Toilet Keeps Running: How to Fix It
A toilet that never stops running is usually one of two faults: water leaking past the flush valve into the pan, or a fill valve that won't shut off. Telling them apart takes two minutes and points straight at the fix.
A continuously running toilet wastes a surprising amount of water and is one of the most common repairs a plumber is called to. The noise comes from water constantly moving through the cistern, and the cause is almost always a failed valve — either the outlet (flush) valve letting water leak into the pan, or the inlet (fill) valve failing to close.
This guide shows how to work out which of the two it is with a simple test, then how to fix the common causes: worn flush-valve seals, a failed fill valve, an incorrectly set water level spilling into the overflow, and debris holding a valve open. It also covers the water-regulations angle on overflows and warning pipes.
Key takeaways
- Two main causes: water leaking past the flush valve into the pan, or a fill valve that won't shut off.
- The dye test — a few drops of food colouring in the cistern — quickly shows if the flush valve is leaking into the pan.
- A fill valve that keeps running usually needs the water level reset or the valve washer/cartridge replaced.
- Water escaping via the overflow or warning pipe signals a fill valve not shutting off, or a set level that is too high.
- Modern flush valves and fill valves are cheap and quick to swap — often faster and more reliable than rebuilding old parts.
Diagnose it: flush valve or fill valve?
First work out which valve is at fault. Put a few drops of food colouring into the cistern water and wait without flushing. If colour appears in the pan after a few minutes, water is leaking past the flush (outlet) valve seal — the cistern slowly empties into the pan and the fill valve keeps topping it up, so it runs continuously.
If the pan stays clear but the cistern keeps filling and you can hear or see water going into the overflow, the fill (inlet) valve is not shutting off, or the water level is set too high. Those are the two families of fault, and each has a distinct fix.
The dye test settles it
Colour in the pan means a leaking flush valve. Water going to the overflow means a fill valve fault. Two minutes with food colouring saves guessing.
Fixing a leaking flush valve
A flush valve leaks when its seal or flapper perishes, or when grit sits on the seat and holds it fractionally open. Isolate the supply, flush to empty the cistern, and inspect the flush-valve seal or diaphragm washer. Clean the seat and replace the seal; if the valve body itself is worn or brittle, fit a new flush valve rather than nursing an old one.
On older syphon-type cisterns the diaphragm washer inside the syphon perishes and the toilet flushes weakly or runs — replacing the diaphragm cures it. On modern drop (flapper) valves the whole cartridge is usually swapped as a unit.
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Fixing a fill valve that won't stop
If the fill valve keeps running, first check the water level — it should sit around 20 to 25 mm below the overflow. Many valves adjust with a screw or clip; lowering the level so it stops before reaching the overflow is often all that's needed. If the valve dribbles even at the correct level, the washer or cartridge has failed.
Isolate the supply and replace the fill-valve washer or, more reliably, the whole fill valve — modern bottom- or side-entry valves are inexpensive and quick to fit. Check the inlet for debris while you are there, as grit lodging on the seat is a common cause of a valve that won't seal.
Overflows, warning pipes and the regulations
A cistern that overfills should not simply pour water into the pan indefinitely; the internal overflow on modern valves is designed to discharge into the pan as a warning that the fill valve has failed, and older installations use an external warning (overflow) pipe. Either way, a running overflow is a sign to fix the fill valve, not to ignore.
Under the Water Supply (Water Fittings) Regulations 1999, WCs must not waste, misuse or contaminate water, and a continuously running toilet is exactly the kind of waste the regulations exist to prevent. Fitting compliant fill and flush valves and setting the level correctly keeps the installation both efficient and compliant.
A running toilet wastes water
The Water Regulations require fittings not to waste water. A toilet that runs constantly can waste hundreds of litres a day — fixing it is a compliance point as well as a comfort one.
From guidance to action
Frequently asked questions
How do I tell if the flush valve or the fill valve is causing it?
Do the dye test: add a few drops of food colouring to the cistern and wait without flushing. If colour seeps into the pan, the flush valve is leaking. If the pan stays clear but water runs to the overflow, the fill valve is failing to shut off.
What level should the water in the cistern sit at?
Generally around 20 to 25 mm below the overflow, or the marked water line on the fill valve. Setting it too high sends water to the overflow; setting it correctly lets the fill valve close and stops the running.
Is a constantly running toilet a water-regulations issue?
Yes. The Water Supply (Water Fittings) Regulations 1999 require fittings not to waste water, and a toilet running continuously wastes a significant volume. Repairing the failed valve and setting the level correctly restores both efficiency and compliance.
Related guides
How to Fix a Dripping Tap
Cure a dripping tap: tell washer taps from ceramic-cartridge taps, isolate safely, replace the worn part, and fix leaks at the spout or base.
The Water Regulations 1999 Explained
Fluid categories, backflow prevention and notification — the Water Fittings Regs in plain terms.
No Hot Water: Causes and Fixes
Diagnose no hot water by system type — combi diverter valves, cylinders, immersion elements, thermostats and airlocks — and put the fault right.
