Pump size calculator
A pump is sized by flow and head. This turns your volume and run time into the gallons per minute you actually need, then tells you whether the pipe in the ground can carry it.
Pumps are sized by flow and head, not horsepower. Work out the gallons per minute you need, check the pipe can carry it, then read the pump's curve at that head.
Flow needed at your run time
46gpm
- One turnover in 8 h needs
- 46 gpm
- Assumed total dynamic head
- 35 ft
- Safe flow for this pipe
- 45 gpm
- Running cost, variable speed
- $2–$6/month
The pipe is the limit, not the pump
46 gpm through 1.5 in pipe runs the water fast enough to whistle, cavitate and cost you head. Lengthen the run time instead of buying a bigger pump.
Electricity at 17.3¢/kWh — US EIA, 2025 average residential price, read 2026-09-04. Wattage per gpm is shown as a range because it depends on the pump curve, not on us. (3,412 BTU per kWh, for the heater comparison.)
Horsepower is the wrong number
Nobody buys a pump to own horsepower; they buy it to move water through a filter against resistance. Two numbers decide it: the gallons per minute you need, and the feet of head your plumbing puts in the way. A 1.5 HP pump on a long run with a heater and a solar bank can deliver less flow than a 1 HP pump on a short, fat pipe.
Take the flow figure this tool gives you, find it on the manufacturer’s pump curve at the head shown, and read the speed. On a variable-speed pump that is the setting you programme — not the highest one it will do.
The pipe you already have
Suction pipe is a hard ceiling. Water above roughly six feet per second is noisy, cavitates at the impeller and burns head you paid for. Most residential pools were plumbed in 1½ inch, which tops out near 45 gpm no matter what you bolt onto it. If the tool flags your flow as over-piped, the fix is a longer run time at a lower speed, which also happens to be cheaper.
Slower is cheaper than you think
Halving pump speed roughly halves the flow but cuts the power draw far more steeply. Running twice as long at half speed moves the same water for a fraction of the bill — which is why the running-cost figure above is a range and why variable speed pays back.
Before you replace a pump
Check the basket, the lid o-ring and the impeller first. A pump that lost flow overnight has a blockage or an air leak, not a sizing problem — the repair guide walks the symptoms in order.