Unicel C-4637 Replacement Filter Cartridge for 37 Square Foot Rainbow, Waterway Plastics, Morgan Spas, Leisure Bay Spas, Custom Molded Products Diameter: 4 15/16-inch; length: 13 5/16-inch Top end cap: open, 2-1/8-inch inner diameter; bottom end cap: open, 2-1/8-inch inner diameter Pleat count: 158; filter media: 4-ounce reemay 2040; filter area: 37 square feet Get more efficient filtration and better water clarity with Unicel filters Longer time between cleanings; meets oem specifications