If your toilet keeps running after you’ve flushed it, it’s usually a sign that something inside the cistern isn’t working as it should.

It’s a common issue and often more of an annoyance than anything serious, but it can waste a surprising amount of water if it’s left unresolved.

In most cases, the problem comes down to one of a few simple components inside the cistern.

What’s Actually Happening

When a toilet is working properly, water fills the cistern, stops at the correct level, and stays there until the next flush.

If it keeps running, it usually means water is continuously flowing from the cistern into the bowl.

That can happen if the cistern isn’t sealing properly, or if it keeps refilling when it shouldn’t.

Common Causes

There are a few typical reasons this happens.

Worn or Faulty Flush Valve

Inside the cistern, the flush valve (or seal) controls the release of water when you flush.

Over time, this seal can wear out or become slightly misaligned. When that happens, water can slowly leak into the pan even when the toilet hasn’t been flushed.

Water Level Set Too High

If the water level in the cistern is set too high, it can spill into the overflow.

This creates a constant trickle of water into the bowl, which often looks like the toilet is still running

Fill Valve Not Shutting Off Properly

The fill valve controls how the cistern refills after a flush.

If it doesn’t shut off correctly, the cistern will keep taking in water, which then escapes into the overflow.

Limescale or Debris

In some cases, a build-up of limescale or debris can affect how the internal parts move.

This can stop valves from sealing properly or prevent them from closing fully.

General Wear Over Time

Like any plumbing component, parts inside a toilet cistern don’t last forever.

Seals, valves and fittings can gradually wear out, especially in older toilets, and eventually need replacing.

Simple Checks You Can Make

If you’re comfortable lifting the lid of the cistern, it’s often possible to see what’s going on.

Things to look for:

• Is water constantly flowing into the overflow?

• Is the water level sitting too high?

• Does anything look misaligned or not sealing properly?

Sometimes a small adjustment to the float or repositioning a seal can stop the issue.

When It’s More Than a Simple Fix

If the toilet keeps running after you’ve checked it, it’s usually a case of replacing one of the internal parts.

It’s not normally a big job, but if the cistern is concealed (hidden behind a wall or unit), access can be more limited and the repair can be a bit more involved.

Leaving it can lead to ongoing water waste, so it’s worth getting it sorted sooner rather than later.

Final Thoughts

A running toilet is usually caused by a small issue, but it’s worth sorting early.

Left alone, it can quietly waste a lot of water over time.

In most cases, it comes down to a worn seal, a valve not shutting properly, or the water level being set too high.