Size S: Cap opening diameter is around 4″ (the cap circle is around 32centimeter) Size M: Cap opening diameter is around 5″ (the cap circel is around 39centimeter) Size L: Cap opening diameter is around 6″ (the cap circel is around 47centimeter) Helmet Exclusively for pets! Made of high impact ABS plastic the same plastic used in construction hard hats. Designed with your dogs safety and comfort in mind, “The Pet Helmet” can protect a pet’s head from wind, windblown objects and other irritants when riding on a motorcycle, in a car, truck or boat. Helmet is Excellent for blind dogs or any pet that requires protection from minor head trauma when bumping into things. Does your dog have stand-up ears? Not to worry…..”The Helmet” was designed so that it does not rest flat on the head. The customizable foam pad that is included fits between the ears, not over them. This prevents “The Helmet” from pushing the ears flat to the head. The adjustable nylon strap lets you custom fit “The Helmet” for almost any pet dog or cat.