"Turnover" is the time it takes your pool pump to move a volume of water equal to the whole pool through the filter. It is the most useful single number for judging whether a pump is right for a pool, how long to run it, and whether a cloudy pool is a filtration problem or a chemistry problem.
The formula
turnover time (hours) = pool volume (gallons) ÷ flow (gallons per minute) ÷ 60
Turned around, the flow needed for a given turnover time is:
flow (GPM) = pool volume (gallons) ÷ turnover hours ÷ 60
In metric, use litres and litres per minute — the arithmetic is identical. 1 GPM is 3.785 L/min; 1 m³/h is 16.7 L/min.
| Pool volume | 6-hour turnover | 8-hour turnover | 10-hour turnover |
|---|---|---|---|
| 10,000 gal (37,854 L) | 27.8 GPM | 20.8 GPM | 16.7 GPM |
| 15,000 gal (56,781 L) | 41.7 GPM | 31.3 GPM | 25.0 GPM |
| 20,000 gal (75,708 L) | 55.6 GPM | 41.7 GPM | 33.3 GPM |
| 25,000 gal (94,635 L) | 69.4 GPM | 52.1 GPM | 41.7 GPM |
| 30,000 gal (113,562 L) | 83.3 GPM | 62.5 GPM | 50.0 GPM |
| 40,000 gal (151,416 L) | 111.1 GPM | 83.3 GPM | 66.7 GPM |
What turnover time should you aim for?
For residential pools, one turnover in about 8 hours is the most common design target, and many pool builders size equipment so the pool can turn over at least once a day. Public and commercial pools are governed by local health codes, which often require turnovers of 6 hours or less — and much faster for spas and wading pools, which have a lot of bathers per gallon. Check what applies where you are.
One turnover does not filter every drop. Filtered water mixes back into unfiltered water, so after one turnover roughly 63% of the original water has been through the filter, after two about 86% and after three about 95%. That is why heavily used pools benefit from more than one turnover a day.
Flow is lower than the label
A pump’s rated flow is measured against little resistance. Your system has a filter, maybe a heater and chlorinator, pipe runs, elbows, valves and returns — together, the system’s "total dynamic head". The higher the head, the lower the flow. The same pump that moves 80 GPM in one system may move 50 in another.
- Best: a flow meter on the return line reads the real flow.
- Good: read the filter pressure gauge, convert it to feet of head (about 2.31 ft per psi) and add the suction side, then look up the flow on the pump curve.
- Rough: time how long the pump takes to fill a known volume, for example by running a waste line into a measured container.
Remember also that filter pressure rises as the filter loads up with dirt, which reduces flow. Clean or backwash when the pressure is about 8–10 psi above its clean reading, or as the filter manufacturer recommends.
How long should the pump run?
At least long enough for one turnover a day, and longer in hot weather, during heavy use or after a storm. With a single-speed pump that often means 8–12 hours. Splitting the run into two blocks (morning and late afternoon) keeps chlorine distributed and skimmers working.
Variable-speed pumps
A variable-speed pump changes the answer — and the electricity bill. Pump power falls much faster than flow when the motor slows down (roughly with the cube of speed by the pump affinity laws), so running at half speed for twice as long moves the same water for a fraction of the energy. That is why variable-speed pumps are usually set to run long hours at low speed, with short high-speed periods for skimming, vacuuming, a heater or a spa. Many jurisdictions now require variable-speed or multi-speed pumps for new installations.
Worked example
A 16 × 32 ft pool averaging 4.75 ft deep holds 18,193 US gallons. For an 8-hour turnover it needs 18,193 ÷ 8 ÷ 60 = 37.9 GPM of real flow. If the system moves 45 GPM at high speed, one turnover takes 6.7 hours. At a low speed delivering 25 GPM, one turnover takes 12.1 hours — and still uses much less electricity.
Above-ground pools
Above-ground pools usually ship with a pump and filter sized for them. A 24 ft round with a 52 in wall holds about 12,970 gallons; turned over in 8 hours that is 27 GPM. Small cartridge-filter pumps that come with frame pools often turn the pool over less often, which is one reason those pools need careful chlorine levels and regular filter cleaning.
Turnover and water clarity
If the water is cloudy, check the cheap things first: is the pump running long enough for at least one turnover? Is the filter clean? Then test the chemistry — free chlorine, pH and stabilizer. More turnover won’t fix a pool with no free chlorine, and perfect chemistry won’t fix a pool whose pump runs two hours a day. See pool chlorine basics.