Fix a running toilet (won't stop filling)
Save £80–150 — no plumber needed
A toilet that keeps running wastes 200–400 litres of water a day. It is almost always the floating arm [float arm] or the rubber seal [flapper valve] — both are cheap and straightforward to fix.
Last updated: February 2025
Only basic tools needed — most homes already have them.
Before you start
A running toilet constantly trickles water into the bowl or down the safety overflow pipe [overflow tube] after flushing. This is almost always one of two things: the floating arm [float arm] is set too high, or the rubber seal [flapper valve] is not sealing.
You will be working inside the tank [cistern] — the tank at the back of the toilet that stores water before each flush. It contains only clean water. Nothing unpleasant.
You do not need to turn off the mains supply. There is a shut-off valve [isolation valve] (a small slotted screw on the pipe behind the toilet) that controls just this tank.
Know what you're looking at
You don't need to understand everything — just match what you see to what's happening.
What is your toilet doing right now?
Fix A
Water trickles constantly
You can hear running water even when nobody has flushed, or water is dripping from an outside pipe
Fix B
Water ripples in the bowl
You see slow movement or a shimmer in the toilet bowl between flushes, and the tank keeps quietly refilling
Fix C
Tank fills slowly or runs after flushing
The refill noise continues longer than 2 minutes after a flush, or the tank never seems to finish filling
Quick glossary — plain English for the technical terms in this guide
- Tank [Cistern]
- The tank behind or above the toilet that fills with water after each flush and stores it ready for the next one.
- Floating arm [Float / float arm]
- A plastic ball or cup on the end of an arm. It floats on the surface and rises as the tank fills — when it reaches the right height it shuts off the water inlet valve.
- Water inlet valve [Fill valve]
- The valve (usually on the side of the tank) that lets water in after a flush. The floating arm controls when it opens and closes.
- Ball-float valve [Ballvalve]
- An older style of water inlet valve where the float is a hollow metal or plastic ball on a long arm. Modern versions use a cup-shaped float on a vertical shaft instead.
- Rubber seal [Flapper valve]
- The rubber seal at the bottom of the tank. When you flush it lifts to let water into the bowl, then drops back down and seals so the tank can refill.
- Safety overflow pipe [Overflow tube]
- A safety pipe inside the tank. If the water level gets too high, water drains down this pipe and exits outside the house (the trickle you sometimes hear from an outside wall).
- Shut-off valve [Isolation valve]
- A small valve on the pipe behind the toilet. Turn the slot in the screw head 90° so it sits across the pipe — this shuts off the water supply to just this toilet.
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Tools & materials
- ✓Towel or cloth— to rest the cistern lid on and catch any drips
- ✓Flat-head screwdriver— for adjusting the shut-off valve [isolation valve] and some ball-float valves [ballvalves]
- !Replacement rubber seal [flapper valve]— only needed if cleaning does not fix the seal — take the old one to match the size
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Fix A
Lower the water level
Technical name: floating arm [float arm] adjustment (ball-float valve [ballvalve])
What's happening: The water in the tank [cistern] is filling too high and spilling down the safety overflow pipe [overflow tube] before the valve can shut off. You'll often hear this as a trickle from a pipe on an outside wall.
This looks more complicated than it is — you are bending a plastic arm or turning a screw.
- Lift the lid off the tank [cistern] and rest it on a towel nearby.
- Gently lift the floating arm [float arm] upward with your hand while the toilet is still running.
- If the water stops while you hold the arm up, the arm is set too high — that is your problem. Carry on to the next step.
- Lower the arm. On a traditional ball-float valve [ballvalve] (a long metal arm with a hollow ball at the end): bend the arm gently downward — a few degrees is enough. On a modern version (a plastic cup that slides on a vertical shaft): find the adjustment dial or clip on the side of the valve and turn it to lower the cutoff point.
- Open the shut-off valve [isolation valve] a quarter turn and watch where the water stops filling.
- The water should stop at least 25 mm below the top of the safety overflow pipe [overflow tube] — the tall central pipe inside the tank. If it still fills too high, lower the arm a little more and test again.
- Replace the lid. A correctly set tank is completely silent between flushes.
Cost: free. Time: under 5 minutes. No parts needed.
Fix B
Check the rubber seal
Technical name: rubber seal [flapper valve] inspection and replacement
What's happening: The rubber seal [flapper valve] at the bottom of the tank [cistern] is not sitting flat. Water is slowly leaking through into the bowl, which keeps triggering the tank to refill. You'll see gentle rippling in the bowl between flushes.
You won't break anything doing this — the rubber seal just clips on and off.
- Lift the lid off the tank [cistern] and rest it on a towel.
- Press down firmly on the rubber seal [flapper valve] — the disc at the bottom of the tank over a circular opening.
