Some of the most popular top manifold filters on the market are Pentair's FNS Plus, Hayward's ProGrid and are easily identified by the multi-port or backwash valve on the side of the tank. These grids are offered in five different sizes and are easily identified by the height of the grid. Just take the height of your grids and double it to determine the square footage of your filter. If you have a 24" tall grid, then you have a 48 sq. ft. filter - it's that easy!