Moderna Universal Size Replacement Filter Sheet for Cat Litter Boxes How much fun is it trying to figure out if you have the right size replacement filter to fit the specific hole in your cat's litterbox? Not much. Take the hassle out of buying incorrect sizes. You can cut this charcoal filter sheet to any size you need: round, square, or rectangle. You'll always have the perfect size replacement filters on hand. Item Specifications: Size: 6.3" x 6.3"