To choose the proper pump for any pool, you're going to have to familiarize yourself with the recirculation system to find out just how big a pump you really need. You'll want to know how much water you want to move, how far and how fast you want to move it how much resistance you're going to encounter along the way. Pump manufacturers sum up all of these factors into two basic measurements and rate their pumps accordingly. It is usually measured in gallons per minute or gpm. The vertical distance that the water has to travel obviously affects the performance of a pump.
Static head accounts for just a small portion of the total head in a recirculation system.
The majority of head loss is caused by dynamic head. As it flows, the water encounters friction or drag from the- pipe and resistance from each piece of equipment and each turn it has to make. The amount of friction head is determined by the diameter of the pipe and the speed of the water flowing. Using 2-inch pipe, only 4.3 feet of friction head would develop for every 100 feet. And with 3-inch pipe, only .65 feet of friction head would develop for every 100 feet.