Dogs within a specific breed can have a wide variety of head measurements. Just because a muzzle was made to fit all Rottweilers or all Dobermans or all Cocker Spaniels doesn't mean that it will. You will need to measure your dog's head to ensure the right size is selected.
Below, we'll go through how to measure your dog's muzzle size to make sure you have the right muzzle size.
Measure yoru dog's nose length. This will be from the top of the nose to half an inch below the eyes. You can see an example in the photo below:
Example: The dog in this photo has a 4-inch nose length.
Measure the circumference of the nose. This should be done at the same point--half an inch below the eyes--all the way around the mouth while it is close.
Example: The dog in this photo has a circumference of about 11.5 inches.
If you need a more visual way of sizing a muzzle for your dog, check out this video on finding your dog's muzzle size: