Toilet Keeps Running? Here's Why and How to Fix It
Reviewed by the Devon Leak Detection team. Last updated June 2026
The short answer
A toilet keeps running when something stops the cistern sealing properly, usually a worn flapper or flush valve, a faulty fill or float valve, or water escaping down the overflow. A dye test confirms a silent leak. Left alone, it can quietly waste hundreds of litres a day.
That faint hiss from the bathroom, the trickle into the pan that never quite stops, the cistern that refills itself in the middle of the night. A running toilet is one of the most common plumbing niggles in Devon homes, and one of the easiest to ignore. The trouble is that an unattended cistern can pour clean water into the pan day after day, and on a metered supply you pay for every litre of it. The good news is that most running toilets come down to one of a few inexpensive parts, and many can be sorted in an afternoon.
How a toilet cistern is meant to work
Lift the cistern lid and the mechanism is simpler than it looks. When you press the flush, a valve at the bottom of the tank opens and releases the stored water into the pan. As the tank empties, a float drops and opens the fill valve, letting fresh water in from the mains. The water rises, the float rises with it, and at the set level the fill valve closes again. A separate overflow pipe sits as a safety net: if the tank ever overfills, water escapes down the overflow rather than spilling onto the floor.
A toilet that keeps running is simply a cistern where one of those steps has failed. Either water is leaking out of the bottom faster than it should, or fresh water keeps flowing in and never switches off. Pinning down which one is happening is the key to a quick fix.
The most common causes of a running toilet
A worn flapper or flush valve seal
The flapper, or the rubber seal on a drop valve, is the part that lifts when you flush and drops back to plug the hole afterwards. Over the years it goes stiff and brittle, or limescale builds up on the seat it rests against. Either way the seal is no longer watertight, and a thin stream of water slips past it into the pan around the clock. This is the single most common reason a toilet trickles between flushes, and it is often the cause behind a leak you can hear but never quite see.
A faulty fill valve or float
If the fill valve fails to shut off cleanly, the tank keeps taking on water even after it is full. Sometimes the float is set too high, so the water rises above the correct level. Sometimes the valve itself is worn or gritty and never seats properly. In both cases you tend to hear a constant low hiss as mains water keeps creeping in. A float arm that has slipped, or a modern float cup set at the wrong height, can cause the same thing.
Water escaping down the overflow
When the fill valve will not close, the water level climbs until it spills over the top of the overflow pipe. On modern internal-overflow cisterns this runs straight into the pan, which looks exactly like a running toilet. On older homes you may instead see a steady drip from an overflow pipe poking out through an external wall. Either is a clear sign that the fill valve or float needs attention rather than the flush seal.
Signs you might notice:
- A faint hiss or trickle that carries on between flushes
- Ripples or a thin stream of water moving in the pan when nobody has flushed
- The cistern topping itself up on and off through the day or night
- A drip from an overflow pipe outside the house
- A metered water bill that has crept up with no obvious explanation
Confirm a silent leak with the dye test
Some leaks are too slow to hear but still waste plenty of water. The dye test is a simple way to catch them. Take off the cistern lid and add a few drops of food colouring to the water in the tank, then leave the toilet alone for ten to fifteen minutes without flushing. When you come back, check the pan. If colour has appeared in the bowl, water is creeping past the flush seal and you have a leak to fix. If the bowl stays clear, the flush valve is holding and any running is more likely down to the fill side.
The quicker the colour shows up, the bigger the leak. It is a five-minute check that tells you exactly where to look, and it saves you replacing parts that were never the problem.
How much water a running toilet wastes
A running toilet rarely feels urgent because the water disappears quietly down the pan. The numbers tell a different story. Water industry estimates suggest a leaking loo can waste somewhere around 200 to 400 litres a day, and the worst cases run far higher. The industry also reckons a meaningful share of UK toilets, in the region of one in twenty or more, are leaking to some degree without their owners realising.
If your home is on a water meter, that waste lands directly on your bill, and over a year a single running toilet can add a noticeable sum. If you would like a precise figure for your own supply, your meter readings or your account with South West Water will show the trend. The practical takeaway is the same either way: fixing a running toilet promptly usually pays for itself.
Fix it or replace it
Once the dye test points you at the culprit, the repair is usually straightforward. If the flush seal is leaking, turn off the supply at the isolating valve, flush to empty the tank, then lift out the old flapper or drop-valve seal. Give the seat it rests against a gentle clean to clear any limescale, fit the matching replacement, and check the chain or linkage has just a little slack so the seal can drop fully home. Turn the water back on and watch the level settle.
If the fill valve is the problem, start with the easy adjustment: lower the float so the water stops a couple of centimetres below the top of the overflow. If the valve still will not shut off, it is worn and a replacement is the reliable fix. Fill valves are inexpensive and most are designed to swap out without specialist tools. When you do replace a part, take the old one to the merchant or note the toilet make, as fittings vary between cisterns.
Most running toilets are happily within reach of a confident homeowner. Call in a professional if the cistern is cracked, the isolating valve is seized, the same fault returns soon after a repair, or you simply would rather have it done properly first time. And if your bill has climbed but the dye test comes back clear and every toilet looks fine, the water may be escaping somewhere you cannot see. That is the point to consider a proper leak investigation rather than swapping parts on guesswork.
Frequently Asked Questions
A leak that runs at night but seems fine by day is usually a slow one. With no flushing for hours, even a small drip past the flush seal becomes obvious. A dye test will confirm it. The fault is the same as a daytime leak, the quiet house simply makes it easier to hear.
A running toilet is rarely an emergency, but it is not worth leaving either. The water goes down the pan rather than onto your floor, so there is no immediate damage, yet it can quietly waste a great deal of water. Aim to look at it within a few days rather than living with it for months.
Yes, if your home is metered. Every litre that trickles into the pan is metered water you pay for, and a steady leak can add a meaningful amount over a year. If you are on a fixed unmetered charge it will not change your bill, but it still wastes treated water that is worth saving.
Use the dye test. Colour appearing in the pan points to the flush seal or flapper. If the bowl stays clear but you can hear a constant hiss, or the level keeps rising toward the overflow, the fill valve or float is the more likely cause. The test tells you which side to work on first.
Call someone in if the cistern is cracked, the isolating valve is seized, the fault keeps returning after repairs, or you would simply rather it was sorted properly. And if your bill has risen but every toilet checks out clear, the water may be escaping elsewhere, which is the time for a proper leak investigation.
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