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A dropping water level is common and usually explainable
You walk past the pool one morning and notice the waterline looks a little lower than it did last week. Straight away the worry starts. Is something broken? Is water draining into the garden? Will it cost a fortune to fix?
Take a breath. Some pool water loss is completely normal. Every outdoor pool loses water to the air, and plenty lose a little more through splashing, backwashing and heavy use. The trick is knowing how much is normal for your pool, and spotting when the numbers stop adding up.
In this guide we’ll explain the everyday reasons a level drops, show you a simple test you can do with a bucket, and help you decide when it’s time to get an engineer involved.
Why it pays to keep an eye on the level
The waterline matters more than you might think. If it falls too far, your skimmer can start drawing in air instead of water. That can make the pump run dry, which isn’t good for it. Keeping the level steady also keeps your chemistry more predictable, because every top up dilutes what’s already in there.
Evaporation is the most common reason for a lower waterline
Water is always escaping from the surface of an uncovered pool. You can’t see it happening, but on a warm, breezy day it adds up surprisingly quickly.
Several things speed evaporation up:
- Warm water, especially if it’s warmer than the air around it
- Wind blowing across the surface
- Low humidity and dry, sunny weather
- A large surface area compared with the depth
- Water features, jets and fountains that throw water into the air
- Leaving the pool uncovered overnight
On a cool, still, damp day you may barely notice any change. On a hot, windy day in August, a heated pool can lose a noticeable amount between morning and evening. Both are normal.
How the seasons change things
In spring and autumn the gap between warm water and cold night air is often at its widest. That’s when heated pools tend to see the most pool water loss through the surface, even if the day itself doesn’t feel especially hot. In high summer, sunshine and wind do more of the work. If you’d like to understand the science in more depth, our guide to pool evaporation goes into the detail.
Covers make a real difference
A good cover is the simplest way to cut evaporation. It sits on the surface and stops water escaping into the air. It also keeps heat in and leaves out. Many owners are surprised by how much steadier their level becomes once they get into the habit of covering the pool whenever it isn’t being used. You can read more about the options in our guide to automatic pool covers.
Everyday pool water loss that isn’t a leak
Evaporation isn’t the only innocent explanation. Pools lose water in lots of small, ordinary ways that are easy to forget about.
Splashing and swimmers
Every time someone climbs out, they take water with them on their skin and swimwear. It sounds like very little, but a family swimming several times a day can account for a fair bit of water over a week, especially if towels and wet feet are carrying water away from the poolside too. Kids jumping in, cannonballs and water fights can send a surprising volume over the edge. After a busy weekend with friends round, a lower level is no surprise.
Backwashing and cleaning
If your filter is backwashed, some water goes to waste as part of that process. How much depends on the size of the filter and how long you run the backwash for, so a thorough clean will usually take more than a quick one. Vacuuming to waste does the same. It’s worth making a note each time you do either, so you don’t mistake it for something more serious later.
Rain and overflow
Heavy rain can raise the level, and some pools have an overflow that drains the excess away. When the weather dries up again, the level can look like it’s dropped quickly, when really it’s just settling back to normal.
The bucket test is a simple way to compare
If you’re still unsure, the bucket test is the easiest check you can do at home. It compares the water your pool loses with the water a bucket loses in the same conditions. Because both are sitting in the same sun and wind, they should evaporate at roughly the same rate.
Here’s how it works:
- Fill a sturdy bucket with pool water to about 10 centimetres from the top.
- Stand the bucket on the first or second step of the pool so it sits in the water. It needs to be steady and not float.
- Mark the water level inside the bucket with a strip of tape or a waterproof pen.
- Mark the pool level on the outside of the bucket too.
- Turn off any autofill and avoid swimming or topping up for the test period.
- Leave it for 24 hours with the pump running as normal.
- Come back and compare how far each level has dropped.
If the bucket and the pool have dropped by about the same amount, evaporation is the likely cause. If the pool has dropped noticeably more than the bucket, something else is going on.
Running it twice gives you a clearer answer
Try the test once with the pump running and once with it switched off. Leave a day or so between the two so the results don’t blur together. If the pool loses more with the pump on, the issue may be on the pressure side of the plumbing. If it loses more with the pump off, it could be on the suction side. This is useful information to pass on to an engineer, even if you don’t know exactly what it means.
Tips for a fair result
- Pick a calm, dry day if you can, because heavy rain will spoil the result
- Make sure nobody moves or knocks the bucket
- Use a measuring tape rather than guessing by eye
- Write the numbers down so you can compare them later
Signs that pool water loss might be a leak
Sometimes the test simply confirms what you already suspected. Other times it points towards a leak. Either way, there are a few other signs worth looking out for.
