Sizing may be based on the dog’s weight or their waist size. Here at jjdiapers, our sizes are based on the your dogs weight. We find this ensures a much more precise fit to measure your dog. If your dog is at the very end of the range or in between ranges, we’d recommend going a size up. Our dog diapers can be adjusted by using the Velcro straps to make the waist tighter as we have 2” on the outside of the diaper, this gives room to tweak the fit.