Toilet Bowl Water Level Low? Causes and What to Do

A low water level in the toilet bowl usually means water is being siphoned out of the pan's trap, either by a partial blockage in the toilet or soil pipe, or by a blocked vent or stuck air admittance valve that lets other appliances pull the water out as they drain. Other causes are a weak or short flush that doesn't refill the pan, a crack in the pan that lets water seep out, toilet paper or a cloth hanging over the trap and wicking the water away, or evaporation in a toilet nobody uses. The way to tell them apart is to watch when the level drops: straight after flushing, when another appliance drains, or slowly over hours.

Why the level matters

Every toilet pan has a bend that holds water. That water is the trap seal, and its job is to stop gas from the drains coming into the room. Building regulations guidance on drainage, Approved Document H, expects drainage and its ventilation to be designed so that trap seals aren't lost when appliances drain. When the level in the pan drops, the seal gets shallower, and if it drops far enough you'll notice a drain smell in the bathroom. The causes behind a low level, especially blockages and cracks, also tend to get worse if they're left.

Tests to find the cause

Put a small piece of masking tape on the inside of the pan at the normal water line, or take a photo, so you can see how far the level moves. Then:

What you see	Most likely cause
Flush is slow, swirls or rises before draining, and the level ends up low	A partial blockage in the pan, the pan connector or the soil pipe
Level drops, bobs or gurgles when the bath, basin, shower or washing machine drains	A blocked soil stack vent or a stuck air admittance valve
Level drops slowly over several hours, with damp, staining or a smell at the base	A crack in the pan or its trap
Flush is weak and the cistern doesn't fill to its marked water line	A cistern or fill valve problem, so the flush doesn't refill the pan
Level is low in a spare toilet or after you've been away	Evaporation
Level drops even though nothing else is going on, and there's paper or something hanging into the water	Wicking over the trap

To check for a crack, add a few drops of food colouring to the water in the pan and leave it for an hour or two without flushing. If coloured water appears around the base, behind the pan or on the ceiling below, the pan is leaking.

The common causes

A partial blockage

When there's a partial blockage just past the trap, the water from the flush can't get away cleanly. It fills the pipe, and as it finally drains it siphons water out of the trap behind it, leaving the level low. The flush usually looks slow, swirls for longer than normal or rises before going down. A flanged toilet plunger often clears a blockage in the pan itself. If it keeps coming back, or other appliances are also slow, the blockage is further down the drain. Our guide to unblocking a toilet covers what to try, and when a blocked toilet needs a plumber covers when to stop.

A blocked vent or air admittance valve

As water runs down the soil pipe, air has to get in behind it. Normally it comes in through the vent at the top of the soil stack, or through an air admittance valve, a one-way valve often found in a loft, boxing or the top of an internal stack. If the vent is blocked by leaves or a nest, or the valve is stuck shut, the draining water pulls air through the nearest trap instead, which is often the toilet. That sucks water out of the pan. It usually happens when the bath, shower or washing machine empties, and you may hear gurgling. Our guide to a gurgling toilet goes into how to tell a vent problem from a blockage.

A weak or short flush

The pan is refilled by the tail end of the flush. If the cistern isn't filling to its intended level, the flush is short and may not leave enough water behind. Cisterns are required to be marked inside with a line showing the intended flush volume. If the water stops well below that line, the fill valve needs adjusting or replacing. A dual flush toilet that's mostly used on the small flush can also leave a low level after heavy use. Our guides to toilet cistern problems and a flush button that isn't working cover both.

A cracked pan

A hairline crack in the pan, often in the trap where you can't see it, lets water seep out slowly. The level drops over hours rather than straight after a flush, and there's often damp, staining or a smell around the base. In a flat, the first sign can be a stain on the ceiling below. A cracked ceramic pan can't be reliably repaired and needs replacing. If water is appearing at the base but the pan isn't cracked, see our guide to a toilet leaking at the base.

Wicking

Toilet paper, a cleaning cloth, a toilet rim block or hair caught in the trap and hanging over the edge can draw water out of the trap little by little, like a wick. It's easy to miss and easy to fix: pull it out wearing gloves and flush.

Evaporation

In a toilet that isn't used for weeks, the water simply evaporates, and faster in a warm room. When you come back there's a drain smell and a low level. Flushing it restores the seal. For a spare bathroom, flushing the toilet and running the taps every week or two keeps the seals full. Our guide to drain smells in an unused bathroom covers this in more detail.

What you can do yourself

• Check for and remove anything hanging over the trap.
• Try a flanged toilet plunger if the flush is slow or swirls.
• Lift the cistern lid and check the water reaches the marked line. If it doesn't, the fill valve may need adjusting.
• Do the food colouring test if the level drops slowly with no obvious reason.
• If you have an air admittance valve you can reach safely, check it isn't covered, boxed in tightly or stuck. Don't go onto the roof to look at a stack vent.

When to call a plumber

Call a plumber if the level keeps dropping after you've tried the basics, if other sinks, baths or showers are also slow or gurgling, if the food colouring test shows a leak, if there's a smell of drains in the bathroom, or if water is showing on the ceiling below. A pan that's cracked needs replacing, and a vent or soil pipe problem needs someone who can get to it safely.

We charge £95 an hour for labour, plus a £60 call-out that's waived when the job runs over 2 hours. Parts are extra and we confirm the price before any work starts. Our toilet repair service covers pans, cisterns and connectors across London, and the pricing page explains how jobs are charged.

Frequently Asked Questions