How the calculation works
The pool sets the demand: required flow (GPM) = volume ÷ (turnover hours × 60).
The plumbing sets the ceiling. Water moving faster than about 6 ft per second through suction pipe gets noisy, wastes energy and strains fittings. At that limit 1½ inch pipe carries about 37 GPM and 2 inch about 62 GPM. A pump that can push more than the pipe can carry does not make the pool cleaner — it just makes the plumbing work harder.
Worked example
A 20,000 gallon pool on an 8-hour turnover needs about 42 GPM. That is more than 1½ inch suction plumbing should carry, so the answer is not a bigger pump — it is a longer turnover at a lower flow, or 2 inch pipe. A variable-speed pump run gently for 10 or 12 hours does the same job without touching the plumbing.
Assumptions
- Maximum velocity of 6 ft/s in suction pipe, the figure normally used for residential plumbing.
- Schedule 40 PVC internal diameters.
- One turnover per day.
Limitations
- This gives flow, not horsepower. Two pumps of the same HP deliver very different flow depending on the resistance in the system, so flow is the figure to shop on.
- Heaters, salt cells, solar panels and long pipe runs all add resistance, which cuts real flow below the pump curve.
- Suction pipe is the constraint here. Return plumbing tolerates slightly higher velocity.
Questions people ask
What size pump do I need for a 20,000 gallon pool?
About 42 GPM for an 8-hour turnover. In horsepower terms that is usually a 1 to 1.5 HP pump, but flow is what matters — HP is a motor rating, not a promise of output.
Is a bigger pump better?
No, and it is a common expensive mistake. Water forced through the filter too fast is filtered less well, and an oversized pump costs more to run every hour of its life.
Are variable-speed pumps worth it?
Usually. Power draw falls much faster than flow as speed comes down, so running at half speed for twice as long moves the same water for a fraction of the electricity. In many places they are now required on new installs.
Why does my pipe size matter?
Because it caps flow no matter what pump you fit. Pushing 60 GPM through 1½ inch pipe means high velocity, cavitation noise and wasted power — the water simply cannot get through any faster.