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Pool Pump Size Calculator

Work out the flow rate your pool pump needs for your turnover time, plus a simplified total dynamic head and motor size, so you can compare pumps on their performance curves.
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6–8 h is typical for residential pools.

Suction plus return runs.

From spec sheets (head loss at your flow).

Only used for the motor-size estimate.

Required flow rate

41.7 GPM

To turn over 20,000 gallons in 8 hours, choose a pump that delivers 41.7 GPM at about 19.4 ft of total dynamic head.

Total dynamic head
19.4 ft
Pipe friction
4.4 ft
Pipe velocity
4 ft/s
Hydraulic power
0.2 HP (0.15 kW)
Estimated motor size
0.41 HP at 50% efficiency

How it's calculated

flow (GPM) = pool gallons ÷ (turnover hours × 60) pipe friction = Hazen-Williams, C = 150 for PVC h (ft per 100 ft) = 0.2083 × (100/C)^1.852 × GPM^1.852 ÷ d^4.8655 fittings = each 90° elbow ≈ 30 pipe diameters of straight pipe TDH (ft) = pipe friction + equipment loss + height difference water horsepower = GPM × TDH ÷ 3,960
  • d is the inside diameter of Schedule 40 PVC (ASTM D1785): 1.61 in for 1.5", 2.067 in for 2", 2.469 in for 2.5", 3.068 in for 3".
  • The elbow allowance (L/D = 30) comes from Crane Technical Paper 410, the standard reference for fitting losses.
  • Water speed is checked against the limits in ANSI/APSP/ICC-15 (residential pool energy efficiency): no more than 6 ft/s in suction lines and 8 ft/s in return lines.
  • Equipment loss is your filter and heater head loss at the design flow, from their spec sheets. A dirty filter adds more.

This is a simplified TDH: it lumps suction and return into one length and ignores valves and inlets you do not enter. For a new build, have the designer run a full hydraulic calculation.

Worked example

A 20,000 gallon pool, 8-hour turnover, 2" pipe, 100 ft of pipe with 10 elbows, 15 ft of filter and heater loss, equipment at water level:

  1. Flow: 20,000 ÷ 480 = 41.7 GPM.
  2. Elbows: 10 × 30 × 2.067 ÷ 12 = 51.7 ft, so 151.7 ft of equivalent pipe.
  3. Friction: 2.87 ft per 100 ft × 1.52 = 4.4 ft.
  4. TDH: 4.4 + 15 + 0 = 19.4 ft.
  5. Water horsepower: 41.7 × 19.4 ÷ 3,960 = 0.2 HP; at 50% efficiency the motor delivers about 0.41 HP.

Look for a pump whose curve passes through 42 GPM at about 19 ft. Then check that the flow stays within your filter's rating with the filter size calculator, and keep pipe velocity at 4 ft/s or below the limits above.

Flow needed by pool size and turnover

Pool size6 h8 h10 h
10,000 gal28 GPM21 GPM17 GPM
15,000 gal42 GPM31 GPM25 GPM
20,000 gal56 GPM42 GPM33 GPM
25,000 gal69 GPM52 GPM42 GPM
30,000 gal83 GPM63 GPM50 GPM
40,000 gal111 GPM83 GPM67 GPM

Maximum flow by pipe size

Pipe (Sch 40)Suction (6 ft/s)Return (8 ft/s)
2 in63 GPM84 GPM
3 in138 GPM184 GPM
1.5 in38 GPM51 GPM
2.5 in90 GPM119 GPM

Frequently asked questions

What size pump do I need for a 20,000 gallon pool?

For an 8-hour turnover you need about 42 GPM; for 6 hours, about 56 GPM. Typical residential plumbing puts that flow at roughly 40–60 ft of head, which most 1 to 1.5 HP pumps can deliver. A variable-speed pump lets you run lower flows most of the day.

Is a bigger pool pump better?

No. An oversized pump pushes water faster than the filter and pipes are designed for, which raises head, noise and energy use, and can exceed the filter's design flow. Size for the flow you need at your real head, not the biggest horsepower.

How many hours a day should I run my pool pump?

Long enough to circulate the whole pool roughly once a day, which is what the turnover time in this calculator represents. With a variable-speed pump, running longer at low speed uses far less energy than a short run at high speed.

What is total dynamic head (TDH)?

TDH is the total resistance the pump works against, expressed in feet of water: friction in the pipes and fittings plus the losses through the filter, heater and valves, plus any height the water has to climb. A pump's flow depends on it, so you need both GPM and TDH to read a pump curve.

Single-speed or variable-speed pool pump?

Variable-speed. Pump power falls roughly with the cube of speed, so running at half speed takes about one-eighth of the power while moving half the water. US Department of Energy efficiency standards in force since July 2021 have made variable-speed motors the norm for most new in-ground filter pumps.

See all Pool Equipment Sizing Calculators.

Last updated: · Written and maintained by Raul Vaquerizo