Rim eats into the measurements provided. I had to take the litter box to a plastic store to cut the rim to make it worthwhile. Rim makes the cat litter box about 16 x12 and after cutting the rim to be flush with the box is now 20 x 14.5 on the inside of the cat box. 22 x 15.5 would be a perfect cat box.
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Catit Cat Pan with rim by Sherry from Tulsa, OK06/27/2014
Finally found a litter box with sides high enough for my kitten not to scratch out litter. He gets very active covering up his mess and normally gets litter all over. Really like this pan.