Turnover is a schedule, not a pump size
Turnover is the volume divided by the flow through the filter. Two turnovers of a 20,000 gallon pool is 40,000 gallons a day, which is 28 GPM if the pump never stopped and 56 GPM if it runs half the day. Every combination of flow and hours that multiplies to 40,000 gallons satisfies the same schedule, and that is the whole point of this page: the pump does not have to be sized to a number of hours you picked for other reasons.
Worth keeping straight is what a turnover actually buys. The pool is not a queue, and filtered water returning to it mixes immediately with water that has not been filtered. After one nominal turnover a meaningful fraction of the original water has never passed the filter. That is why targets sit at more than one turnover a day rather than exactly one, and it is also why chasing an exact number is not worth much. The pool volume calculator gives you the gallons this all starts from.
Why the plumbing decides the flow
Velocity in a pipe is flow divided by area, and in the units used here it is GPM times 0.4085 divided by the inside diameter in inches squared. Two inch Schedule 40 has an inside diameter of 2.067 inches, so 60 GPM through it moves at 5.7 feet per second. Push 90 GPM through the same pipe and it is 8.6 ft/s.
| Pipe | Inside dia | GPM at 6 ft/s | GPM at 8 ft/s |
|---|---|---|---|
| 1-1/2 in Sch 40 | 1.610 in | 38 | 51 |
| 2 in Sch 40 | 2.067 in | 63 | 84 |
| 2-1/2 in Sch 40 | 2.469 in | 90 | 119 |
| 3 in Sch 40 | 3.068 in | 138 | 184 |
Suction lines are conventionally held to a lower velocity than returns because the suction side is where a pump gets into trouble. Water arriving too fast against too much resistance drops in pressure at the impeller eye, vapour bubbles form and collapse, and the pump gets noisy and loses flow. These are design practice figures rather than a specification you can look up, which is why the limit is an input here.
The practical consequence is the one people find annoying. A larger motor on undersized plumbing does not deliver much more water, because the resistance rises roughly with the square of flow while the pump curve falls off. What the extra power mostly produces is noise, heat and an electricity bill. If the flow is short, the fix is on the pipe side — a bigger suction line, fewer sharp fittings, a cleaner filter — and the pressure drop calculator shows how quickly friction climbs as a line gets small for its flow.
The affinity laws, and why slow wins
Centrifugal pumps follow three relationships closely enough to plan around. Flow scales with speed. Head scales with the square of speed. Power scales with the cube of speed. That last one is where the money is.
Halve the speed and you get half the flow for one eighth of the power. To move the same volume of water you now run twice as long, so the energy for the day is one eighth times two, which is one quarter. Energy for a fixed volume of water scales with the square of the speed. At 60 percent speed it is 36 percent of the energy; at 40 percent speed, 16 percent. Nothing else in pool operation offers a lever like that.
There are limits on how far down you can go, and they are real rather than theoretical. Skimmers need enough surface flow to actually skim. Heaters usually have a minimum flow below which they will not fire. Suction cleaners and in-floor systems need pressure. Salt cells and chlorinators need flow to prove before they produce. A sensible variable-speed schedule is usually a long low-speed block for filtration with a short high-speed block for whatever needs the flow, rather than one speed all day.
What the run hours do not cover
Filtration hours are a circulation question and nothing more. They do not fix water that is out of balance, and running the pump longer will not compensate for a filter that needs cleaning or a skimmer basket nobody empties. Rising filter pressure with falling return flow is the signal that the filter is loading, and it changes every number on this page while it happens.
Bonding and grounding of pool equipment is licensed electrical work, and the reason is that water and people are in the circuit. No wiring procedure appears anywhere on this site. If a pump, heater, light or panel needs anything beyond a plug in a working receptacle, that is an electrician's job.
Questions people ask
How many hours a day should a pool pump run?
Long enough to move the water you decided to move, which is a different question for every pool. Work it from the volume: two turnovers of 20,000 gallons is 40,000 gallons, and at 60 GPM that is 11 hours, while at 30 GPM it is 22. The common advice of eight hours is a guess about a typical single-speed pump on a typical pool, and it stops being useful the moment your flow is not typical. Bather load, debris, sun and temperature all push the number up, and the honest way to tune it is to start from a turnover target and adjust based on how the water actually looks and tests over a few weeks.
Does a variable-speed pump really save that much?
The physics behind the claim is sound and unusually favourable. Power goes with the cube of speed while flow goes with speed, so the energy to move a fixed volume of water goes with the square of speed. Running at half speed for twice as long uses a quarter of the energy for the same turnover. Whether you see that in the bill depends on whether the low speed still does the jobs the system needs — heater minimum flow, skimming, a cleaner — and on how much of the day you can spend there. The savings are largest on pools that were running an oversized single-speed pump for short bursts, which is most of them.
Is a bigger pump better?
Almost never on existing plumbing. The pipe, fittings, filter and heater set a resistance curve, and the pump curve intersects it at one point. A larger impeller moves that point somewhat, but resistance grows roughly with the square of flow, so the extra flow you gain is a fraction of the extra power you spend. Meanwhile the higher velocity on the suction side brings noise and cavitation risk, and the filter sees water moving through it faster than it filters well. If the flow is genuinely inadequate, the productive changes are larger suction pipe, fewer sharp elbows, a cleaner filter and a properly sized filter, in roughly that order.
What flow rate should I enter if I do not have a flow meter?
The best free estimate comes from the pump curve and two gauges. Read the filter pressure on the return side and the vacuum on the suction side, convert both to feet of head and add them, then read the flow off the pump curve at that total head. If you have neither gauge nor curve, take the pump nameplate flow and treat it as optimistic by a wide margin, because it is measured at a head your plumbing does not provide. A flow meter in the return line is inexpensive relative to the decisions it informs, and it is the only reading that tells you when the filter has loaded enough to matter.
Should I run the pump at night or during the day?
That is an economics question rather than a water quality one, unless your rate has time-of-use pricing, in which case the answer is whatever avoids the peak window. There is a case for daytime running on pools that use a chlorinator or salt cell, because sunlight consumes free chlorine and having circulation and production happening while the sun is on the water lines them up. There is a case for night running on an unheated pool in hot weather, since circulation at night moves warm surface water down and exposes it to cooler air. Neither effect is large enough to override a meaningful difference in the electricity rate.