Real-world flow also depends on pipe size and filter resistance — treat this as the sizing target, and the pump label's flow curve as the spec to match against it.
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The sizing formula
needed GPM = gallons ÷ (run hours × 60)
One turnover a day is the residential standard, and the math is humbling: pools need far less pump than the horsepower arms race suggests. The extra capacity of an oversized single-speed pump doesn't clean better — water can only pass through the filter so fast — it just converts electricity into noise and bearing wear.
A worked example
15,000 gallons over an 8-hour run: 31 GPM. Nearly any pump sold today hits that at low effort — which is the argument for variable speed: run 24 hours at a whisper (better filtration, quieter, pennies) or 8 hours at medium, and save the high RPM for vacuuming day. Check what that does to your bill with the pump run time calculator — cube-law savings are the best deal in pool equipment.
Matching pump to filter and pipes
Two ceilings to respect: your filter's design flow (on its label — exceeding it degrades filtration) and your plumbing (1.5" pipe tops out around 42 GPM sensibly; 2" around 73). The right pump is sized to the smallest of: your turnover need, the filter's rating, and the pipe's comfort zone. Bigger than that is spend without benefit.
Frequently asked questions
What size pump does my pool need?
Enough GPM to turn the water over once in 8–10 hours: gallons ÷ (hours × 60). A 15,000-gallon pool needs only ~25–31 GPM — which a modest pump delivers easily. Most residential pools are over-pumped.
Is a bigger pool pump better?
Usually the opposite. Power rises with the cube of speed, so an oversized single-speed pump burns dramatically more electricity to push water faster than your filter can even use it. Right-sized (or variable-speed run slow) wins on cost and on filtration quality.
What horsepower is right for my pool?
For typical residential plumbing, 1 HP moves roughly 50–80 GPM — more than most pools need. If you're replacing a legacy 1.5–2 HP single-speed, a variable-speed unit run at low RPM will filter the same water for a fraction of the bill.
Do I need more flow for a heater, cleaner, or spa?
Sometimes: heaters have minimum flow requirements (check the manual), suction cleaners like decent flow, and spa jets are the real GPM hogs. Variable-speed pumps solve this elegantly — high RPM for spa hour, low RPM for the other 23.