This collar is constructed of nylon cord with stainless steel hardware. Measure the dog's neck just under the jaw and right behind the ears. Use a string or cloth tape measure. The string should be VERY SNUG. It should be as tight a measurement as you can get it. It is VERY IMPORTANT that you have the tightest possible measurement located at the top of the neck directly below the jawline. If this collar is being used as a corrective collar then order it 1-2 sizes smaller. The hardware on the collar adds 2 inches. If you are using this as a back up collar then order the size you measured. This is not intended to give a painful correction.