How the math works
Chlority mathTurnover flow (GPM) = gallons ÷ (turnover hours × 60). A 20,000-gallon pool on an 8-hour turnover needs about 42 GPM.
Chlority mathMakers state a pump's maximum flow at almost no resistance. Real plumbing pushes back (total dynamic head: pipe length, elbows, the filter and any heater), so we list pumps whose maximum flow is 1.5× the turnover flow for in-ground pools and 1.25× for above-ground pools, and not so far above it that a single-speed pump is oversized. Filters are listed when their stated maximum flow is at least the turnover flow.
LimitsFor an exact match, estimate your total dynamic head and read the flow off the maker's pump curve, and check the pump's voltage and plumbing size against your pad. Since 2021, federal rules have made most standard-size in-ground filter pumps variable speed.
Questions
What size pump do I need for a 20,000-gallon pool?
About 42 GPM for an 8-hour turnover. With typical in-ground plumbing that usually means a 1 to 1.5 HP pump, or a variable-speed pump that can run slower for most of the day.
Is a bigger pool pump better?
No. An oversized single-speed pump uses more power, can exceed the filter's rated flow and pushes water through the filter too fast. Variable-speed pumps avoid this by running at the speed you need.
Do I have to run the pump 24 hours a day?
No. One full turnover a day is a common minimum; many owners run a variable-speed pump longer at low speed, which uses far less energy than a shorter run at high speed.