Why does my toilet keep running after flushing?

Because water is escaping from the cistern faster than the fill valve can shut off, so the valve never reaches its stop point. In almost every case one of two components is at fault, and both are cheap and straightforward to replace.

The first is the flush valve seal at the bottom of the cistern — the rubber flapper or, in a modern dual-flush unit, the seal around the siphon or drop valve. Over years the rubber hardens, distorts or collects limescale, and it no longer seats cleanly. Water trickles continuously into the pan, the level drops, and the fill valve keeps topping it up. The classic test is to add a few drops of food colouring to the cistern and leave it for twenty minutes without flushing. If colour appears in the pan, the flush valve seal is leaking.

The second is the fill valve itself, which fails to close fully at the set level. Here water usually escapes down the overflow pipe instead, and you will see it running into the pan from the overflow rather than seeping from the flush valve. Adjusting the float down sometimes buys time, but a worn diaphragm needs the valve replaced.

A few simpler causes are worth checking first. A flush handle or button that sticks slightly can hold the valve open. A chain that is too short holds the flapper off its seat; too long and it tangles underneath. A float set too high sends water straight to the overflow.

This is worth fixing promptly rather than living with. A continuously running toilet can waste hundreds of litres a day, which shows up quickly on a metered water bill.