Deluxe Aluminum Dog Door Extra Large 10 1/2" x 15" Opening Dogs up to 100lbs Wall Installation use Wall Kit # WKDXL Door Sizes and Pet Weight: If your cat is smaller than about 15 lbs or your dog smaller than about 8 lbs, a flap size of around 5" x 7". If your dog is between 8 lbs and 35 lbs consider a flap size around 8" x 11". If your dog is between 35 lbs and 80 lbs consider a flap size around 10" x 15". If your dog is over 80 lbs consider a flap size of 15” x 20”. Exact Measurement: 1. Measure your pets width - Simply open a door the minimum width needed for the widest part of your pet to comfortably walk through, then measure the door opening and add an inch. You can also stand above your pet and use a ruler to measure the widest portion of your pet, then add an inch. It is important to add an inch or so, that way your pet will not rub on the door as you pet passes through. 2. Determine your pets height measure from the top of your dogs shoulders to dogs belly or chest add 2 "