Short answer: run your pool filter long enough for the pump to circulate the entire volume of water at least once, ideally twice a day. For a typical family pool in peak season, that works out to roughly 8 to 12 hours of filtration a day — not the 4 hours most factory timers are set to. Since filtration is the biggest running cost after heating, it pays to know not just how long, but when and with what to filter.
The basic rule: the whole volume of water 1–2 times a day
Filtration has one job: push all the water in the pool through the filter before algae and bacteria get a chance to multiply. One complete filtration of the pool’s volume is called a turnover. In practice: outside the peak of the season, one turnover a day is enough; in summer, once the water climbs above 24 °C and the pool sees regular use, plan on two.
You can work out the required filtration time with a simple formula:
filtration time (h) = pool volume (m³) ÷ actual filtration flow rate (m³/h)
Example: a 7 × 3.5 m pool with a depth of 1.2 m holds roughly 29 m³. A sand filter with a 6 m³/h pump needs 29 ÷ 6 ≈ 5 hours for one turnover. For two turnovers, that comes to around 10 hours of filtration a day.
Watch out for one common mistake: the figure printed on the pump’s box is the maximum flow rate with no load. The actual flow rate, once you factor in resistance from the filter, pipework and a dirty filter medium, is usually 20–30% lower. So if the manufacturer states 8 m³/h, plan on roughly 6 m³/h in real conditions. If you want to be on the safe side, size your filtration so that one turnover takes no more than 5–6 hours.
Why “4 hours a day” isn’t enough in the heat
Most budget pool kits and timers come preset to 4 hours of filtration a day. That may be fine early in spring when the water is below 18 °C — but in July it’s a reliable recipe for trouble. There are three reasons:
- Water temperature. The rate at which algae and bacteria multiply rises with temperature. Water at 28 °C is a far more biologically active environment than water at 20 °C, and without enough circulation, microorganisms multiply faster than the filter and chlorine can remove them.
- Bather load. In summer you swim more often and in bigger groups. Every swimmer brings sunscreen, sweat and organic debris into the water, which uses up disinfectant and feeds algae.
- Sunlight. UV radiation breaks down free chlorine — on hot, sunny days it can burn off most of a day’s dose. Warm water, low chlorine and still water together are exactly what algae need.
The result is usually cloudy water that turns green. If that’s already happened, we’ve covered how to fix it in our article on green pool water — but it’s cheaper to simply extend filtration before the water turns. Shock chlorination plus a flocculant together cost a few tens of euros and a day’s work; a few extra hours of filtration cost mere cents.
How long to filter by water temperature — a rule-of-thumb table
Pool owners have a handy rule of thumb: “water temperature divided by two.” The number of filtration hours per day equals half the water temperature in degrees Celsius. It’s not a standard or a scientifically derived formula, but a proven practice that tracks the fact that water’s biological activity rises with temperature. Always treat it as a minimum and compare it against the turnover calculation above — whichever figure is stricter wins.
| Water temperature | Recommended daily filtration | Situation |
|---|---|---|
| up to 16 °C | 4–6 hours | spring, autumn, pool not in use |
| 18–22 °C | 8–10 hours | start and end of season, occasional swimming |
| 24–26 °C | 10–12 hours | peak season, regular swimming |
| 28 °C and above | 12–14 hours, or even round the clock at lower output | heatwave days, full bather load, kids, parties |
During a heatwave, after a storm, or after a weekend with a packed pool, don’t hesitate to run filtration for a full 24 hours as an exception. Longer circulation never hurts the water — the only thing that goes up is your electricity bill.
When to run the pump: daytime or at night?
From the water’s point of view, the answer is clear: filter mainly when the pool is under the most strain — that is, during the day, when the sun is out and you’re swimming. Circulating water spreads chlorine evenly, carries debris to the filter as soon as it enters the water, and keeps the surface from overheating in still layers. Algae also photosynthesize during the day; still, sunlit water is exactly what they thrive on.
From your wallet’s point of view, though, it looks different. If you’re on a dual-rate electricity tariff, you pay noticeably less per kilowatt-hour during off-peak hours — which tempts many owners to run filtration only at night. But filtering purely at night is a mistake: it leaves the water without circulation during the day, exactly when the strain is greatest, and chlorine dosed into still water doesn’t distribute well.
A sensible compromise looks like this:
- Split filtration into 2–3 blocks. For example, 4 hours in the morning, 4 hours in the afternoon and early evening (the busiest time), and the rest on the cheap night tariff. Shorter breaks between blocks beat one long one.
- Keep the main block during the day. At least half of your daily filtration time should run between morning and early evening, especially in sunny weather and after chlorinating.
- Use the night as a top-up. Cheap electricity is great for a second turnover, backwashing the filter, or running the heater — not as the only time the water moves.
