Basic steps for selecting a pool pump:

How many gallons of water do I need to turn over in one day? (Twice your pool volume)

How many hours per day will I run my pump? (Entirely up to you - 8-12 is typical)

Flow rate required = (gallons) divided by (hours) divided by (60 minutes per hour) = gallons per minute

Assume a system TDH of 45-65 feet. If your circulation system has a lot of valves in it, a lot of piping elbows, and/or a long run from the equipment to the pool, choose a number on the higher end of that range. Make your best guess. (It should be noted that this is a shortcut, but one that is fairly well accepted by the board).

Look at the pump curve for a particular pump you are considering. How much flow will it deliver at the TDH you selected? Compare that to the flow rate required you calculated above.

Note that two-speed pumps will have two pump curves, usually on the same graph. At low speed, use a TDH approximately one-fourth (25%) of the TDH you selected above. Look at the pump curve for a particular pump you are considering. How much flow will it deliver at the TDH you selected? Compare that to the flow rate required you calculated above.