Why Is My Pool Losing Water? Leak vs. Evaporation
A dropping water level is the first thing that makes a pool owner worry. But not all water loss means you have a disaster on your hands. Some loss is just natural evaporation.
Before you panic, your first task is to figure out exactly what you are dealing with. Here is how to tell the difference, including a simple test you can run yourself tonight.
The Quick Answer
Normal pool evaporation typically accounts for 3mm to 5mm of water loss per day (about the thickness of a pound coin). This can increase slightly in hot, dry, or windy weather. If your pool is losing noticeably more than 5mm a day, or if it continues to lose water during cold, damp weather, you likely have a leak.
A simple 24-hour DIY bucket test will confirm the truth. If it points to a leak, professional leak detection can find the exact spot without digging up your garden.
First, Rule Out Evaporation with the Bucket Test
This is the exact test that structural pool engineers will tell you to run first. It compares your pool’s water loss against a controlled volume of water sitting in the exact same environmental conditions.
How to Run a 24-Hour Bucket Test:
Position the Bucket: Fill a standard plastic bucket with pool water and place it on a shallow pool step or sun shelf. Submerge it halfway down so the water inside the bucket stays at the exact same temperature as the pool water.
Mark the Levels: Using waterproof tape or a permanent marker, draw a line showing the water level inside the bucket. Then, draw a second line on the outside of the bucket showing the exact pool water level.
Set the Conditions: Turn off your pool pump and filtration system, and make sure nobody uses the pool for the next 24 hours. Disable any automatic water top-up systems.
Compare the Drop: After 24 hours, check both marks. If the pool water level has dropped noticeably lower than the water level inside the bucket, evaporation is not your problem. You have a leak.
Tip: For the most accurate result, run this test over two or three days if the weather is highly changeable or windy.
Key Signs It Is a Leak, Not Evaporation
The water level keeps dropping during cool, humid, or completely still weather when evaporation rates should be near zero.
You find yourself topping up the pool with a hose far more frequently than in previous seasons.
You notice unexplained damp patches, spongy lawns, or sudden green algae growth near the pool shell or the plant room.
The water loss continues at an aggressive, steady rate regardless of changes in weather.
Where Pool Leaks Hide: Structural vs. Plumbing
A swimming pool leak falls into one of two categories: it is either in the physical structure or buried deep within the pipework. Telling them apart requires professional equipment.
Leak TypeCommon Physical SymptomsDetection MethodRepair ImpactStructural Leak
(Shell, Liner, Tiles)• Cracks in concrete shell
• Tears or wrinkles in vinyl liner
• Loose or missing pool tiles• Hydrostatic dye testing
• Underwater dive inspectionsLocalized patching, often completed underwater without draining the pool.Plumbing Leak
(Underground Pipes)• Water loss accelerates when pump runs
• Air bubbles in return jets
• Unexplained soggy patches on the lawn• Line pressure testing
• Acoustic micro-microphonesTargeted excavation restricted strictly to the pinpointed pipe location.
Plumbing leaks are particularly tricky because the entire pipework network is buried deep underground. Guessing where the leak is leads to digging up perfectly good concrete or patio stones in the wrong place.
Why the Next Step Is Professional Leak Detection
Once your bucket test confirms a leak, the big question is where. Finding it is not a DIY job.
We use specialized hydrostatic pressure testing and ultra-sensitive acoustic equipment to pinpoint underground pipe failures down to the exact centimeter. Because we can track leaks through solid concrete, we ensure any eventual repair requires digging in only one single, targeted spot.
The Independence Advantage: We focus entirely on detection and testing. We do not sell repair services or installation packages. Because we have no financial interest in selling you expensive repair contracts, our only job is to give you a 100% honest, accurate report on where the problem lies.
Frequently Asked Questions
How much water loss in a pool is normal?
Normal evaporation typically accounts for 3mm to 5mm of water loss per day. Hot, dry, or windy conditions can increase this amount slightly.
How do I know if my pool is leaking or just evaporating?
Perform a 24-hour bucket test. Place a bucket of pool water on a step, match the water levels inside and outside the bucket, and turn off the pump. If the pool level drops faster than the bucket, you have a leak.
Can you find a pool leak without draining it?
Yes. Professional leak detection teams use electronic equipment, acoustic listening devices, and line pressure tests to locate underground leaks safely without draining the pool water. Draining a pool poses structural risks to the pool shell and wastes thousands of liters of treated water.
Why is my pool losing water only when the pump is on?
This usually points to a pressure-side plumbing leak in your return lines. The pressurized water forces its way out of small cracks or loose pipe fittings when the filtration system is active.
Think You Have a Leak? Let Us Find It
If your bucket test has pointed to a leak, do not start digging up your garden or patio. Let our expert engineers pinpoint the exact fault using non-invasive line pressure testing. We will find the source of the issue without draining your pool, keeping your repair fast, targeted, and as cost-effective as possible.
Call Our Leak Detection Team on 01234783569 or email us at info@swimmingpoolsurveys.co.uk to book your pressure test today.