The short answer
A 18,000 gallon pool turned over in 8 hours needs a pump flow of 37.5 gallons a minute, with 1-1/2 in suction and 1-1/2 in return pipe to keep the water within code speeds. California caps filtration flow at a 6 hour turnover (50 gpm for this pool).
Flow by pool size
Gallons a minute by pool size and turnover; pipe sizes for an 8 hour turnover.
| Pool | 6 hours | 8 hours | 10 hours | Pipe (8 h) |
|---|---|---|---|---|
| 10,000 gal | 27.8 | 20.8 | 16.7 | 1-1/2 / 1-1/2 in |
| 15,000 gal | 41.7 | 31.3 | 25 | 1-1/2 / 1-1/2 in |
| 20,000 gal | 55.6 | 41.7 | 33.3 | 2 / 1-1/2 in |
| 25,000 gal | 69.4 | 52.1 | 41.7 | 2 / 2 in |
| 30,000 gal | 83.3 | 62.5 | 50 | 2 / 2 in |
| 40,000 gal | 111.1 | 83.3 | 66.7 | 2-1/2 / 2 in |
How it works
The formula
water speed (ft/s) = 0.4085 × gpm ÷ pipe inside diameter²
Assumptions
- Flow cap and pipe speeds from California's 2022 Energy Code, §150.0(p): filtration flow no more than a 6 hour turnover or 36 gpm, whichever is greater; water no faster than 6 ft/s in suction lines and 8 in returns. Other states have no such cap, but the speeds are sound practice everywhere.
- Pipe is Schedule 40 PVC: 1-1/2 in (1.61 in inside), 2 in (2.067 in inside), 2-1/2 in (2.469 in inside), 3 in (3.068 in inside), 4 in (4.026 in inside).
- The 8 hour default is an example; type the turnover you want.
- Pumps must meet the federal weighted energy factor (WEF) rule for dedicated-purpose pool pumps made since July 19, 2021.
Worked example: 18,000 gallons, 8 hours
- 18,000 ÷ (8 × 60) = 37.5 gpm
- In 1-1/2 in pipe: 0.4085 × 37.5 ÷ 1.61² = 5.9 ft/s
Too big or too small
- Too big: wasted power and a strained filter. A pump that moves more than the filter is rated for pushes dirt through and costs more to run.
- Too small: cloudy water. The pool never turns over in a day.
- Variable speed wins: run slow most of the day and fast only for cleaning or a spa.
Questions people ask
What size pool pump do I need?
Start from the flow: a 18,000 gallon pool turned over in 8 hours needs 37.5 gallons a minute; in 6 hours, 50. Then pick a pump whose curve delivers that flow at your system's resistance (head). Bigger isn't better: a variable-speed pump run slowly moves the same water for far less power.
How long should a pool pump run?
Long enough to filter the water once a day or so. California's energy code caps the filtration flow at the rate that turns the pool over in 6 hours (or 36 gpm, if higher), which pushes pumps to run slower and longer.
What size pool pipe do I need?
Keep the water slow: California's code limits it to 6 feet a second in the suction line and 8 in the return. At 37.5 gpm that's 1-1/2 in suction and 1-1/2 in return pipe (Schedule 40 PVC).
Do pool pumps have efficiency rules?
Yes. Since July 2021, federal rules require dedicated-purpose pool pumps to meet a minimum weighted energy factor (WEF); ENERGY STAR certified pumps beat it. Variable-speed models are the efficient choice.