How to Fit a Martingale Collar
To fit a Martingale collar, start by slipping it over your dog's nose and head with the outer loop at its largest size. Then, pull the collar up behind your dog's ears, making sure the metal rectangles are at least 2 inches apart. After that, pull up on the D ring, where you would attach the leash, and make sure that the adjustable part of the collar closes. Once you've adjusted the collar, check that it's not too tight by seeing if you can put 1 or 2 fingers underneath it.
Use a soft tape measure or piece of string to measure your dog’s head. 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. Add 2” to the largest measurement to allow for growth. If you are purchasing a buckle Martingale you will only need a neck measurement. Take your first head measurement. Take your tape measure and measure the circumference of your dog's head starting from under their jaw up around to the front-base of their ears. Take your second head measurement. Measure, starting from where their neck meets their jaw, up around and over their ears. Both of these first two measurements should give you a complete measurement of their head. The Martingale collar only needs to fit over your dog's head and will mostly be fitted around their neck. Measure around the base of your dog’s neck. This will give you the size of their neck and will inform you as to what size Martingale you will need. Choose a collar size where your measurement is in the middle of the size range. For example, if your dog's neck is 15 inches, choose a size that is 13-18 inches rather than 15-20 inches. Familiarize yourself with the design of Martingale collars. Understanding the design of the collar will help you make the best fitting for your dog. Locate the two loops of the collar, the large outer loop and the small control loop where the leash attaches to a D ring. The larger loop is the adjustable part of the collar and should be adjusted to fit your hound correctly. Slip the collar over your dog’s nose and head. Make sure the nylon adjuster is at its largest size before slipping the collar on your dog. If you have a buckle Martingale, you can simply place the collar around the neck and connect the buckle on the side release. Pull the collar up just behind the dog’s ears. It is crucial to adjust the collar here because this is the point at which the collar could slip off the dog's head. The two metal rectangles should be at least two inches apart from one another. Pull up on the D ring. Pulling up on the D ring will test how well you've fitted the collar. If the hardware on the collar is touching (where the control loop meets the larger loop around your dog's neck), the collar is too loose and needs to be tightened. The control loop should close when you pull up on the D ring. Test the fit with your fingers. Be sure to leave around one to two fingers of room on the loop around the dog's neck when the control loop is closed so that the collar isn't too tight. Adjust the slide buckle on the outer loop so that the hardware is approximately 2" apart. Look to make sure the collar is properly fitted. A properly fitted Martingale collar should rest around the middle of the dog's neck. If it is still tightened up behind their ears, the collar is too tight and might cause them discomfort. Check the fit of the collar each time it is placed on your dog. Martingale collars do stretch over time and need to be adjusted and occasionally completely refitted. 
Use a soft tape measure or piece of string to measure your dog’s head. Take your first head measurement. Take your second head measurement. Measure around the base of your dog’s neck. Familiarize yourself with the design of Martingale collars. Slip the collar over your dog’s nose and head. Pull the collar up just behind the dog’s ears. Pull up on the D ring. Test the fit with your fingers. Look to make sure the collar is properly fitted. Check the fit of the collar each time it is placed on your dog. 