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A pool looks like a one-off purchase of water. It is not: it is a surface that loses water every day, and the loss is bigger in exactly the weather when you want to use it.
Key figures
- about 58 percent More outdoor water used by homes with pools
- 24 hours Bucket-test period for comparing pool water loss
- up to 50 percent DOE maximum heating-bill reduction with a pool cover
Where the water goes
The Department of Energy’s Building America guide is direct: the main source of water loss in swimming pools and spas is evaporation from the water surface. The same guide notes that single-family homes with a pool use about 58 percent more outdoor water than homes without one.
Evaporation rises with water temperature, with air movement over the surface, and with dry air. Heating a pool increases it. Wind increases it. A cover stops most of it.
The other losses are smaller and more controllable:
- Splash-out, which depends on how the pool is used.
- Backwashing a sand or DE filter, which sends water to waste. Cartridge filters avoid this entirely.
- Leaks, which are the reason to measure rather than guess.
The bucket test, which settles the question
If you are topping up more than you expect, find out whether it is evaporation or a leak before calling anyone.
- Fill a bucket with pool water and set it on a step so it is partly submerged.
- Mark the water level inside the bucket and on the pool wall.
- Leave the pump running normally for 24 hours, with nobody swimming.
- Compare. Both levels dropping about the same amount is evaporation. The pool dropping faster than the bucket is a leak.
Repeat with the pump off to narrow a suspected leak to the plumbing rather than the shell.
It cuts evaporation, which saves water. It prevents night heat loss, which the Department of Energy’s guide says can reduce heating bills by up to 50 percent. And it saves chemicals, because less water leaves and less sunlight burns off chlorine.
Reducing the loss without ruining the pool
- Cover it when it is not in use. The single biggest lever.
- Lower the target temperature in the shoulder season.
- Plant or build a windbreak on the prevailing wind side.
- Switch to a cartridge filter if you are replacing the filter anyway and water is expensive where you live.
- Keep the water level at mid-skimmer. Too high wastes water through the overflow, too low starves the pump.
Refilling and the chemistry that follows
A large top-up changes the water chemistry: it dilutes stabiliser, changes hardness and shifts pH. Test after any significant refill rather than assuming the previous balance still holds. That is a small point that prevents a week of cloudy water.
Related reading: Compare a Pool Pump Repair With the Complete Replacement and Why Your Next Pool Pump Has to Be Variable Speed.
Questions people ask
How much water does a swimming pool lose per week?
It depends on surface area, temperature, wind and humidity, so measure yours with the bucket test rather than using a national figure. Evaporation is the main source of loss.
How do I know if my pool is leaking?
Use the bucket test. If the pool level falls faster than the water in a bucket sitting in it, you have a leak rather than ordinary evaporation.
Does a pool cover save water?
Yes, and that is its main job. Because evaporation is the largest source of loss, a cover addresses water, heat and chemicals at the same time.
Do homes with pools use much more water overall?
The Department of Energy's Building America guide puts outdoor water use at about 58 percent higher for single-family homes with a pool.
Where these figures come from
- Water Savings in Pools and Spas Building America Solution Center, US Department of Energy · basc.pnnl.gov
Figures last checked September 2026.
