A toilet constantly running is usually a worn flapper valve letting water leak from the cistern into the bowl, a float set too high so the fill valve never shuts off, or a faulty fill valve itself. Less often it is a cracked overflow tube. Most causes are cheap parts you can replace yourself, but a running toilet wastes water non-stop until it is fixed.
In This Guide
Why your toilet keeps running
A toilet constantly running means water is escaping somewhere it should not, either dripping from the cistern into the bowl or trickling out through the overflow. Lift the lid and watch what happens after a flush finishes. If the water level in the cistern keeps dropping and the fill valve keeps topping it up, or if water is trickling into the bowl even though nobody has flushed, you have found the fault, even if you have not found the cause yet.
Three parts cause almost every case: the flapper, the float, and the fill valve. All three are cheap, all three wear out with age, and all three are usually a five-minute job to check.
The flapper valve: the most common cause
The flapper sits at the bottom of the cistern and seals the outlet to the bowl. Every time it lifts and drops back down, it wears a little. Over months and years the rubber hardens, warps, or picks up a build-up of limescale along the seal, and it stops sitting flush. Once that happens, water seeps past it constantly, refilling itself in a loop that never truly stops.
Check the flapper
Turn off the water at the isolation valve, flush to empty the cistern, then lift the flapper out and run a finger round the seal. If it feels stiff, cracked, or coated in scale, that is your answer. Replacement flappers are sold to fit most cisterns and take a few minutes to swap.
We see this most weeks on older cisterns where nobody has touched the internals since the toilet was fitted. It is one of the simplest fixes in the house, and one of the most commonly ignored because the noise it makes is easy to tune out.
The float and fill valve
The float rises with the water level and tells the fill valve when to shut off. If the float is set too high, or has a small crack letting water in and making it sit lower than it should, the fill valve never gets the signal to stop. Water then trickles endlessly out through the overflow tube into the bowl.
The fill valve itself can also fail on its own, with worn internal seals that let water past even once the float has risen to the correct height. If adjusting the float arm downward does not stop the running, the valve itself is the more likely culprit.
Water running when nothing is in use
A cistern that refills itself every few minutes with nobody near the bathroom is losing water around the clock. It adds nothing to your bill directly since most Scottish homes pay for water through council tax rather than a meter, per mygov.scot, but the waste itself, and any resulting damp around a leaking seal, is still worth sorting quickly.
- Flapper: hardened, warped or scaled rubber seal at the base of the cistern.
- Float: set too high, or waterlogged and sitting lower than it should.
- Fill valve: worn internal seals that let water pass even at the correct float height.
- Overflow tube: cracked or poorly seated, letting water bypass the shut-off entirely.
The dye tablet test
If you are not sure whether water is actually leaking from the cistern into the bowl, a dye tablet or a few drops of food colouring settles it. Drop it into the cistern, wait about fifteen minutes without flushing, then look in the bowl. Colour appearing in the bowl water confirms the flapper is not sealing.
Run the test properly
Add the dye to the cistern only, not the bowl. Do not flush during the wait. Check the bowl at the ten and fifteen minute marks. Any tint at all means water is passing the flapper seal, even if you cannot hear it running.
The overflow tube
Less common, but worth ruling out: the overflow tube inside the cistern is there to carry excess water safely into the bowl if the fill valve ever fails to shut off. If that tube has cracked or come loose from its fitting, water can bypass the normal shut-off point and run continuously even with a perfectly good flapper and float. It is usually a case of reseating or replacing the tube, which again is a straightforward parts swap rather than a plumber-only job.
Toilet fixed but still hearing water?
If you have replaced the flapper and float and water is still running somewhere in the house, it may not be the toilet at all. We can trace it without lifting a single tile.
Why a silent leak matters more than you think
A running toilet is one of the most common silent leaks in a home, and one of the easiest to shrug off because it does not flood anything. But water running constantly for weeks can soften flooring, damage a subfloor, or feed damp into a wall cavity if the cistern itself has a hairline crack rather than a worn seal. It is also, simply, waste that costs nothing to fix once you know which part has failed.
If your toilet is one of several signs pointing to a bigger problem, such as damp patches, a spinning meter, or unexplained damage elsewhere in the house, it is worth checking our wider guide to plumbing leak detection to see whether the toilet is the whole story or just the symptom you happened to notice first.
When it is not the toilet at all
Sometimes people fix every part in the cistern, the running stops for a day, then starts again, or they hear water moving in the walls even with the toilet isolated. That points to something else: a hidden pipe leak, a failing valve elsewhere in the system, or water hammer from a fault in the supply line. If you have already replaced the parts and the sound persists, it is worth having it checked properly rather than replacing parts on a guess.
Our guide on water hammer and why pipes knock covers one common cause of ongoing noise that has nothing to do with the cistern at all, and our page on finding your stopcock is useful if you need to isolate water quickly while you work out what is actually happening.
Frequently Asked Questions
Usually the flapper valve is not sealing properly, letting water leak from the cistern into the bowl, which triggers the fill valve to keep topping it up. A worn or scaled flapper is the most common cause and is cheap to replace.
Turn off the water, flush to empty the cistern, and check the flapper seal by hand. If it feels hard, warped, or coated in limescale, it is no longer sealing properly and needs replacing. A dye tablet test also confirms it.
Drop a dye tablet or food colouring into the cistern, wait about fifteen minutes without flushing, then check the bowl. Colour in the bowl water confirms the flapper is letting water pass even if you cannot hear it running.
Most Scottish households pay for water through council tax rather than a meter, per mygov.scot, so a running toilet usually will not increase your bill directly. It is still wasted water and worth fixing promptly.
In most cases, yes. Flappers, floats and fill valves are inexpensive parts sold to fit standard cisterns, and swapping them is usually a five to ten minute job once the water supply is isolated.
If the running continues after new parts are fitted, the fault may not be the toilet at all. It could be a hidden pipe leak or a valve issue elsewhere in the supply, which is worth having traced properly.
No, it is not urgent in the way a burst pipe is, but it should not be left indefinitely. Constant running wastes water and, if the cistern itself is cracked, can eventually damage flooring or feed damp into a wall.
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