The Unicel C4440 cartridge is 40 square feet and it can be used to replace Sundance Spas 6540-495, Pleatco PSD40-4, and Filbur FC-2710. It is 4 5/8 inches in diameter and 16.75 inches long. It features 158 pleats and 4 media, with a top castle 0-inch inner diameter and bottom open 2 1/16 inch inner diameter. Unicel cartridge filters have been recognized as the best pool filter cartridge replacements in the industry. They have been recognized year after year for their quality and performance with their products.