The short answer
Start from flow, not horsepower. Required flow (GPM) = gallons ÷ (turnover hours × 60): a 20,000-gallon pool on an 8-hour turnover needs about 42 GPM. A pump's maximum flow is measured at almost no resistance, so look for one whose curve delivers your flow at your system's total dynamic head; our pump size calculator allows 1.5× for typical in-ground plumbing.
Turnover flow by pool size
| Pool volume | Flow needed |
|---|---|
| 10,000 gal | about 21 GPM |
| 15,000 gal | about 31 GPM |
| 20,000 gal | about 42 GPM |
| 25,000 gal | about 52 GPM |
| 30,000 gal | about 63 GPM |
Why bigger isn't better
An oversized single-speed pump uses more power, can exceed the filter's rated flow and pushes water through the filter too fast. A variable-speed pump such as the Hayward TriStar VS or Pentair SuperFlo VS avoids this by running at the speed you need.
Check the rest of the system
- The filter's rated flow must be at least the pump's flow.
- Match plumbing size (1.5 or 2 in) and voltage (115V or 230V).
- Heaters and salt cells need a minimum flow to run; check their makers' figures.
SafetyPump wiring and bonding are electrical work; use a licensed electrician.
Products mentioned
Questions readers ask
What size pump for a 15,000-gallon pool?
About 31 GPM for an 8-hour turnover; typical in-ground plumbing means a pump rated for about 45 GPM or more at zero head.
Does horsepower tell me the flow?
Only roughly. Two pumps with the same horsepower can move different flows; compare the maker's flow curve at your head.