Why Pool Losing Water? A Comprehensive Investigation

As a pool owner, you’ll be aware that the water level drops continuously, and you need to top up the pool regularly. Pools lose water for various reasons, most of them completely normal and to be expected. However, when the level suddenly drops faster than before, it could spell disaster if you don’t find out why and solve the problem. It’s important to know exactly why your pool is losing water.

Pools can lose water in three ways. Evaporation is the most common reason, but water may be lost simply by excessive splashing from kids playing or over-enthusiastic water features. Thirdly, and most seriously, there may be a leak in the circulation system or the pool structure itself.

The first task to be tackled is to determine whether or not the pool is losing water at an abnormal rate and, if so, what the reason is. I’ll give you information regarding normal water loss, then look in detail at the causes of an abnormal drop in water level. If there’s a leak, I’ll explain how to trace it and then how to solve the problem before any severe damage is caused.

Why Does A Pool Lose Water?

If all pools lose water, how does one know if there’s a problem? First, let’s establish what a trouble-free pool will lose:

The Normal Loss Of Water Measured

The average loss of water out of a pool is about ¼ -inch per day, or just under 2 inches a week. At the height of summer, this could double, which in an average-size (33 feet X 18 feet) residential pool can amount to about 1300 gallons weekly. The calculation, with all measurements in feet, is:

Length x Width x Drop in water level x 7.48

(the last figure is the number of gallons in a cubic foot.)

Loss Of Pool Water Through Evaporation

Evaporation is a natural process and occurs when molecules at the surface move fast enough to break away and become a gas. The evaporation rate can vary substantially depending on the elements of nature – wind, air temperature, humidity, and sunlight.

  • The wind over the pool carries water vapor away from the surface, drying the air and encouraging evaporation.
  • The hotter the air surrounding the pool, the faster the molecules on the surface move and the more they break away and evaporate.
  • Sunlight will have the same effect, transmitting energy and heat to the pool’s surface to encourage evaporation.
  • Humidity, when low, allows the air to absorb more water vapor, so it is inversely proportional to the rate of evaporation.

These factors mean that evaporation levels can vary substantially in different regions, and the Climate Prediction Center of the National Weather Service, as well as some state governments, produce evaporation maps for the whole United States.

How To Reduce Evaporation Of Pool Water

Based on the figures above, evaporation could total around 60,000 gallons of water in a year. Then there’s a loss of chemicals to consider. So, reducing evaporation means saving money, too.

1. Pool covers are highly effective, reducing evaporation by up to 95%, and help to keep the pool free of dust and debris.

2. Shading the pool, whether with a shade tent or awning or by planting shade plants around the perimeter, will reduce the water temperature and the air temperature, both of which affect the rate of evaporation.

3. Turn off any water features, such as fountains, sprays, and bubblers, which aerate the water and increase the movement of the water molecules, thus encouraging evaporation.

4. Don’t heat the water. You may enjoy a warm pool, but ideally, the water temperature should be at least 2 degrees colder than the surrounding air to minimize the rate of evaporation.

Loss Of Pool Water Through Leaks

While evaporation is a normal phenomenon, loss of water through leaks indicates faults in the structure or in the circulation system of the pool, and these need to be sorted out as soon as possible to avoid significant damage and substantial repair costs.

How Do I Know If My Pool Is Leaking?

Apart from the drop in the water level, other signs of a leak may be visible excess moisture in the ground around the pool or constant changes in the chemical balance of the water.

The Bucket Test for Leaks

This is a simple but reliable way of testing for a leak before you try and locate it.

  1. Fill a bucket to approximately five inches from the top, and mark the water level with a felt-tip pen.
  2. Place the bucket on the first or second step or the pool’s tanning ledge, and again mark the water level on the outside of the bucket. It should be immersed at least five inches deep.
  3. Turn off the auto-fill system, if you have one, and leave the pump off for the first 24-hour test.
  4. After 24 hours, check the water level inside and outside the bucket. If the outside level has dropped more than the inside, it shows a leak, possibly in the pool’s structure.
  5. Rerun the same 24-hour test, this time with the pump running normally. There may not have been a leak shown in the first test, but if there is now a difference in the two water levels, it indicates that the leak is in the filtration system, probably in the pressurized return line.

What Are The Possible Causes Of Leaks?

Leaks in an inground pool could be:

  • Structural, with possible cracking of the walls or floor of the pool.
  • If it’s a vinyl-lined pool, there could be a tear in the lining
  • In an older pool, the seals around pool lights, wall jets, skimmer box, or main drain could have deteriorated so that they are no longer water-tight.
  • The piping in the entire system, from inlets through the filter and pump to the return line, may develop leaks at joins or in the pipes themselves.

How To Locate A Leak In The Pool

Checking where the leak is situated is mainly a visual exercise, so be prepared to go underwater with a pair of goggles.

The skimmer is the most common place for leaking to occur, so I recommend checking this first. Cracks in the walls or floor of the skimmer box can be repaired with an epoxy sealant, but if the seal to the outlet pipe has failed, the skimmer might have to be replaced, and I would get a professional to do this job rather than try it myself.

The pump, filter, multiport valve, and other equipment, such as a chlorinator, should all be checked visually with the pump running to check for leaking seals, which can be replaced relatively easily.

Leaking underground pipes, especially from the main drain, won’t show up on a visual inspection and might be difficult to repair – that’s when a professional pool repairer is the best alternative.

Leak detection dyes can be invaluable in pinpointing the exact location of a leak in a lining or pool shell. You’ll need your goggles for this one – go underwater, find the suspected crack, and, using a syringe, squirt a few drops of dye close to it. If the dye moves towards and into the crack, you’ve found the leak.

Leaks in an inground pool can have a destructive part to play if left unattended, as the surrounding soil supporting the weight of the pool could be slowly eroded away. If that happens, your pool could be destroyed, so don’t leave it to chance – get a pool specialist to fix that leak.

Loss Of Pool Water Through Excessive Splashing

I’ve left the third reason for the loss of pool water until now, partly because it’s not a significant cause and also because there’s not much you can do about kids jumping in and out of the pool and splashing while they play. As a parent and a pool owner, as long as they play safely, kids will be kids!


Evaporation and excessive splashing are two harmless reasons for losing pool water, and at least evaporation can be reduced. When leaks have been detected and located, though, and structural failure or underground leaks are possible reasons, repairs are essential. They should be done without delay as the situation will only worsen until the costs to remedy the damage far outweigh that of the original repair.