Unicel 50sqft Rainbow Inline Cartridge Diameter=4-15/16in.; Length=13-5/16in.; Top=2-1/8in.; Bottom=2-1/8in. Used on many manufactures from Cal Spa, Phoenix Spas, Waterway,Weslo, Image, Maxx, Artesian, Baja, Bullfrog Spas and many more! NOTE: 50 sq.ft. C-4950 is an upgrade from the 25 Sq.ft. C-4326 abd C-4625 Cartridges. NOTE:Image and Weslo spas used in most models from 2003 to present Cross Reference 100533, 17-175-1595, 172380, 201987300, 25391, 25393, 3301-2145, 6540-471, C4950, PENR172380, PRB50-IN, PRB50IN, R172380