Pool Pump Size Calculator
Enter your pool's gallons and the turnover time you're targeting. You get the required flow rate in GPM and the smallest typical pump size that covers it.
8-10 hours is the standard residential target.
Required flow rate
42 GPM
20,000 gal ÷ 8h
Suggested pump
3/4 HP
typical range 40-60 GPM
| Pump size | Typical GPM range | Covers your 42 GPM? |
|---|---|---|
| 3/4 HP | 40-60 GPM | Yes |
| 1 HP | 55-75 GPM | Yes |
| 1.5 HP | 70-95 GPM | Yes |
| 2 HP | 85-110 GPM | Yes |
| 2.5 HP | 100-130 GPM | Yes |
| 3 HP | 115-150 GPM | Yes |
Required GPM by pool size, 8-hour turnover
The standard residential target. A 10-hour turnover needs proportionally less flow; a faster turnover needs more.
| Gallons | Required GPM | Typical pump |
|---|---|---|
| 10,000 | 21 GPM | 3/4 HP |
| 15,000 | 31 GPM | 3/4 HP |
| 20,000 | 42 GPM | 3/4 HP |
| 25,000 | 52 GPM | 3/4 HP |
| 30,000 | 63 GPM | 1 HP |
| 40,000 | 83 GPM | 1.5 HP |
Typical single-speed pump GPM ranges
Published planning ranges for common residential plumbing (roughly 1.5-2" pipe, 40-60 ft total dynamic head) -- your exact plumbing changes the real number.
| Pump size | Typical GPM range |
|---|---|
| 3/4 HP | 40-60 GPM |
| 1 HP | 55-75 GPM |
| 1.5 HP | 70-95 GPM |
| 2 HP | 85-110 GPM |
| 2.5 HP | 100-130 GPM |
| 3 HP | 115-150 GPM |
Frequently Asked Questions
What is pool turnover, and why does it matter?
Turnover is the time it takes your pump to circulate a volume of water equal to your entire pool once. 8-10 hours is the standard residential target -- fast enough to keep sanitizer and filtration working through the whole pool at least once a day, without running the pump around the clock.
How is the required flow rate (GPM) calculated?
Gallons ÷ (turnover hours × 60 minutes) = gallons per minute. A 20,000-gallon pool on an 8-hour turnover needs about 41.7 GPM -- the pump has to move that whole volume through the filter and back in 8 hours.
Why does the same GPM number map to a range of pump sizes, not one exact size?
A pump's real output depends on total dynamic head -- pipe length, diameter, elevation, fittings, and filter resistance -- which varies by plumbing, not just pool size. The ranges here are typical published figures for common residential plumbing; your specific system's pump curve, matched against your actual head, is the only fully exact answer.
Is a bigger pump always better?
No -- oversizing wastes electricity (pump cost scales with horsepower) and can even hurt water quality by moving water through the filter too fast for good contact time. Sizing to the turnover target, not the largest pump available, is the standard approach.
Does a variable-speed pump change this math?
The turnover target and required GPM are the same either way -- a variable-speed pump just lets you run at that GPM efficiently at low RPM instead of a single-speed pump's fixed output, which is usually oversized for normal circulation and only needed at full speed for tasks like vacuuming.
These calculators use published seeding rates, extension-service fertilizer guidance, and standard pool-chemistry dosing figures, and are for planning. Read the product label; it governs the dose. Pool chemicals are hazardous when mixed or mishandled, and a pool store's water test beats any estimate here.