How to fix a toilet that keeps running

By DIY Plumbing Guide Editorial Team · Published 20 October 2026

Why toilets run on, how to diagnose the fill valve, flush valve or overflow, and step-by-step fixes for lever, push-button and dual-flush cisterns.

A toilet that keeps running, or refills every so often without being flushed, wastes water and money if you are on a meter. The cause is almost always inside the cistern, and you can usually diagnose it in a few minutes. This guide covers the most common faults and how to fix them.

How a cistern works?

A cistern has two main parts:

a fill valve (inlet valve), which lets water in and shuts off when a float rises to the right level;
a flush valve (or older siphon), which releases water into the pan when you flush, then reseals.

Modern cisterns usually have an internal overflow that drains into the pan, which is why a fault often shows as a constant trickle into the bowl rather than water outside.

Step 1: Work out which part is at fault

Take the cistern lid off carefully (it is heavy and breakable). On push-button cisterns, you may need to unscrew the button first.
Look at the water level. If it is up to or above the overflow, or the top of the overflow tube, the fill valve is not shutting off.
If the level is below the overflow but water still trickles into the pan, the flush valve seal is leaking.

A quick test: put a few drops of food colouring in the cistern and do not flush for half an hour. If colour appears in the pan, water is leaking past the flush valve.

Fixing the fill valve

Adjust the float: on most modern valves, a screw or clip lowers the float so the valve shuts off earlier. Set the water level below the overflow, usually marked inside the cistern.
Clean the valve: grit or limescale can stop it sealing. Turn off the water at the isolating valve on the supply pipe, take off the valve's cap and rinse the diaphragm or washer.
Replace the valve: if cleaning does not work, replacement fill valves are widely available and fit most cisterns. Check whether you need a bottom-entry or side-entry valve.

Fixing the flush valve

Turn off the water at the isolating valve and flush to empty the cistern.
Most push-button flush valves twist or unclip from their base. Lift it out.
Check the rubber seal (washer) at the bottom. If it is worn, misshapen or dirty, clean it or replace it with a matching seal.
Refit the valve, turn the water back on and test.

If the seal between the cistern and pan, or the valve base itself, is leaking, the cistern may need removing, which is a bigger job. Consider a plumber.

Older lever cisterns with siphons

If a lever flush has trouble flushing rather than running, the siphon's diaphragm (flap valve) may be worn. Replacing it involves removing the siphon from the cistern, so turn off the water and drain first.

Water coming out of an external overflow pipe

Older cisterns have an overflow pipe through the wall. Water dripping from it points to the fill valve not shutting off, so check and adjust the float as above. Overflow pipes outside can also come from loft tanks, so check where it leads.

When to call a plumber?

Leaks from the base of the toilet or between cistern and pan.
Cracked cistern or pan.
No isolating valve, and you are not comfortable turning off the main supply.

Frequently asked questions

How much water can a leaking toilet waste?

Leaking toilets are a significant source of waste in homes; on a water meter, it can noticeably raise bills. Some water companies offer free checks or devices.

Is the dual-flush button the problem?

Sometimes the button mechanism sticks and holds the valve open. Check it moves freely.

Sources and further reading

Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.