How this calculator works
Turnover time is how long the pump takes to move the whole volume of the pool through the filter once. The flow it needs is the volume divided by the turnover time in minutes. The pipe is chosen so that the water speed stays at or below 8 feet per second, which is 2.448 × the square of the inside diameter × 8 gallons per minute. The filter area follows from the flow rate each type of filter can take.
Worked example: a pool of 20,000 gallons with an 8 hour turnover needs 41.7 GPM (2,500 gallons per hour), which fits 1½ in pipe, and a sand filter of 2.8 ft² or a cartridge filter of 111 ft². At a 6 hour turnover it needs 55.6 GPM, and at 12 hours 27.8 GPM. A 32 × 16 ft pool with an average depth of 5 ft holds 19,150 gallons and needs 39.9 GPM, and a pool of 40,000 gallons needs 83.3 GPM through 2 in pipe.
Flow needed for common pool sizes
Gallons per minute at 6, 8 and 12 hour turnover.
| Pool | 6 hours | 8 hours | 12 hours | Pipe (8 hours) |
|---|---|---|---|---|
| 10,000 gal | 28 GPM | 21 GPM | 14 GPM | 1½ in pipe |
| 15,000 gal | 42 GPM | 31 GPM | 21 GPM | 1½ in pipe |
| 20,000 gal | 56 GPM | 42 GPM | 28 GPM | 1½ in pipe |
| 25,000 gal | 69 GPM | 52 GPM | 35 GPM | 2 in pipe |
| 30,000 gal | 83 GPM | 63 GPM | 42 GPM | 2 in pipe |
| 40,000 gal | 111 GPM | 83 GPM | 56 GPM | 2 in pipe |
Tips before you buy
- A turnover of 8 hours is the usual target for a residential pool: the pump moves the whole volume of the pool through the filter once in 8 hours, which is three times in 24 hours.
- A variable-speed pump can run for 12 to 24 hours at a low speed and use a fraction of the electricity of a single-speed pump.
- Do not oversize the pump: too much flow wastes power, stresses the filter and can exceed the suction-line speed limit set for safety (usually 6 ft/s on the suction side).
- Match the filter to the pump: the filter must handle at least the pump's flow, or it will be bypassed or damaged.
- Clean the skimmer and pump baskets and backwash or clean the filter when the pressure rises 8 to 10 psi above the clean reading.
Tools & materials for this project
As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
What size pool pump do I need?
Divide the pool volume by the turnover time in minutes. A 20,000 gallon pool with an 8 hour turnover needs 42 gallons per minute, then pick a pump whose curve gives that flow at your system's head.
How many GPM for a 20,000 gallon pool?
41.7 GPM for an 8 hour turnover, 55.6 GPM for 6 hours and 27.8 GPM for 12 hours.
What is turnover time?
The time for the pump to circulate the whole volume of the pool once. Eight hours is standard for a home pool, and public pools need a shorter time set by code.
What size pipe for a pool pump?
For 42 GPM, 1½ in pipe keeps the water at or below 8 feet per second. 1½ in carries up to about 50 GPM, 2 in up to 84 and 2½ in up to 119.
How big should the filter be?
A sand filter at 15 GPM per square foot needs 2.8 ft² for this flow, a cartridge filter at 0.375 GPM per ft² needs 111 ft², and a DE filter at 2 GPM per ft² needs 20.8 ft².
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.