Pentair 25021-0223S Small Inner Cartridge Replacement Sta-Rite System 3 SM-Series S7M400 Pool and Spa Cartridge Filter Small inner cartridge replacement Fits Pentair Sta-Rite System 3 SM-Series pool and spa cartridge filter model S7M400 136-square feet filtration area Measures 11-inch length by 31-1/4-inch height by 11-inch width