The toilet keeps running — the water won't stop
A quietly leaking toilet can waste hundreds of litres a day without you noticing — and most people live with it for months. The part that stops it is cheap, and the repair takes no experience at all.
The problem
You hear water running minutes after the flush finishes, or continuously. Or you see a thin trickle running down the inside of the bowl. Or the cistern refills itself in short bursts every so often ('phantom flushing'). Or you have to press twice to get a proper flush.
Likely causes
A worn flapper / flush valve seal
The number one cause by a wide margin. The rubber seal at the bottom of the cistern hardens and scales up over time, stops sealing properly, and lets water trickle from the cistern into the bowl continuously. The part costs very little.
The float is set too high
If the water level is above the overflow tube (the open vertical pipe in the middle), water pours into it constantly. Nothing is broken — it just needs adjusting. Dropping the float a couple of centimetres fixes it.
A failed fill valve
The valve that refills the cistern no longer shuts off completely when the water reaches its level, so it keeps trickling. It's replaced as a whole unit and it's fairly easy.
A short or snagged chain
On models with a chain, if it's too short or caught under the flapper, the flapper can't close fully. The cheapest repair in existence: adjust the chain.
Limescale on the valve seat
In hard water, scale builds on the rim the flapper seals against. Even a new flapper can't seal on it. Scrub the seat smooth with an abrasive sponge (never metal).
Fix it — step by step
- Find the leak — the dye testTake the cistern lid off and put a few drops of food colouring (or some coffee) into the cistern — not the bowl. Don't flush. Wait 20-30 minutes.If colour appears in the bowl without flushing, the flapper is definitely leaking. It's the clearest test there is.
- Check the water levelIt should sit 2-3cm below the top of the overflow tube. If the water is touching or running into that tube, that's your problem — and it's an adjustment, not a replacement.There's usually a moulded line inside marked WL or Water Line. Set it to that.
- Lower the floatOn modern floats (the cup that slides on a rod) there's a screw or a plastic clip — turn or slide it to lower the shut-off point. On old ball-on-an-arm floats, gently bend the arm down or turn the adjusting screw.Drop it a centimetre, flush, look, repeat. Not all in one go.
- Check the chainIt needs a small amount of slack — about a centimetre. Too tight and it holds the flapper slightly open all the time; too loose and it snags underneath it.If there's excess chain dangling, clip it off or hook it up. It loves to get trapped under the flapper.
- Shut the water off and empty the cisternClose the small valve on the wall under the cistern. Flush to empty it. Sponge out what's left.If the valve is seized, don't force it. Close the house main instead.
- Inspect and replace the flapperThe flapper is the round rubber seal at the bottom of the cistern. Pull it off (usually it just unclips, or hooks over two ears). Rub it with a finger — if it leaves a black slime, or feels hard and brittle, replace it.Take the old one to the shop. Sizes are near-standard but not identical, and matching by eye doesn't work.
- Clean the valve seatThe rim the flapper closes onto must be perfectly smooth. Scrub it with an abrasive sponge or a cloth and vinegar. Run a finger round it — it should feel smooth, with no gritty scale.Never use wire wool or sandpaper — the scratches make the leak permanent.
- Reassemble, turn the water on, and re-run the dye testFit the new flapper, open the valve slowly, let it fill. Then repeat the dye test: colouring in, wait half an hour. The bowl should stay clear.If it still leaks with a new flapper and a clean seat, the flush valve body itself is worn → replace the whole assembly or call a plumber.
Tools & parts you'll need
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When to call a professional
Call a plumber if: water is leaking from the base of the toilet onto the floor (that's the wax ring underneath — it means lifting the whole pan, a much bigger job); the cistern itself is cracked or weeping; the isolation valve is seized and won't close; you've replaced both the flapper and the fill valve and it still leaks; or the bowl backs up when you flush (a drain blockage — an entirely different problem).
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How much water does a running toilet actually waste?
A leak you can barely hear can pass hundreds of litres a day — the figure genuinely shocks people. And it isn't just noise; it's water you paid for going into the sewer. The part that stops it costs less than a meal.
What is 'phantom flushing'?
The cistern leaks very slowly into the bowl, so the level drops, so the fill valve kicks in for a few seconds to top it up, then shuts off. Fifteen minutes later it does it again. It's a certain sign of a leaking flapper — even if you never hear running water.
I have to press twice for a proper flush
Usually the water level in the cistern is too low (raise the float), or the chain has too much slack so the flapper doesn't lift far enough. Both are two-minute fixes.
The new flapper still leaks
Very likely the valve seat is scaled or scratched, so the new flapper can't seal against it. Scrub it with an abrasive (never metal) sponge and retest. If it's genuinely scored, the answer is a complete flush-valve replacement.
There's water on the floor around the toilet
That's a different problem entirely. Water at the base usually means the wax ring underneath has failed, and fixing it means lifting the pan. It can also just be condensation on the cistern in hot weather. If it's the seal, call a plumber — and treat water around the base as potentially contaminated.
Can I put a coloured cleaning tablet in the cistern?
Better not to. In-cistern chemical tablets attack the rubber flapper and seals and dramatically shorten their life. Clean the toilet from the top, not from inside the cistern.