True story, honest! My beloved late Mother-in-law Janis, actually trained her cat to use the human toilet! I don’t know how, but she did, bless her soul. We just use the clumping stuff, but rule of thumb is 2 boxes for every cat. Our Mr. Blue just turned 16 years old last week. He weighs 21 lbs! He has 2 boxes side-by-side and uses both. Hubs & I have a very small ranch home. I keep those boxes clean daily.