Unicel C-4311 Replacement Filter Cartridge for 45 Square Foot Spa Filter Diameter: 4 15/16-inch; length: 12 1/4-inch Top end cap: open, 2-1/8-inch inner diameter; bottom end cap: open, 2-1/8-inch inner diameter Pleat count: 212; filter media: 3-ounce reemay 2033; filter area: 45 square feet Get more efficient filtration and better water clarity with Unicel filters Longer time between cleanings; meets oem specifications