Why does my toilet keep running?

Telling a leaking flush valve from an overfilling inlet, the dye test that identifies it in minutes, and what each fix involves.

Difficulty: beginner
Time: 1 hr
Read: 3 min
Safety: caution

Short answer

Put food colouring in the cistern and wait without flushing. Colour appearing in the pan means the flush valve or flapper is leaking. If the water level is at the overflow instead, the inlet valve is at fault.

A running toilet is one of two faults, and a few drops of food colouring tells you which in ten minutes.

Safety

Isolate the water supply before removing any part — at the isolating valve on the supply pipe if there is one, otherwise at the stopcock. Ceramic cisterns crack easily under leverage, so never force a fitting. A continuously running toilet can waste hundreds of litres a day and will show on a water meter.

What you'll need

Food colouring
An adjustable spanner
A replacement washer, flapper or valve as identified

Step by step

Do the dye test.A few drops of food colouring in the cistern. Do not flush. If colour appears in the pan within fifteen minutes, water is leaking past the flush valve.
Check the water level.It should sit around 20mm below the overflow. Water at or above the overflow means the inlet valve is not shutting off.
Adjust the float first.The simplest fix. Most modern valves have a screw or clip that lowers the shut-off level. If the level was just too high, this is the whole repair.
Clean the inlet valve.Grit and limescale on the valve seat stop it sealing. Isolate the water, dismantle the valve and clean the diaphragm and seat — this fixes many inlet faults.
Replace the inlet diaphragm.A cheap rubber washer inside the valve. It perishes over years and is the most common inlet failure. Take the old one to a merchant to match it.
Check the flush valve seal.For a leaking flush valve, the seal or flapper is usually perished or has grit under it. Lift it out, clean the seat, and inspect the rubber.
Replace the flush valve seal.Usually a simple rubber ring that lifts off. Replacing the seal is far cheaper and easier than the whole valve, and it fixes most flush valve leaks.
Check the flush button mechanism.On push-button cisterns, the button rod can stick partially depressed, holding the valve slightly open. Check it returns fully.
Look at limescale in hard water areas.Scale on valve seats and seals is the usual reason parts fail early. Descaling the components extends their life considerably.
Replace the whole valve if parts are unavailable.Older cisterns often have obsolete parts. A modern universal inlet or flush valve fits most cisterns and costs little.

Tips

The food colouring test identifies which valve is at fault in fifteen minutes and costs nothing.
Water at the overflow level means inlet valve. Water disappearing into the pan means flush valve.
Most repairs are a rubber washer, not a whole valve.

Common mistakes

Replacing the whole cistern mechanism without diagnosing — The fault is usually a single perished washer or grit on a seat. Replacing everything is expensive and often does not address a limescale problem that will recur.
Forcing a fitting on a ceramic cistern — Ceramic cracks under leverage and a cracked cistern is a replacement rather than a repair. Isolate the water and work gently.

If it doesn't work

Constant trickle into the pan

Cause: Flush valve seal perished or grit on the seat — Fix: Lift out the seal, clean the seat, and replace the rubber if it is hardened.

Water running into the overflow

Cause: Inlet valve not shutting off — Fix: Lower the float setting first. If that does not work, clean or replace the diaphragm.

Cistern fills very slowly

Cause: Partially blocked inlet or a nearly closed isolating valve — Fix: Check the isolating valve is fully open and clean the inlet filter.

Fault returns within months

Cause: Limescale in a hard water area — Fix: Descale the components when servicing, and expect to replace seals more often.

Noise but no visible leak

Cause: Inlet valve chattering — Fix: Partially closing the isolating valve reduces flow and often silences it.

Questions people ask

How much water does a running toilet waste?

A continuously leaking flush valve can waste several hundred litres a day. On a water meter it shows up quickly and it is one of the most cost-effective repairs in a house.

How do I know which valve is faulty?

Food colouring in the cistern. Colour appearing in the pan without flushing means the flush valve. Water at the overflow level means the inlet valve.

Do I need a plumber?

For a washer or a valve replacement, generally not — it is a straightforward job with the water isolated. If the cistern is cracked or the pipework needs altering, that is different.