Timer, smart plug, or a variable-speed pump?
Switching filtration on and off by hand doesn’t work — you’ll forget, you’ll be away, it’ll rain. Automation is the first and cheapest step toward savings on filtration:
- A mechanical or digital timer (from about €10–20) switches filtration on in fixed blocks. Cheap, reliable, no smart features.
- A smart plug (from about €15–30) does the same plus lets you control it from your phone, track consumption, and set up scenarios — for example, extending filtration on hot days based on the forecast. Just make sure it’s rated for the current draw and suitable for outdoor use.
- A variable-speed (inverter) pump is the biggest investment, but also the biggest saving. Instead of a “full power / off” cycle, it runs for long hours at lower speed.
Why does a variable-speed pump save so much? A pump’s power draw doesn’t fall linearly with speed — it falls roughly with the cube of the speed (the so-called affinity laws). A pump running at half the flow rate draws only a fraction of the power, so the same volume of filtered water costs you significantly less electricity — typically half to two-thirds less. Slower flow also filters more gently and more quietly.
| Filtration mode (2 turnovers/day, €0.24/kWh) | Daily consumption | Seasonal cost (5 months) |
|---|---|---|
| Single-speed 550 W pump, 10 h/day | approx. 5.5 kWh | approx. €200 |
| Same pump, half the time on the night tariff (avg. €0.18/kWh) | approx. 5.5 kWh | approx. €150 |
| Variable-speed pump, 20 h/day at low speed | approx. 2–2.5 kWh | approx. €75–90 |
Treat these figures as a model example for a pool of around 30 m³ — exact numbers depend on your pump, pipework and electricity price. But the trend is clear: a variable-speed pump running long and quiet works out cheapest and is best for the water. For how to tie filtration into a smart home, consumption monitoring and automated scenarios, see our detailed article on smart pool automation.
Common mistakes that ruin the water and the electricity bill
Most water problems during the season aren’t caused by bad chemistry, but by the wrong filtration schedule. The most frequent culprits:
- Filtration set too short. Leaving the factory default of 4 hours a day running all summer. The water deteriorates slowly and quietly — until one heatwave weekend finishes the job.
- Filtering only at night for cheap electricity. You save a few cents a day, but the water sits still during exactly the hours the sun and swimmers stress it most.
- A clogged filter. Dirty sand or a blocked cartridge can easily cut flow in half — the pump runs, you pay for the electricity, but turnovers aren’t actually happening. Backwash a sand filter roughly once a week, and clean or replace a cartridge as it gets dirty. You’ll find the differences between the two systems and their maintenance intervals in our comparison of sand vs. cartridge filtration.
- Undersized filtration. A small cartridge pump from a basic pool kit physically can’t manage one turnover of 30 m³ of water even in 12 hours. The fix is a more powerful sand filter, not longer suffering with a weak pump.
- Switching filtration off while on vacation. A week of still water in July almost guarantees a green homecoming. Leave filtration running on a timer, and shock-chlorinate lightly before you leave.
Frequently asked questions
Is 4 hours a day enough to filter a pool?
Only with cold water outside the season. In summer, once the water is above 24 °C, plan on 10–12 hours a day so the pump can filter the entire volume at least once, ideally twice. Short filtration is the most common cause of cloudy, green water.
Is it better to filter continuously, or in blocks?
The water benefits most from circulation that runs as long as possible — ideally a variable-speed pump running at low output for, say, 20 hours a day. With a single-speed pump, filter in 2–3 blocks spread across the day, topped up with the night tariff; one long block only at night is a mistake.
Can filtration run while we’re swimming?
Yes, and it should. Swimming introduces the most contaminants into the water, and running filtration carries them straight to the filter instead of letting them settle to the bottom. Only switch it off for backwashing, vacuuming to waste, or servicing.
Does the “water temperature divided by two” rule always apply?
It’s a handy rule-of-thumb minimum, not a standard. Always check with the calculation that your pump can actually manage 1–2 turnovers of the volume in that time — with weaker filtration, the required time works out longer than the rule suggests. Add more during heatwaves, storms, and heavy bather loads.
Does a variable-speed pump pay off for a smaller pool too?
Yes, the percentage saving is similar — typically half to two-thirds of filtration consumption. For a smaller pool the absolute amount is lower, so payback takes longer; work it out from the difference in consumption, run time, and your electricity price.
Conclusion: hours of filtration are cheaper than rescuing the water
Properly set filtration is the cheapest pool chemistry there is: 1–2 turnovers of the volume a day, the main run during daylight, a clean filter, and smart switching based on your tariff. If your current pump can’t manage the turnovers you need, or you want to switch to economical variable-speed operation, you’ll find the full range of pumps, filtration systems and accessories in our pool technology category — and more practical guides in our Guide section.
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