For above ground pool pump and filter systems, the manufacturer will pair the proper horse power pump with the proper size filter. So all you really need to do is measure your pool and follow the rule above. That will guide you to the correct size pump. Now if you are replacing an existing pump and not the filter, you will need to make sure you get a pump designed for above ground pools. Above ground pool pumps are usually postioned below the water line (i.e. on the ground next to your pool). Because they have to force water upward, pumps for above ground pools are designed differently than those used for in-ground pools (where the pump is typically above the water line of the pool). When replacing a pump for an above ground pool, just make sure you purchase one that is categorized as an "above ground" pump. Most of the time, you will see pumps divided into two categories, above ground and in-ground.