- The level keeps dropping at the same rate even when the pool is covered
- You’re topping up far more often than you used to
- Soggy patches, sinking paving or unusually green grass near the pool
- Cracks in the surround or around fittings
- Air bubbles in the return jets, or the pump struggling to prime
- Algae that keeps coming back despite good chemistry, because fresh water keeps diluting the sanitiser
- The level always stops at the same point, such as just below a light or skimmer
That last one is a useful clue. If the water drops to a certain line and then stays there, the leak is often somewhere around that height.
What to avoid doing
It’s tempting to start pulling things apart to find the problem. Please don’t. Digging near pipework or taking fittings off can make things worse and may affect any guarantees you have. The same goes for adding sealing products you’ve read about online without advice. Leave the investigation to people who do it every day.
When to call an engineer about pool water loss
If your comparison shows the pool is losing clearly more than the bucket, or you’ve spotted any of the warning signs above, it’s time to get some help. The same applies if you simply can’t make the numbers make sense.
A qualified engineer will usually start with the easy things. They’ll look at the equipment, the pipework they can reach and the fittings around the pool. They may use specialist methods to trace water movement or test sections of pipe. Finding a leak early tends to make the repair simpler, and it saves you wasting water and chemicals in the meantime.
What to have ready
The more you can tell an engineer about your pool water loss, the quicker they can narrow down the cause. It helps if you can share a few details when you get in touch:
- How long you’ve noticed the problem
- Roughly how much the level drops each day
- Your bucket test results, with the pump on and off
- Any changes you’ve made recently, such as new equipment or landscaping
- Photos of anything that looks out of place
Our guide to a swimming pool leak explains more about where leaks tend to happen and how they’re usually fixed.
How HiLife can help
We look after pools as part of our aftercare, servicing and repairs work. If you’re worried about your level, talk to us about what you’re seeing. We can talk it through, suggest checks and arrange a visit if it looks like something needs looking at properly.
Keeping your level steady day to day
A few simple habits make it much easier to spot a problem early and keep your pool running smoothly.
- Cover the pool whenever it isn’t in use
- Check the level at the same time each day, especially in summer
- Keep a simple log of top ups, backwashes and heavy use
- Look around the surround and equipment area every so often
- Book regular servicing so small issues get caught early
- Keep the level around the middle of the skimmer opening
Once you know your pool’s normal pattern, anything unusual stands out straight away. A notebook by the plant room door or a note on your phone is all you need. There’s no need for anything fancy. That’s the best early warning system you can have, and it costs nothing but a minute or two of your time.
Choosing a pool that’s easier to look after
If you’re planning a new pool, it’s worth thinking about maintenance from the start. A one-piece pool has no joins in the shell itself, which helps keep things simple. Pairing it with a good cover and well-planned equipment makes daily care easier still. You can explore our full range of pools to see what might suit your garden.
Pool water loss questions we get asked a lot
How much water loss is normal?
It varies with the weather, the water temperature, wind and whether the pool is covered. The best way to judge your own pool is to use the bucket test and compare like with like.
Can I leave the autofill on during the test?
No. Turn it off, otherwise it will top the pool up and hide the real drop. Switch it back on once you’ve finished.
Does a heated pool lose more water?
Usually, yes. Warmer water evaporates faster, particularly on cool evenings. A cover helps a lot here.
Will a small leak fix itself?
Unlikely. Leaks tend to get worse over time, and water escaping into the ground can cause other problems. It’s better to have it looked at early.
Should I keep topping up while I wait for an engineer?
Keep the level high enough for the skimmer to work, so the pump doesn’t run dry. Make a note of how much you add, as that helps with diagnosis.
Is it safe to swim if I think there’s a leak?
In most cases a slow drop in level isn’t a safety concern, but if you see anything unusual with electrics, fittings or the surround, stay out and get advice first.
Getting to the bottom of pool water loss
A falling waterline isn’t always bad news. Most of the time it’s simply the weather, a busy weekend or a backwash. A quick test with a bucket will usually tell you if you’re dealing with something normal or something that needs attention. If the numbers don’t add up, don’t wait. Getting an engineer involved early keeps repairs simpler and gets you back to enjoying the pool sooner.
Call us on 01483 323 676 or email hello@hilifepools.co.uk for a friendly, no-pressure chat, or get in touch online and we’ll come back to you.
