Size S: Suitable for dogs with a head circumference of about 14 inches. Range of strap: 16.5-18.5. Size M: Suitable for dogs with a head circumference of about 16 inches. Range of strap: 18.5-22.5. Size L: Suitable for dogs with a head circumference of about 18.5 inches.Range of strap: 22.5-25. Get used to it quickly: Designed according to dogs head feature, there are 2 ear holes in the dog helmet, will stay well on dog's head. Sponge is also added inside this bike helmet, will make your dog loves this comfortable and safety helmet soon. Ultra hard material for safety: This dog hat helmet made of lightweight but sturdy ABS material and soft sponge, all give your dogs head the best protection when dog hit or bitten by other animals. Stay on head well: The ear holes and adjustable strap increased stability of the helmet, will not fall off in normal daily use. Suitable for: Any situation where a dog needs protection, such as driving, motorcycling, biking, walking. Also a good choice for daily dress.