Pool Pump Size Calculator (GPM and Turnover)

Calculator: flow from gallons and turnover time

Turnover time is how long it takes the pump to push a volume of water equal to the whole pool through the filter. Flow needed (gpm) = pool gallons ÷ (turnover hours × 60).

With the example values, 20,000 gallons in 8 hours needs 41.7 gpm (9.5 m³/h). Enter your own gallons and the turnover time you want. If you don't know your gallons, start with the pool volume calculator. For an above-ground pool of about 10,000 gallons, enter 10000 and read the flow the same way. Intex and similar kit pumps are covered in the Intex pool pump guide.

Turnover time: what the standards say

• The U.S. Building America Solution Center (run by the Pacific Northwest National Laboratory, PNNL) summarizes the residential energy standard: the pump must be able to turn the whole pool over within 12 hours. It also says that the turnover time should "not be less than six hours" in normal operation.
• For sizing pipes, the energy-efficiency standard ANSI/APSP/ICC-15 uses a 6-hour turnover or 36 gpm, whichever is greater (ICC CodeNotes). For the example pool, that design flow is 55.6 gpm.

So an 8-hour turnover falls within those limits. Running the pump longer at a lower speed is usually cheaper than a fast turnover (see the last section). How many hours a day to run it is a separate question, covered by the pump run time calculator.

Pipe size limits the flow

Water moving too fast through a pipe wastes energy as friction and makes the pump louder and work harder. ANSI/APSP/ICC-15 sets the limits for residential filtration piping: 6 feet per second (ft/s) in suction pipes (pool to pump) and 8 ft/s in return pipes (pump back to pool). The ICC table gives the most flow each Schedule 40 PVC pipe size can carry at those speeds:

• 1.5 in pipe: 38 gpm suction, 51 gpm return
• 2 in pipe: 63 gpm suction, 84 gpm return
• 2.5 in pipe: 90 gpm suction, 119 gpm return
• 3 in pipe: 138 gpm suction, 184 gpm return

For the example flow of 41.7 gpm, 1.5 in suction pipe is too small and 2 in is enough. A 1.5 in return pipe is enough. At the standard's design flow of 55.6 gpm, both sides need 2 in pipe.

What this means when you buy: a bigger pump on small pipes can't move much more water. It mostly raises pressure, noise, and the power bill. Check your pipe size first. It is usually printed on the pipe near the pump.

Why we don't size pumps by horsepower

Horsepower tells you how big the motor is, not how much water reaches your pool. The real flow depends on the resistance of your system, called head: pipe length and size, fittings, the filter, a heater, and how dirty the filter is. The same pump can move quite different flows in two pools.

To pick a pump, find the flow you need here, then read it off the pump maker's performance curve at your system's head. A pool professional can measure or estimate head. We do not give a horsepower table, because one horsepower number per pool size would be a guess.

Pool filter pumps made since July 19, 2021, must meet a federal minimum efficiency score, the weighted energy factor (WEF), under 10 CFR 431.465. The same rule says above-ground pumps with a built-in sand or cartridge filter must ship with a timer. A higher WEF means more water moved per kWh, so compare it between pumps of the same size.

Variable-speed pumps: same water, less energy

Pump power drops much faster than speed. By the pump affinity laws, power changes with the cube of speed. At the example setting of 50% speed:

• The pump moves 50% of the flow and uses about 13% of the power.
• It has to run 2.0 times as long for the same turnover.
• For the same turnover, it uses about 25% of the energy.

These are ideal numbers. Real savings are lower, because the motor and drive lose some efficiency at low speed. PNNL gives a measured example: slowing a pump about 30%, from 3,450 to 2,400 rpm, cut its power by about 70%, from 2,000 W to 593 W. PNNL puts the typical saving of a variable-speed pump at 50% to 90% of pump energy. To see what that means in dollars, use the pool ownership cost calculator.

Next step after the pump: match the filter to the same flow with the pool filter size calculator.

FAQ

What size pool pump do I need?

One that delivers your pool's gallons ÷ (turnover hours × 60) at your system's head. A 20,000-gallon pool turned over in 8 hours needs about 41.7 gpm. Check that your pipes can carry it, then pick the pump from its maker's flow curve.

How many gpm do I need for my pool?

Divide your gallons by the turnover minutes. For an 8-hour turnover, that is gallons ÷ 480: 20,000 gallons needs 41.7 gpm. The pipe-sizing standard uses a 6-hour turnover or 36 gpm, whichever is greater.

What horsepower pump do I need for my pool?

Horsepower alone can't answer it, because the same motor moves different flows in different plumbing. Find the flow you need first, then choose a pump whose performance curve gives that flow at your system's head. Among pumps that can do that, compare the federal efficiency score (WEF).

Is a bigger pool pump better?

Usually not. More flow than your pipes can carry raises pressure and power use without much more filtering. A pump sized for your flow, run longer at low speed, uses less energy for the same turnover.

Sources

• International Code Council, CodeNotes: Velocity limits in the 2021 ISPSC (APSP-15 6 and 8 ft/s, design flow, Table 1 pipe flows): iccsafe.org (PDF)
• PNNL, Building America Solution Center, Variable-speed pool pumps (turnover limits, 2,000 W to 593 W example, 50–90% savings): basc.pnnl.gov
• eCFR, 10 CFR 431.465, Energy conservation standards for dedicated-purpose pool pumps (July 19, 2021; WEF; timers on integral-filter pumps): ecfr.gov
• U.S. CPSC, Don't swim with shocks (GFCI on pumps, qualified electrician, monthly tests): cpsc.gov