Pool Pump Size Calculator
Find the pump flow your pool needs to turn the water over. Enter your volume in gallons and a turnover time to get the required GPM and GPH. Need your gallons first? Use the volume calculator.
Not sure? Calculate your pool volume first.
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required GPM
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GPH
8
turnover (hours)
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low-speed GPM
Two things to check. Your filter must be rated for at least this flow, or water moves through too fast to be cleaned. And real flow is reduced by plumbing, fittings, the filter, and lift (total dynamic head), so match the pump curve: pick a pump that delivers your target GPM at your system’s head, not its best-case open-air number.
Matched pump and filter gear
Sized to your turnover. Starting points; see our reviews for specific picks.
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How to Size a Pool Pump
Sizing a pool pump starts with one idea: turnover. Turnover is circulating your entire pool volume through the filter once. The common target is one turnover in about 8 hours, which keeps the water clear and the chemistry evenly mixed without running the pump around the clock. To find the flow you need, divide your pool volume in gallons by your turnover time in hours to get gallons per hour (GPH), then divide GPH by 60 to get gallons per minute (GPM). GPM is the number pump and filter specs are written in, so it is the figure you shop with.
Here is the math in practice. A 20,000-gallon pool on an 8-hour turnover needs 20,000 divided by 8, or 2,500 GPH. Divide 2,500 by 60 and you get about 42 GPM. That is the target flow your circulation system should move. Pick a shorter turnover and the required GPM climbs; pick a longer one and it falls. The calculator above runs these numbers for you and sizes the gear to match.
Single-Speed vs Variable-Speed Pumps
Once you know your target GPM, the next choice is pump type. A single-speed pump runs at one fixed, high flow. It reaches your turnover quickly, but it is loud and it draws a lot of electricity every hour it runs, which adds up fast over a season. For a single-speed pump, your calculated GPM is simply the flow you size to.
A variable-speed pump changes the equation. Because you can dial the speed up or down, you can run longer hours at a lower, quieter, far cheaper flow and still move the same total volume each day. If your 8-hour target is 42 GPM, you could instead run near half that flow for roughly 16 hours and reach the same daily turnover. Power draw rises steeply with speed, so cutting the speed in half cuts energy use far more than half. That is why variable-speed pumps cut energy cost dramatically and usually pay back their higher purchase price, and why many regions now require them above a certain size. The calculator shows that lower long-run speed for you when you choose a variable-speed pump.
Match the Filter, and Match the Pump Curve
Two checks keep your sizing honest. First, the filter. Every filter has a maximum flow rate, and your pump should not exceed it. Push more GPM than the filter is rated for and water races through too fast to be cleaned properly, and you can damage the filter. Size the pump to your turnover, then pick a filter rated at or above that flow. Cartridge, sand, and DE filters all publish a flow rating, and a slightly larger filter runs longer between cleanings.
Second, head. The GPM from the formula is your target, but real-world flow is always lower because of resistance in the system: pipe length and diameter, elbows and fittings, the filter, a heater or salt cell, and the height water has to be lifted. Together these add up to total dynamic head. A pump rated for a big GPM in open air will deliver less once it is plumbed into your equipment pad. The fix is to match the pump curve. Manufacturers publish a flow-versus-head chart for each pump, so you choose one that delivers your target GPM at your system’s actual head rather than at its best-case number. When in doubt, a pro can measure your head and confirm the curve.
Put it together and pump sizing is straightforward: confirm your gallons, pick a turnover, compute the GPM, choose single-speed or variable-speed (variable-speed almost always wins on cost and noise), and then match a filter and a pump curve to that flow. Get those right and your water stays clear with the least possible energy and noise.
Plan the rest of your setup:
Frequently Asked Questions
How do I figure out what size pump my pool needs?
Start with your pool volume in gallons and the turnover time you want, usually about 8 hours. Divide the gallons by the hours to get gallons per hour (GPH), then divide by 60 to get gallons per minute (GPM). That GPM is the flow your pump and filter need to move. A 20,000-gallon pool on an 8-hour turnover needs about 2,500 GPH, or roughly 42 GPM. This calculator does that math and sizes the gear to match.
What is pool turnover and why 8 hours?
Turnover is the time it takes your pump to circulate the entire pool volume through the filter once. The common target is one turnover in about 8 hours, which keeps the water moving, the chemicals mixed, and the debris filtered without running the pump 24 hours a day. Some owners aim for two turnovers a day in summer. A shorter turnover needs more flow (higher GPM), while a longer turnover needs less.
Should I get a single-speed or variable-speed pump?
Variable-speed pumps win on cost and noise. A single-speed pump runs at one high flow, so it hits your turnover quickly but draws a lot of power and is loud. A variable-speed pump lets you run longer hours at a lower, quieter speed to move the same daily volume, often cutting energy use by more than half. Many regions now require variable-speed pumps above a certain size, and the energy savings usually pay back the higher purchase price.
Does the pump GPM need to match my filter?
Yes, your filter has a maximum flow rate, and the pump should not exceed it. If the pump pushes more GPM than the filter is rated for, water moves through too fast to be cleaned well and you can damage the filter. Size the pump to your turnover target, then choose a filter rated at or above that flow. Bigger filters generally run longer between cleanings, so erring larger on the filter is usually a safe call.
Why is my real flow lower than the calculated GPM?
The calculated GPM is your target, but real flow is reduced by head, the resistance from pipes, fittings, the filter, the heater, and the height water is lifted. A pump rated for 60 GPM in open air might deliver far less once plumbed. That is why you match the pump curve: manufacturers publish flow versus head charts, so you pick a pump that delivers your target GPM at your system’s total dynamic head, not its best-case number.
Can I run a variable-speed pump on low all the time?
Mostly yes, and that is the point. Running on a low speed for longer hours moves the same daily volume while using a fraction of the energy, since power draw rises steeply with speed. Many owners run low most of the day for filtering and circulation, then bump to a higher speed only when vacuuming, running a heater, or operating a salt cell or spa jets that need more flow. Set a schedule that hits your daily turnover.
How many gallons per minute is a turnover for my pool size?
Divide your gallons by your turnover hours, then by 60. On an 8-hour turnover, a 10,000-gallon pool needs about 21 GPM, a 15,000-gallon pool about 31 GPM, a 20,000-gallon pool about 42 GPM, and a 30,000-gallon pool about 63 GPM. Enter your own numbers above for an exact target, and remember to confirm your gallons with the volume calculator before sizing any pump.