[header] How to fit a martingale collar [title] Use a soft tape measure or piece of string to measure your dog's head. [step] Measure the largest part of your dog's head and the middle of their neck. If your dog's head is larger than their neck, you'll need a collar that can fit comfortably over their head. 
If their neck is larger than their head, you'll need a collar that fits comfortably around their neck. [substeps] Add 2 " to the largest measurement to allow for growth.