A family pool with a volume of 30 m³ uses roughly 45–55 m³ of water in a typical year if you refill it every spring, and only 15–25 m³ if you leave the water in over winter and simply treat it in spring. At an average combined water and sewage price of around €5/m³ (an indicative European price), that is the difference between roughly €250 and €100 a year. And the biggest item during the season is not filling the pool — it is evaporation, which you can reduce by up to 95%.
What makes up a pool’s water consumption?
A pool does not consume water in one big draw but in five smaller ones that add up over the year. Anyone who wants to manage their consumption should know them individually:
- Initial fill — the entire pool volume in one go. For a 30 m³ pool, that means 30 m³ of water. It is not necessarily an annual item, though (see below).
- Evaporation — the main item during the season. An uncovered pool typically loses 3–5 mm of water level per day, and even more on hot, windy days.
- Sand filter backwashing — each backwash with a rinse cycle sends roughly 200–300 litres down the drain.
- Splash-out and carry-out — jumping in, children playing, water carried away on swimwear and towels. It seems trivial, yet it adds up to 1–3 m³ per season.
- Partial water changes — deliberately draining and refilling part of the volume, typically because of high chlorine stabiliser or dissolved solids.
The good news: four of the five items can be reduced significantly. The only one you cannot do anything about is the water carried out on swimwear.
How much water does evaporation take? Numbers that surprise
Evaporation depends on water and air temperature, humidity, wind and sun exposure. For an uncovered outdoor pool in summer it typically runs at around 3–5 mm of water level per day; industry overviews (e.g. Latham Pool Products or International Coverpool) report values of over 6 mm per day for hot, dry days. Wind speeds up evaporation considerably because it constantly carries the moist air away from the surface.
Millimetres sound harmless — converted into litres, they no longer do. An 8 × 4 m pool has a surface area of 32 m². One millimetre of evaporation over that area means 32 litres of water:
- 3 mm/day = 96 litres a day, i.e. about 2.9 m³ per month,
- 5 mm/day = 160 litres a day, i.e. about 4.8 m³ per month,
- over the May–September season (about 150 days) that adds up to 14–24 m³ — half to three quarters of the volume of an average family pool.
If your water level is dropping faster than the few millimetres a day mentioned above, it may not be evaporation but a leak. How to tell the two apart reliably (the bucket test) and where pools most often leak is covered in our article Pool losing water: evaporation or a leak?.
Model calculation: what a 30 m³ pool uses per year
Let us work with a specific model pool: in-ground, 7 × 3.5 m, average depth 1.25 m — a volume of roughly 30 m³ and a surface area of 24.5 m². The assumptions are clearly stated so you can adjust them for your own pool:
- May–September season (150 days), pool without a cover,
- average evaporation of 3.5 mm/day (a mild Central European summer, averaging sunny and cooler days),
- sand filtration, backwash once a week at 250 litres, 20 weeks,
- splash-out and carry-out of 2 m³ per season, normal family use.
| Consumption item | Calculation (assumptions) | Volume per year | Cost at €5/m³ |
|---|---|---|---|
| Spring fill | full volume of 30 m³ (only if you drain the pool) | 30 m³ | €150 |
| Evaporation (May–September) | 24.5 m² × 3.5 mm × 150 days | ca. 13 m³ | ca. €65 |
| Sand filter backwashing | 20 × 250 l | 5 m³ | €25 |
| Splash-out and carry-out | estimate, family use | 2 m³ | €10 |
| Total with a full refill | — | ca. 50 m³ | ca. €250 |
| Total without draining | water stays in, only topped up | ca. 20 m³ | ca. €100 |
The cost is based on an indicative European combined water and sewage price of around €5/m³ including VAT. Depending on your country and provider, your rate may easily differ by €1–2/m³ in either direction — for a precise calculation, use the rate from your own water bill.
If you have your own well, the maths is different: well water costs only the electricity for pumping, but it comes with risks — iron, manganese and hardness can turn a pool brown within hours. Before you run a hose from the well to the pool, read how to fill a pool with well water without unpleasant surprises.
How to cut water consumption to a minimum
1. Cover the surface — evaporation drops by up to 95%
The single most effective measure. A solar cover or a pool enclosure reduces evaporation by up to 95% because it stops the surface from being in contact with moving air. Of the model 13 m³ of evaporation, only about 1 m³ remains — and as a bonus, a cover retains water heat and keeps debris out, so the filtration works less. You will find a comparison of covers, slatted covers and enclosures in our article pool enclosure vs. solar cover.
