Start with turnover
Turnover is the time needed to pass a volume of water equal to the whole pool through the filter once:
Turnover (hours) = pool volume in gallons ÷ (flow rate in gallons per minute × 60)
If you do not know your volume, the tables on the pool volume page cover common sizes and give the formulas for anything unusual. Flow rate is harder: it is not the pump's horsepower and it is not the number on the motor label. It is determined by the pump curve, the plumbing size, the filter and the total resistance of the system. A flow meter reads it directly; failing that, a reasonable estimate for a typical residential system is 40 to 70 gallons per minute, and installers can usually give a figure for the system they built.
Worked example: a 20,000-gallon pool with a system moving 50 gallons per minute moves 3,000 gallons per hour, so one turnover is about 6.7 hours.
One turnover is a floor, not a target
Filtered water returns into the pool and mixes with unfiltered water rather than displacing it. The practical consequence is that one nominal turnover filters roughly two-thirds of the water, two turnovers roughly ninety per cent, and so on. That is why a pool run at exactly one turnover a day looks acceptable rather than good, and why two turnovers is the usual recommendation for the hot part of the season.
| Conditions | Turnovers per day | Typical hours (6–7 hr turnover) |
|---|---|---|
| Cool water, light use, spring or autumn | 1 | 6–8 |
| Normal summer, moderate use | 1.5 | 10–12 |
| Hot weather, heavy use, or water above 85 °F | 2 | 13–16 |
| Treating algae, cloudiness or slime | Continuous | 24 |
| After a storm or heavy debris load | Continuous until clear | 24 |
Temperature changes everything
Sanitiser demand rises steeply with temperature. As a rough working figure, every 10 °F rise in water temperature roughly doubles chlorine consumption. Biological growth accelerates on the same curve. A pool that is comfortable at eight hours in June is often struggling at eight hours in August with nothing else having changed — and the owner reaches for chemicals when the answer was hours.
This is the single most common reason a pool that "was fine all season" turns in a heatwave.
Day, night, or split?
Run the majority of the hours during daylight. Sunlight is destroying free chlorine while the sun is up, bather load is concentrated in daytime, and debris blows in during the day. If your electricity tariff strongly rewards off-peak use, split the schedule — for example a long daytime block and a shorter overnight block — rather than moving everything to the night.
One caveat for cold climates: during freezing conditions on an open pool, continuous overnight running is a freeze-protection measure, because moving water resists freezing and standing water in pipes does not.
Variable-speed pumps invert the usual advice
With a single-speed pump, hours cost money in a straight line. With a variable-speed pump they do not, because power consumption falls roughly with the cube of speed while flow falls roughly in proportion to it. Halving the speed halves the flow but cuts the power draw to something closer to an eighth.
The practical consequence is that running longer and slower is usually both cheaper and better. Many variable-speed installations run close to continuously at low speed and step up only for vacuuming, backwashing, running a pressure-side cleaner or clearing debris after a storm. Continuous gentle circulation eliminates the daily period of stagnation that intermittent operation creates, and stagnation is where problems start.
One thing to watch: some equipment needs minimum flow to function. Heaters have a minimum flow rate, salt chlorine generators have a flow switch, and suction- or pressure-side cleaners need a certain flow to move. If your low-speed setting is below any of those thresholds, that equipment simply will not operate.
Signs you are running too little
- Water is clear in the morning and slightly hazy by evening.
- A persistent faint haze that clarifier fixes for a day.
- Algae appearing first in shaded corners and behind the ladder.
- Chlorine present in the morning test and absent in the afternoon test.
- Debris settling on the floor overnight rather than reaching the skimmer.
Signs you are running more than you need
- Water is consistently clear, balance holds for a week, and the filter pressure climbs very slowly.
- No haze at any point in the day, even after heavy use.
If both of those are true for a few weeks, reduce the schedule by an hour at a time and watch the water rather than cutting it in half at once. Efficiency measures for pool water and energy use more generally are covered by the EPA's WaterSense programme.
Related: circulation, filtration questions, pool volume tables.