There are two adjustable straps including the head strap and the chain strap. Firstly, put the head strap on your dogs head and adjust to a suitable size for your dogs. Secondly, adjust the chain strap to a proper size. Legal Disclaimer The dog goggles is suitable for medium or larger dogs, not fits for small dogs. It is normal of measurement error about 1-2cm due to the hand measurement. To report an issue with this product, click here.