Listen to where the water is going
A toilet that keeps running often makes a faint trickling sound long after the flush has finished. Look into the pan without flushing again. A steady ribbon of clean water down the back of the bowl usually means the flush valve is not sealing properly or the cistern is overfilling and sending water through the overflow route. Older cisterns may discharge an overflow outside, while many modern toilets overflow internally into the pan. The sound can be intermittent if the cistern slowly loses water and then the inlet valve opens again to top it up.
Two faults account for many cases
The first common fault is a fill valve that does not shut off at the correct water level. A sticking float, worn diaphragm or scale can allow the cistern to rise too high. The second is a flush valve seal that no longer sits flat, allowing water to escape from the cistern into the pan. Debris on the seal can be enough to cause a slow leak. On concealed cisterns the same principles apply, but access is through the flush plate or service opening rather than by lifting a lid.
Simple observations can save guesswork
Remove the cistern lid only if it lifts easily and there are no concealed fixings you might damage. Watch what happens as the cistern fills. If the water level climbs until it reaches the overflow, the inlet side is the likely problem. If the level stops normally but gradually falls, the flush valve is probably leaking. Adding dye to the cistern is sometimes suggested, but plain observation is usually enough. Avoid bending plastic float arms or adjusting unfamiliar valves at random; modern mechanisms can be less forgiving than old brass ball valves.
Why constant running should not be ignored
The water may be clean, but it is still being wasted continuously. A slow internal leak can pass many litres through the toilet without leaving a puddle on the floor, so it is easy to underestimate. The inlet valve also cycles repeatedly as it replaces lost water, which can make the fault noisier at night. In hard-water areas, scale can build up around moving parts and make a marginal valve worse. The problem is rarely an emergency, but it is one of those small repairs that makes sense to deal with rather than leave for months.
What the repair normally involves
A plumber will identify whether the fill valve or flush valve is responsible, then decide whether the mechanism can be serviced or is better replaced. Sometimes a new seal is all that is required. In other cases the entire valve is inexpensive and replacement is more reliable than dismantling an old scaled mechanism. Concealed cisterns can take longer because access is restricted and replacement parts must fit the existing frame or valve body. The plumber should test several flushes before leaving, not simply wait for the cistern to fill once.
What to mention before booking
Tell the plumber whether the cistern is exposed, close-coupled to the pan or concealed behind a wall or panel. Say whether you can see water running into the bowl and whether the cistern makes repeated filling noises. A photograph of the toilet and flush controls helps identify the type. If the flush button is sticking or difficult to press, mention that as well because it may be part of the same problem. For concealed systems, any visible brand name on the flush plate can be useful when replacement components need to be sourced.
Check the toilet again before the job is finished
A running toilet can appear fixed during the first refill and then start trickling again several minutes later. Ask for the cistern to be flushed a few times and left long enough to prove that the water level remains stable. If the old mechanism is heavily scaled, replacing one seal may expose wear elsewhere, so it is reasonable to ask whether the plumber expects the rest of the valve to remain serviceable. On a concealed cistern, keep a note of the replacement part or model if one is fitted; that information can save time in future. Also check that the flush button sits correctly and that any access panel closes without strain. A successful repair should stop both the visible trickle into the pan and the repeated top-up noise from the inlet valve.
This page is general guidance about toilet keeps running; the cause in an individual home still needs to be established from the actual installation.