Pool Pump and Filter Sizing Calculator
Pumps are sold on horsepower and filters on square feet, and neither number answers the question. Start from the flow one turnover needs, check it against what your pipes can carry, and the pump and filter follow from that.
Not sure? Use the volume calculator first.
Hours to move the whole pool once. 8 to 12 is usual.
Your plumbing
Measure the pipe at the pump. Most pools built before about 2005 have 1½ inch.
Skimmer and main drain lines, counted separately
Suction flow is shared evenly across the lines, which real plumbing only approximates. Size the pump to flow, not horsepower: a bigger pump on the same pipes moves little more water and uses a lot more power.
Result
Enter your pool volume
The fields start on ordinary values so you can see what each one wants. Nothing is worked out until you put your own numbers in.
Pumps are sold on horsepower and filters on square feet, and neither number answers the question. Start from the flow one turnover needs, check it against what your pipes can carry, and the pump and filter follow from that.
Getting an accurate answer
- Measure the pipe where it meets the pump. Most pools built before about 2005 have 1½ inch suction and return lines, which caps the whole system whatever pump sits on it.
- Count suction lines separately: a skimmer line and a main drain line are two. The flow is split evenly between them, which real plumbing only approximates.
- The pump figure assumes about 50 feet of head, a typical residential system. A long run to a raised spa, a heater and a cartridge filter can push that toward 70, and a pump curve read at the wrong head overstates its flow.
- Filters go by the plate rating, which already has the manufacturer's design rate built in. The areas here say which size range to look in.
Flow first
Target flow is gallons ÷ turnover hours ÷ 60. A 20,000 gallon pool over 10 hours needs 20,000 ÷ 10 ÷ 60 = 33 GPM, far less than most installed pumps make at full speed. The pump sizing guide covers why horsepower is the wrong number to start from.
Eight to twelve hours of turnover is the usual range. Longer is fine, and on a variable-speed pump it is cheaper, because moving the same water slowly takes far less power.
What the pipes will carry
The limit is a speed. The code caps residential pool piping at 8 feet per second, and most designers aim for 6 on a suction line. Hayward publishes its own maximum system flow by pipe size, 45, 80, 110 and 160 GPM for 1½, 2, 2½ and 3 inch, which works out a little under 8 feet per second.
The calculator takes the suction lines at 6 feet per second and the return line at Hayward's figure as what the plumbing should carry, and uses the lower of the two. With two 1½ inch suction lines and a 1½ inch return, that is 45 GPM, the return line's limit.
If a turnover needs more than that, run longer rather than buying a bigger pump. A bigger pump on the same pipes moves little more water, only faster and louder.
Sizing the filter
Size the filter to the pump's flow, not the pool's volume, then go one size up. A bigger filter costs a little more once and runs longer between cleanings for years. The filter sizing guide explains why oversizing works the other way from a pump.
Sand is designed at 15 to 20 GPM per square foot, so 33 GPM needs about 1.7 to 2.2 square feet, an 18 to 20 inch tank. D.E. runs around 2 GPM per square foot. Cartridge plates often rate under 1 GPM per square foot, so the area shown is the least it could be, and the plate decides.
Choosing the pump
With the flow settled, the pump is the one that delivers it at your head on a low speed. Without a head calculation, the shortcut is to assume about 50 feet and read the pump's curve there. The pump pages link each maker's manuals.
Variable speed is the answer almost regardless. Size it so the target flow comes at a low speed, and the top speed is there for a cleaner or a spa.
Questions about this calculator
What size pump do I need for my pool?
Work out the flow for one turnover in 8 to 12 hours, gallons divided by hours divided by 60, then check your pipes can carry it. Most residential pools need 40 to 60 GPM, and on 1½ inch plumbing the ceiling is around 40 to 50 GPM whatever the pump. Choose one that delivers your flow at about 50 feet of head on a low speed.
How fast should water move through pool pipes?
No faster than 8 feet per second, the code maximum for residential pool piping, and about 6 on a suction line. Hayward publishes maximum flows a little under 8 feet per second: 45 GPM for 1½ inch pipe, 80 for 2 inch, 110 for 2½ and 160 for 3.
What size filter do I need for my pump?
One whose plate rating is above your pump's flow, then one size up. By area, sand is designed at 15 to 20 GPM per square foot, D.E. around 2 and cartridge often under 1, so 40 GPM means about 2 to 2.7 square feet of sand, 20 of D.E. grid, or 40 or more of cartridge.
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