Introduction
A pool loses water through three routes, and only one of them is unavoidable: evaporation. The other two — backwashing and leaks — are fully or partly controllable, and they are usually where the waste is.
The problem is that most owners have no idea how much they top up. The hose is left running for a few minutes, nobody counts, and at the end of the year there is no way to tell whether consumption was normal or whether a leak had been running for months.
This article explains how to measure it, what is normal, and what reduces each of the three routes.
The Three Routes of Consumption
| Route | Controllable? | What reduces it |
|---|---|---|
| Evaporation | Partly | Cover, water temperature, shelter from wind |
| Backwashing | Yes | Correct frequency, correct duration, filter type |
| Leaks | Yes | Early detection |
There is a fourth, smaller but real route: carry-out — water that leaves the pool on swimmers' bodies, in games and splashes. It is not optimised, it is managed with common sense.
First: Learn to Measure
Without measurement, no optimisation is possible. There are two simple ways.
The mark on the skimmer
Mark the ideal level in the skimmer mouth with tape or pencil. Every week, on the same day and at the same time, see how far it has dropped. A 1 cm drop on an 8 × 4 m pool is about 320 litres.
The calculation: surface area (m²) × drop (m) × 1000 = litres lost.
Example: 8 × 4 = 32 m² of surface. A 2 cm drop = 0.02 m. 32 × 0.02 × 1000 = 640 litres that week.
The meter or the bucket
If you can, fill through a meter or count buckets. Less elegant and more reliable. Always note the figure in your weekly log, next to pH and chlorine.
What Is Normal?
Evaporation depends on air temperature, water temperature, humidity and above all wind. There is no universal figure, but there are useful orders of magnitude for comparison.
In high summer, on an uncovered pool exposed to wind, losing several millimetres of water height per day is common. On a sheltered pool covered overnight, that loss falls substantially.
What matters is not the absolute figure: it is the variation against your own series. If you held 2 cm a week for a month and then suddenly went to 6 cm with no change in weather or use, you have a leak to investigate.
Telling Evaporation from a Leak: The Bucket Test
It is the most useful test in pool maintenance, and it is free.
- Fill a bucket with pool water to about 5 cm from the rim.
- Stand the bucket on a pool step, submerged enough to be stable but with the rim above the water.
- Mark the water level inside the bucket and the pool level outside it.
- Leave it 24 to 48 hours, with the pump running on its usual schedule and nobody using the pool.
- Compare the two drops.
If both drop equally: it is evaporation. There is no leak.
If the pool drops more than the bucket: the difference is a leak.
Repeat the test with the pump off. If the loss reduces a lot, the leak is probably in the pressure pipework, on the return side. See Pool Water Leaks: Detect and Repair and Pool Losing Water: Evaporation or Leak?.
💡 Dica Profissional
Run the bucket test once in June, even if you suspect nothing. You then have a reference figure for evaporation in your pool, at your exposure, in your climate. Next time consumption looks high, you have something to compare against instead of guessing.
Reducing Evaporation
The cover is the only measure with a large impact
Evaporation happens at the surface. Covering the surface is the only intervention that acts directly on the cause, and it makes a substantial difference to water consumption — as well as reducing chemical use and, on heated pools, energy.
The most effective cover for this purpose is one that sits in contact with the water, because it blocks the direct exchange. See Pool Covers: Types and Benefits.
Wind is the most underestimated factor
Wind across the surface accelerates evaporation very significantly. On an exposed pool, a hedge, a fence or a glass panel on the prevailing-wind side reduces consumption more than any product.
Along the coastal strip between Costa da Caparica and Sesimbra, the afternoon nortada is a real and recurring factor in summer water consumption.
Water temperature
The warmer the water, the more it evaporates. On heated pools, every extra degree costs in water and in energy. Dropping the set point by one or two degrees and covering at night reduces both at once.
Reducing Backwash Water
Every backwash discards a considerable volume of water. It is a necessary operation — without it the filter stops filtering — but it is frequently done more often and for longer than needed.
