Is your toilet water level low? Have you just answered a call of nature and noticed that the water level in the toilet bowl is lower than usual? If so, this would usually indicate a problem with your system so you need to get your detective hat on so it can be rectified. Lucky for you, we are here to help make your job finding the problem much easier. There are several things that can cause a low water level in your toilet bowl, and we will cover those along with what to do to save you from calling out a plumber to come and fix it for you.

Note: This article is for UK flush-down toilets and not American-style siphonic toilets.

Low Water Toilet Flushing

Causes of Low Toilet Water

In the UK, there are 4 main reasons why the water level in your toilet will drop and you will need to find out which one of these it is so you can address it sooner rather than later. The 4 reasons are pets, a faulty air admittance valve, a damaged toilet, and a blockage in the system somewhere.


Yes, one of the main reasons for the water in your toilet bowl dropping is that it is being used to quench the thirst of your furry little friends. This is obviously the easiest of the problems to fix although catching them in the act may prove a little more difficult. I once had a lady that was adamant that her dog would not and could not drink from the toilet. After, investigating all other possible causes, it wasn’t until the lady set up a home camera that she caught the dog lifting the seat, taking a drink, and then putting the seat back down!

Although this may be rare, before calling a plumber, you will want to rule this one out.

Faulty Air Admittance Valve

The next reason for a low water level in a toilet bowl is that the air admittance valve is not doing its job properly which is creating a vacuum in the system which is siphoning the water away from the traps. Air admittance valves are one-way valves that are fitted to the top of the soil stacks. When siphoning pressure builds up within the pipe, the valve opens to allow clean air into the pipe, therefore, balancing the pressure.

If this valve becomes blocked or stuck, it is unable to let any air into the system. With no air being allowed back into the system, the pressure causes a suction effect on the water traps visibly reducing the water level. You will want to ensure that the AAV is functioning correctly and is letting air back into the system, if not, you may need to replace it (Amazon link – opens in a new tab).

Note: The water level returning to normal when you remove the AAV could also be a false positive and be an indication of a blockage instead. Instead of allowing air into the system to equalise the pressure, you could be releasing air out of the system to equalise the pressure.

Damaged Toilet

A damaged toilet can be the cause of low water levels although this one is usually quite easy to rule out. Even the tiniest hairline crack in the toilet can lead to the water levels dropping but you are likely to see signs of water on the floor or around the base of the loo so hopefully, you will spot this one quickly. If the toilet has cracked, nothing can be done about it and it must be replaced as the crack could become very bad, very quickly.

If you need some ideas on what toilets are available, we have picked out some great toilets with powerful flushes.


The most common reason for low toilet water levels in your toilet bowl is that there is a blockage somewhere between the toilet and the drain outside. A blockage in the soil stack or at the manhole may be so bad that it has created a vacuum. Blockages are usually caused by things like baby wipes or a build-up of paper or other solids that have made their way into the system. To see if you have a blockage, you will want to open the manhole that the soil stack discharges into and have someone flush a toilet or two and see what the flow is like.

If the flow appears reduced or does not equate to what you are expecting, then you can see that it is blocked. The best thing to clear a blockage yourself is to get some drain rods (Amazon link – opens in a new tab). They are inexpensive and can be fed into the system to dislodge any debris that may be in there causing the water levels to drop.

How to Prevent Low Toilet Water Levels

That covers the main reasons for low water levels in a toilet bowl but there are a few things to keep in mind, so this does not happen again. If you have pets, you should stop them from drinking from the toilet, leave the seat down or keep the door closed so that they are not able to access it very easily. Next up, check your AAV at least annually, and ensure that it is working properly and that the seal has not worn or degraded. After a time, AAVs are known to seize and become stiff, or they can become blocked with dust and debris. Finally, be more thoughtful when it comes to what you flush down the toilet. In the UK, toilet paper is flushed and once wet will almost instantly begin to break down unless you are flushing wads and wads of it at the same time; the same cannot be said for wet wipes or baby wipes.

Wet wipes are not designed to break down on flushing and they are one of the primary causes of blockages in the home.


If you are experiencing toilet water levels lower than usual, it is likely due to a blockage, but it can also be a faulty AAV, a damaged toilet bowl or your cheeky little furry friends. All the issues that we have mentioned are easy enough to resolve yourself without resorting to calling out a plumber. Once you have ruled out pets and damaged porcelain, it is time to check the AAV and also see if there is a blockage. New AAVs are easy to install, and blockages can be reached with a set of drain rods.

4 Reasons for Toilet Water Levels Being Low Infographic

Plumbing Wizard Tips

“Do not put baby wipes or other wet wipes down the toilet!”

“When you check the AAV, it should not be grimy and sticky – it should move freely!”

“If you are not confident enough to tackle these problems yourself – call a plumber!”

“Drain rods are a very handy tool and can save you quite a bit of money, especially if you are having a problem out-of-hours!”

Frequently Asked Questions

What is the air admittance valve installation height?

For AAVs attached to your waste system, they need to be installed at least 600mm above the highest spillover point which is normally the sink.

Why is the water level in my toilet bowl low UK?

Toilets in the UK are different from toilets in the USA. UK toilets use a flush-down system and when you notice the water level dropping, it often indicates a blockage.

What causes the water level to drop in a toilet bowl?

There are various reasons that this can happen. Damaged porcelain, thirsty pets, blockages, and faulty air admittance valves are all things that can cause the water level in your toilet to drop.


Lee Pearce is not just a master plumber; he’s a veritable Plumbing Wizard. With over 30 years of experience in the trenches of pipes and drains, Lee has become the go-to sage for DIY plumbing, saving homeowners thousands in potential call-out charges. As the founder of Plumbing Wizard, he’s dedicated to demystifying the complexities of home plumbing, offering easy-to-follow advice that stands the test of time and pressure. His online blog is a treasure trove of tips, tricks, and tutorials that empower everyday individuals to take charge of their home’s plumbing health. Lee’s practical wisdom is not just about fixing leaks; it’s about imparting confidence and self-reliance. When he’s not writing or elbow-deep in a plumbing project, Lee is passionate about educating the next generation of DIYers, ensuring that practical skills are passed down and preserved.

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