Toilet keeps running: causes and fixes

Lift the lid. The answer is usually right there.

The answer in 2 lines. Turn off the small isolating valve on the pipe feeding the cistern, then lift the lid. If the water rises up to the overflow and runs away, the fill valve (the float valve) isn't shutting off: adjust or replace it. If the level stays below the overflow but water trickles into the bowl, the flush valve seal at the bottom is leaking: clean or replace it. Both are often DIY jobs. A cracked cistern, water on the floor that won't stop, a leaking isolating valve or a concealed cistern you can't reach: call a plumber.

The first 3 steps

Turn off the small isolating valve on the pipe feeding the cistern (a screw slot or lever: turn it so it sits across the pipe). No valve? Use the stop tap.
Take the lid off and look: does the water rise above the top of the overflow pipe, or stay below it?
Flush to empty the cistern and take a photo of the parts and any brand name: it helps you buy the right replacement.

Stop: danger

Macerator toilet (a box with a motor behind the toilet): switch it off at the plug or switch before touching anything, and don't open it yourself: it is electrical.
Water on the floor near a socket or electric heater: touch nothing wet. Switch off at the consumer unit only if it is dry and you can reach it without standing in water; otherwise leave the room and call 999 if there is any danger.

What not to do

Dropping the ceramic lid: it is heavy and breaks easily. Lay it on a towel on the floor. A cracked cistern can't be repaired.
Forcing a stuck isolating valve with pliers: if it snaps, turn the water off at the stop tap.

Where the water goes: two faults, two signs

The water rises to the overflow pipe and runs into it, the cistern hisses or never stops filling: the fill valve doesn't close. It may be set too high, scaled up, or worn. Often DIY.
The level stays below the overflow but a thin trickle runs into the bowl, and the dye test colours the bowl: the flush valve seal (on push-button cisterns) isn't sealing: limescale, a worn washer, or a cable or rod catching. Often DIY.
The toilet seems to refill by itself now and then: a slow leak at the flush valve empties the cistern a little, then it tops up. Same fix.
Water drips from a small pipe on the outside wall: older cisterns have an external overflow. It means the fill valve isn't shutting off.
Water on the floor at the base or under the cistern: the seal between cistern and pan, the supply connection, or a cracked cistern. The connection is often DIY; a cracked cistern needs a plumber.

Older lever-flush cisterns with a siphon rarely leak into the bowl: if one keeps running, look first at the fill valve.

What you can do yourself, and where to stop

Fill valve: adjust the float (a screw, a clip or a bendable arm, depending on the model) so the water stops about 2 to 3 cm below the top of the overflow. If it still runs, clean off the scale, or replace the whole fill valve. How to choose: same entry (side entry or bottom entry, depending on where the pipe comes in), same thread size, and look for a WRAS-approved product (search the WRAS approvals directory by brand or product name).
Flush valve: water off and cistern empty, clean the washer and the seat it presses on, then replace the washer if it is misshapen or hard. Take the old one to the shop, or note the brand and model of the valve.
A tired mechanism: a complete flush valve can be replaced with the water off, following the maker's instructions.
Stop there if the cistern is cracked, a floor leak won't stop, the isolating valve leaks or won't close, the cistern is concealed behind a panel or in the wall and you're not comfortable, or it's a macerator. Call a plumber.

Finding a plumber, and who pays

Plumber: WaterSafe lists approved plumbers by postcode (WaterSafe — Finding a plumber); they are qualified to work to the water fittings regulations. Ask for a price before they start.
If you rent: in England, toilets and their installations are the landlord's responsibility to keep in repair (Landlord and Tenant Act 1985, section 11). Report the fault in writing and let them arrange the repair, unless your tenancy agreement says otherwise.
On a water meter: a running toilet shows up on your bill. Leak allowances from water companies usually cover leaks on the underground supply pipe, not a running toilet: the faster it's fixed, the less you pay.
Water reaching the flat below: tell your home insurer as soon as you can.

Good to know

Do the dye test twice a year, and after any work on the water supply.
In hard-water areas, clean the flush valve washer once a year.
If you are on a meter, read it every month: a rise for no reason is a warning sign.

The figures (latest published)

Yorkshire Water warned in 2025 that a leaky loo could waste twice a household's average daily water use. It is a water company estimate, not an official statistic.
We found no national figure for 2026: the 2025 estimate above is the latest we could verify.

In Leicester, Ben hears water trickling at night. He turns off the isolating valve and lifts the lid: the water is running into the overflow. He lowers the float one notch and turns the water back on: the cistern now stops filling 2 cm below the overflow. The next day, the dye test leaves the bowl clear. The noise has gone.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on 1 November 2026, for England. Confidence: high for the diagnosis and the DIY steps (WaterSafe); high for the landlord's duty under section 11; medium for the water-waste figure, a 2025 estimate from a water company, not an official statistic; medium for leak allowances, which differ between water companies. This guide describes common cases, not a diagnosis.