars should never be left on a dog all day or without supervision and should only be used when training or walking your dog. These should not be a dogs primary collar.