- If the rippling in the bowl stops while you press, the seal is your problem. Carry on.
- Unhook the rubber seal by unclipping it from the two pegs on either side.
- Rinse the seal under a tap.
- Wipe the rim it presses against [valve seat] with a damp cloth — a piece of grit or limescale is often the only problem.
- Clip the seal back on and flush once to test.
- If the bowl still ripples after cleaning, turn off the shut-off valve [isolation valve] — the small slotted screw on the pipe behind the toilet. Turn the slot 90° so it sits across the pipe.
- Flush to empty the tank [cistern].
- Take the old rubber seal to a hardware shop to match the size exactly — seals are not universal.
- Clip the new seal onto the same two pegs.
- Hook the chain onto the lever arm connected to the handle. Leave 1–2 links of slack — too tight and the seal stays slightly open; too loose and it gets trapped underneath when you flush.
- Open the shut-off valve a quarter turn and wait 30 seconds.
- Open the shut-off valve fully.
- Flush once and watch the rubber seal drop back into place. The water in the bowl should be still within a minute.
Cost: free if cleaning fixes it; £10–25 for a replacement seal. Time: 10–15 minutes.
Fix C
Adjust the refill mechanism
Technical name: water inlet valve [fill valve] (ball-float valve [ballvalve]) calibration
What's happening: The part that lets water back into the tank [cistern] after flushing (water inlet valve [fill valve]) is not shutting off at the right level — or is shutting off too slowly. The tank takes an unusually long time to refill, or the refill noise carries on well after the water level should have reached the right height.
This looks more complicated than it is — you are adjusting how high the water fills.
- Turn the shut-off valve [isolation valve] 90° so the slot sits across the pipe — it is the small slotted screw on the pipe behind the toilet.
- Flush to empty the tank [cistern] fully.
- Find the water inlet valve [fill valve] — the tall component on the side of the tank, usually on the left as you face it. On older styles it has a long arm with a ball at the end; on modern ones it is a narrow plastic tower with a cup that slides up and down.
- Lower the cutoff point. On a modern water inlet valve: turn the adjustment screw or slide the float cup down the shaft — this tells the valve to stop earlier. On a traditional ball-float valve [ballvalve]: gently bend the arm downward so the ball sits lower in the water.
- Open the shut-off valve a quarter turn and wait 30 seconds.
- Open the shut-off valve fully.
- Watch the tank fill. The water should stop on its own at least 25 mm below the top of the safety overflow pipe [overflow tube].
- If the tank still will not stop filling even with the float adjusted all the way down, the water inlet valve itself is faulty and needs replacing — a new valve assembly costs around £10–30. If you are not confident with this, call a plumber.
Cost: free for an adjustment; £10–30 if the fill valve needs replacing. Time: 5–30 minutes.
Where beginners go wrong
Replacing the rubber seal [flapper valve] before checking the float. The floating arm [float arm] adjustment costs nothing and takes 30 seconds. Check it first — it is the cause of roughly half of all running toilets.
Buying the wrong size rubber seal [flapper valve]. Seals are not universal. Take the old one to the shop to match it. A seal that is slightly too small or too large will not seal properly.
Chain too tight or too loose. The rubber seal chain needs 1–2 links of slack. Too tight and the seal stays slightly open; too loose and it gets trapped under the seal during flushing.
Not turning off the shut-off valve [isolation valve] before removing the rubber seal [flapper valve]. The tank [cistern] will not stop refilling while the valve is open. Turn the slotted screw 90° across the pipe to shut it off first.
Stop and call a plumber if...
The shut-off valve [isolation valve] (the slotted screw on the pipe behind the toilet) will not close — you will need a plumber to fit a new valve before any other work can be done
Water is leaking from around the base of the toilet, not from the tank [cistern]
The tank will not stop filling even after adjusting the floating arm [float arm] all the way down — the water inlet valve [fill valve] itself may need replacing
The safety overflow pipe [overflow tube] outside the house is discharging water continuously — this is a legal requirement to fix promptly
Cost breakdown
Recommended starter kit
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What you just learned
You now understand how a toilet tank [cistern] works — the floating arm [float arm], the water inlet valve [fill valve], and the rubber seal [flapper valve]. You can diagnose a running toilet, tell the difference between a float problem and a seal problem, and fix both. These skills transfer directly to replacing a full water inlet valve assembly (ball-float valve [ballvalve]), fixing a slow-filling tank, and understanding your home's water system.
This unlocks:
⚠️ Watch out if you rent
Landlords are legally responsible for keeping toilets in working order. Report it in writing first. If they fail to act within a reasonable time (14 days for non-emergencies), contact your local council's Environmental Health department.
How long will this actually take me?
First time
1–1.5 hours
With experience
30 mins
Spend 5 minutes understanding the cistern parts before you start — it saves a lot of guessing.