Backwash on the indicator, not on the calendar. The correct criterion is the gauge pressure: when it rises about 0.3 to 0.5 bar above the clean-filter figure, it is time. Before that, there is no benefit.
The right duration. Watch the sight glass: when the water runs clear, stop. Continuing improves nothing and discards water.
Do not skip the rinse. The rinse position is short — 20 to 30 seconds — and settles the sand. Skipping it sends sand into the pool; extending it wastes water.
See Filter Backwash Step by Step.
Filter type matters. Cartridge filters do not backwash: they are hosed down, which uses far less water. If backwash consumption is a genuine concern and the filter is at end of life, that is a factor to weigh when replacing it. See Pool Filter Types.
Where Backwash Water Can Go
Backwash water is not clean water: it carries the dirt held in the filter and traces of treatment chemicals. Before using it for irrigation or discharging it into the ground, confirm with your câmara municipal which rules apply in your area, and ask for the answer in writing. The rules vary between municipalities, and the local answer is the only one that counts.
The Hidden Effect: Water Topped Up Is Hardness Topped Up
Every litre of new water brings minerals with it. In a hard-water area — and much of the region is — topping up in large volumes means accumulating calcium hardness through the season, because the water evaporates and the minerals stay.
Consequences: deposits on the lining, on the heat-pump exchanger and on the electrolysis cell, and a greater tendency to cloudy water.
In other words: reducing evaporation is not only about saving water. It also slows the rise in hardness, which reduces corrective equipment maintenance.
Common Mistakes
Leaving the hose running unattended. Besides the direct waste, overfilling pushes the level above the skimmer mouth, which cuts off surface skimming and worsens filtration.
Topping up in one large volume. A big fill changes temperature, pH and disinfectant dilution abruptly. Better to top up regularly and in small amounts.
Confusing high consumption with high evaporation. Without the bucket test that is a guess. Many "exceptional evaporations" are leaks.
Backwashing out of weekly habit. If the pressure has not risen, you are discarding water for no reason.
Dismissing children's splashing as nothing. Technically it is minor, but on pools heavily used by children the carry-out is measurable. This is to know, not to restrict.
Frequently Asked Questions
How do I know if my pool has a leak?
Run the bucket test for 24 to 48 hours. If the pool drops more than the bucket, you have a leak. It is the only simple method that separates the two causes reliably.
Does covering the pool reduce water consumption much?
Yes — it is by far the most effective measure, because it acts directly on the surface where evaporation happens. It also brings the most side benefits: chemicals, energy and hardness.
Should I switch the pump off to save water?
No. Insufficient filtration saves no meaningful water and degrades water quality, which you pay for in chemicals. See How to Reduce Pool Maintenance Costs.
Can I reuse backwash water for irrigation?
It depends on what is established in your area and what the water contains. Confirm with the câmara municipal first, and get the answer in writing.
How long should I backwash for?
Until the sight glass runs clear — typically between one and three minutes, depending on the state of the filter. Then 20 to 30 seconds in the rinse position.
Is an automatic top-up system worth installing?
It helps keep the level right and protects the pump, but it carries a risk: if there is a leak, the system quietly tops up for months and the problem never surfaces. If you install one, put a meter upstream and read it monthly.
Conclusion
A pool's water consumption is manageable, but only once it is measured. Anyone who does not write it down cannot tell a hot spell from a leak.
Key points to remember:
- Surface area × level drop × 1000 = litres lost. Note it every week.
- The bucket test separates evaporation from a leak in 24 to 48 hours, at no cost.
- The cover is the measure with the greatest impact on water consumption.
- Backwash on gauge pressure, not on the calendar.
- Topping up a lot of water means accumulating calcium hardness, at a cost in equipment.
If your pool's consumption looks high and you want an answer rather than a guess, the on-site technical diagnosis includes a water-loss test, an installation check and a hardness measurement, with a written report. It costs €45, deductible from your first month under contract.
