How to fix a running toilet
Quick answer: Take the lid off the cistern and see where the water is going. If it leaks into the bowl without flushing, the flush mechanism's washer or siphon is worn. If water keeps flowing in and out of the overflow, the fill valve (float valve) is faulty or set too high. Replace the part, which is cheap.
A running toilet wastes water and costs money if you are on a meter. The sound of a trickle or a cistern refilling on its own every few minutes tells you something inside is worn.
There are two main culprits: the flush mechanism that lets water into the bowl, and the fill valve that tops up the cistern.
What you will need
- Food colouring or a dye tablet
- Adjustable spanner
- Bucket and sponge
- Replacement parts: a siphon diaphragm washer or a universal fill valve (check your cistern's make)
- A cloth or towel
Step by step
Do the dye test
Take the cistern lid off and drop a few drops of food colouring in the water. Do not flush. Wait 15 minutes. If colour appears in the bowl, water is leaking from the cistern into the pan through the flush mechanism.
Look at the water level
Check the water level against the line marked inside the cistern. If it is above the overflow pipe or water is trickling into the overflow, the fill valve is not shutting off.
Turn the water off
Turn the isolation valve on the supply pipe to the cistern a quarter turn with a screwdriver. Flush to empty the cistern and mop up the remaining water with a sponge.
Adjust a float that is set too high
If the float is a ball on an arm, bend the arm down a little or turn the adjuster so it shuts the water off lower. On other types turn the adjuster screw. Turn the water on and see.
Replace a worn siphon washer
On a dual-flush siphon, you may need to undo the nut under the cistern and the flush linkage, lift the siphon out and replace the rubber diaphragm washer at the bottom. Fit the identical size.
Replace the fill valve
If adjusting does not cure it, replace the valve. Undo the nut on the supply pipe and the nut under the cistern, take the old valve out, fit a universal one at the right height and reconnect. Use a UK-approved fill valve.
Turn on and check
Turn the water back on slowly and let it fill. Flush a few times, check the level stops at the mark, and look for leaks around the connections.
Tips
- Take the old part to a plumbers' merchant. Some siphons are brand specific.
- Replace the flush washer and the fill valve together on an old cistern.
- A constantly running overflow pipe outside the house must be fixed promptly. Water suppliers can take action about wasted water.
Stop and get help if
- You cannot turn the water off to the toilet.
- There is a leak from the base of the toilet or the pan connection, or a crack in the china.
- The cistern is a concealed or wall-hung one with no easy access. Ask a plumber.
- The new part still does not stop it running.
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Common questions
Why does my toilet keep refilling?
A worn flush washer lets water into the bowl, so the cistern refills. A faulty fill valve or a float set too high will also cause the cistern to keep running.
How much water can a running toilet waste?
A lot over a year: a steady trickle can use many litres a day. On a water meter it adds to your bill.
Is it OK to fix a toilet cistern myself?
Yes, as long as you can turn the water off at an isolation valve and use only approved parts. Call a plumber for anything involving pipework or a concealed cistern.
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Last reviewed 17 October 2026. This guide is general information for UK homes and is not a substitute for the manufacturer's instructions. If the job involves gas, mains wiring, asbestos, structural work or working at height, or you are unsure, ask a suitably qualified person.