A 15,000 gallon pool that must turn over once in 8 hours needs 31.25 gpm through the pump. Against 35 feet of head that is 0.28 hydraulic horsepower, which lands on a 0.75 hp single-speed pump, and the water moves at 3.0 ft/s in 2 inch pipe. Swapping to a variable-speed pump for the same water cuts the yearly electricity from about 2,130 kWh to 740 kWh.
How the pump size is calculated
Flow comes first. Turnover time is the pool volume divided by flow, so the flow you need is the volume divided by the turnover time:
Flow (gpm) = gallons ÷ (turnover hours × 60)
For 15,000 gal and 8 hours: 15,000 ÷ 480 = 31.25 gpm. The pool turnover calculator runs this backwards from a known flow and lists the targets used for different pool types.
Horsepower depends on the pressure the pump works against, called head and measured in feet of water. DOE's pool pump rule gives the hydraulic power as:
Hydraulic hp = gpm × head (ft) ÷ 3,956
31.25 × 35 ÷ 3,956 = 0.28 hhp. A pump wastes most of its motor power as heat in the motor and in the wet end, so the size on the label runs about twice the hydraulic figure: DOE's baseline pumps needed 1.7 to 2.2 total hp to deliver 0.95 hhp and 3.5 to 3.9 total hp for 1.88 hhp. The calculator doubles the hydraulic value and rounds up to the next standard size (0.5, 0.75, 1, 1.5, 2, 2.5, 3 hp): 0.55 becomes 0.75 hp.
The head is the one number this page cannot know. It is the sum of the pressure drops across the filter, heater, salt cell, valves and pipe, and it climbs as the filter loads with dirt. Read it from a gauge on the pump inlet and outlet, or take it from the pump curve. The 35 ft default is a placeholder. For reference, DOE's test system follows head = 0.0082 × gpm², which is 40 ft at 70 gpm.
Pipe size and water speed
Water speed in a pipe is 0.4085 × gpm ÷ (inside diameter in inches)². Schedule 40 pipe has an inside diameter of 1.610 in for 1.5 inch, 2.067 in for 2 inch and 2.469 in for 2.5 inch (ASTM D1785). The 2024 Model Aquatic Health Code limits speed to 8 ft/s on the pump discharge and 6 ft/s on the suction side. The example runs at 4.9 ft/s in 1.5 inch pipe and 3.0 ft/s in 2 inch. On the suction side, 1.5 inch pipe tops out near 38 gpm and 2 inch near 63 gpm, so a pool with a fast single-speed pump often outgrows 1.5 inch lines.
Pump kWh: single-speed against variable-speed
Pump energy is set by how much water you move and how efficiently. DOE rates pumps in kilogallons per kWh (weighted energy factor, WEF). Its baseline single-speed pumps score 1.74 to 2.69 depending on size; the minimum a variable-speed pump must reach is 6.59 at 1 hhp and rises for smaller sizes. For the example, 5,475 kgal a year (one turnover a day for 365 days) works out as:
| Pump | Energy factor | Electricity | Cost at $0.173/kWh |
|---|---|---|---|
| 0.75 hp single-speed | 2.57 kgal/kWh | 2,131 kWh | $369 |
| Variable-speed | 7.37 kgal/kWh | 742 kWh | $128 |
That saves 1,388 kWh and $240 a year, 65%. It matches the physics: pump power falls with the cube of speed, so at half speed a pump moves half the flow on one eighth of the power (DOE, ENERGY STAR). A single-speed pump of this size runs at about 57 gpm, so it finishes the day's water in about 4.4 hours; the variable-speed pump takes 8 hours at 31 gpm and uses far less. The University of Florida energy guide cites 70% less energy, and ENERGY STAR quotes payback in under two years for typical replacements.
Payback of a variable-speed pump
Payback is the price difference divided by the yearly saving. If your quotes differ by $700, then $700 ÷ $240 is 2.9 years. Change the extra-cost, electricity and days fields to match your quote and your climate. Pumps on pools that run only four or five months save proportionally less. DOE's minimum score for pumps of 0.711 to 2.5 hydraulic hp (6.59 at 1 hhp) is far above the 1.74 to 2.69 of older single-speed pumps, and it applies to pumps made since July 19, 2021, so most new pumps of this size are variable-speed anyway.
How long should the pump run
Eight hours is a common home target (Florida Public Utilities); the University of Florida guide gives up to 12 hours as the outer limit in its state's code. Public pools follow 6 hours or less in the MAHC, which is four turnovers a day. Hot water and heavy use call for more. Check that the filter can pass this flow with the pool filter size calculator, and see the pool heater calculator if you are adding a heater to the loop.
Next steps
Start from your pool gallons if you are unsure of the volume, then size the filter for the flow above. Every tool for this job is in the equipment category.