2. Consider cartridge filtration, or make backwashing more efficient
A cartridge filter is not backwashed — the cartridge is taken out, rinsed and put back, and nothing goes down the drain. For smaller pools this eliminates the entire 4–6 m³ annual item. Sand filtration, on the other hand, offers higher flow rates and convenience; backwash it based on the pressure gauge (a rise of 0.2–0.3 bar), not the calendar, and keep the backwash running only until the water in the sight glass is clear. A detailed comparison of the two systems: sand vs. cartridge filtration.
3. Fix leaks — even a small one is a cubic metre a month
A dripping joint at the pump or a micro-crack in the liner loses an extra 1–3 mm of water level a day — a second, “invisible evaporation”. A leak of just 1.5 litres per hour adds up to more than 1 m³ a month. Check the equipment pit and hose connections regularly; a step-by-step leak-detection procedure is described in our article pool losing water.
4. Top up with rainwater — but do it wisely
Rainwater from a tank is free for topping up and it is soft, which regions with hard water will appreciate. It is well suited to gradually topping up smaller volumes (evaporation, splash-out). When yes: water from a clean roof, a settled tank, topping up no more than 10–20% of the volume, followed by checking and fine-tuning the chemistry. When no: water from overgrown gutters full of organic matter and pollen, the first runoff after a long dry spell, or when you are already fighting algae — the organic matter in rainwater feeds them and drives up sanitiser consumption. You will find products for fine-tuning your values after topping up in the water treatment category.
Does a pool have to be drained every year? No — and that is where you save the most
The biggest saving in the whole table is to stop draining the pool every year. Chemically balanced, continuously treated water will happily last two to three seasons in a pool, and even longer in saltwater pools. Over winter the pool is simply winterised with a lowered water level, and in spring the water is cleaned, filtered and rebalanced — instead of 30 m³ of fresh water, you top up just a few cubic metres.
A partial water change (draining 20–30% and topping up with fresh water) addresses most of the reasons pools used to be fully drained: rising dissolved solids, high chlorine stabiliser (cyanuric acid above ca. 80–100 mg/l), hardening water. A complete change only makes sense in exceptional cases: after a neglected winter with water full of sediment, when repairing the liner or shell, or when the water is chemically “locked” and no longer responds to normal treatment. Exactly when to schedule which change is something our year-round pool maintenance calendar will remind you of.
Where does the water go when you do have to drain?
Pool water is not ordinary rainwater — it contains sanitiser residues. In general, there are two routes:
- Sewer: the foul sewer is the cleanest solution, but drain slowly (on the order of cubic metres per hour, not all at once) so you do not overload the connection. Rules vary by operator — for larger volumes it pays to check the conditions with your local water utility in advance.
- Garden / soakaway: only with water in which the chlorine has broken down first. Leave the pool uncovered without dosing for 5–7 days (UV light breaks chlorine down), then measure free chlorine — below ca. 0.3 mg/l the water will no longer harm the lawn or the soil. Salt water does not belong in the garden at all, and neither does water treated with a winterising product.
Chlorinated water never belongs in the storm drain or a watercourse — the storm network discharges into streams without treatment, and chlorine is toxic to aquatic life.
FAQ: The most common questions about pool water usage
How much water evaporates from a pool per day?
An uncovered outdoor pool in summer typically loses 3–5 mm of water level per day, and over 6 mm in hot, windy weather. For an 8 × 4 m pool that corresponds to roughly 100–160 litres a day. Covering the surface reduces evaporation by up to 95%.
How much does it cost to fill a 30 m³ pool?
At an indicative European combined water and sewage price of around €5/m³, filling a 30 m³ pool comes to roughly €150. In more expensive locations it can be €180 or more; with your own well you only pay the electricity for pumping.
Do I have to drain and refill my pool every year?
No. Chemically maintained water lasts two to three seasons — over winter the pool is simply winterised and in spring the water is cleaned and rebalanced. A partial change of 20–30% of the volume is enough, typically because of high chlorine stabiliser. This saves you the biggest item in your annual consumption.
Can I top up my pool with rainwater?
Yes, rainwater from a clean, settled tank is suitable for gradually topping up smaller volumes — and it is soft, too. Always check pH and sanitiser levels after topping up. Water full of organic matter from clogged gutters is unsuitable — it brings nutrients for algae.
Where am I allowed to drain my pool water?
Ideally slowly into the foul sewer. Into the garden only after the chlorine has broken down (5–7 days without dosing, free chlorine below ca. 0.3 mg/l) — and never salt water. Chlorinated water does not belong in storm drains or watercourses.
A pool’s water consumption is not decided by fate but by three decisions: cover the surface, do not drain unnecessarily, and watch for leaks. With those in place, the annual water bill for a 30 m³ pool stays within a few hundred euros. Everything you need to keep your water in shape for several seasons — from testers and flocculants to algaecides — is in the water treatment category, with more practical guides in our Guide.
