What Usually Causes It
In many cases, a running toilet comes down to one of just a few very common issues inside the tank. The first is usually the flapper — the rubber piece at the bottom of the tank that lifts when you flush and then drops back down to seal the water in. If it doesn’t sit properly, or if it’s slightly worn or warped, water can keep leaking through even after the flush is over. Another common cause is the chain attached to it.
If that chain is too short, tangled, or caught awkwardly, it can stop the flapper from closing all the way. Then there’s the float, which helps control how high the tank fills with water. If the water level is too high, it can keep spilling into the overflow tube and make the toilet run continuously even when nothing looks obviously wrong. That’s why this fix is often easier than people expect. Because once you remove the tank lid and look inside, the problem is usually visible within a minute or two.
And more often than not, it only needs a small adjustment — not a full repair.