By golly, for once the pool store is right! By happy coincidence, on this particular filter, that holds true! The exploded parts diagram confirms it. That's easy. All you need is a tape measure while it's apart.Thanks Kiss4aFrog. Prior to my post, I had just winged it. Everything seems to be working ok. Since then, I purchased a DE scoop. Next time which will be soon, I am going to break it down. I understand from the pool store that twice the length of the grids inside will be the Sq Ft. In other words, the grids should be 12" if it is a 24. I am about to post a new thread because I just received my TF-100. Please jump in there because I need advice. Thanks Again!!!