Why Does My Toilet Keep Running?

A toilet that keeps running after you flush, or runs intermittently on its own, almost always comes down to one of two parts: the flapper isn't sealing, or the fill valve isn't shutting off completely. Both are inexpensive to identify and fix once you know which one it is.

The flapper: the most common cause

The flapper is the rubber seal at the bottom of the tank that lifts when you flush and should drop back down to seal the tank once it's done draining. Over time, it warps, cracks, or develops mineral buildup that prevents a full seal, letting water slowly leak into the bowl, which triggers the fill valve to keep topping off the tank. This shows up as a toilet that runs briefly every few minutes even when no one's used it.

The fill valve: the second most common cause

The fill valve controls how much water refills the tank after a flush and should shut off completely once the tank reaches the right level. A worn or misadjusted fill valve can let water trickle continuously, which usually sounds like a faint, constant hiss rather than the more distinct running-and-stopping pattern caused by a failing flapper.

A simple way to test which one it is

When to call a plumber instead of fixing it yourself

Both parts are widely available and replaceable as a DIY project for many homeowners. It's worth calling for toilet repair if the issue persists after replacing the obvious part, if you're not comfortable working inside the tank, or if the toilet is also showing other issues, like rocking at the base or recurring clogs, alongside the running problem.

Less common causes worth ruling out

If replacing both the flapper and fill valve doesn't fully resolve the running, a few less common causes are worth checking: a fill tube that's positioned incorrectly and siphoning water into the overflow tube, a cracked overflow tube itself, or, less often, a hairline crack in the tank that's allowing a slow, hard-to-notice leak. These are less frequent than a simple flapper or fill valve failure, but they're worth ruling out before assuming a repair didn't work.

Why this is worth fixing promptly, not just eventually

A running toilet is easy to deprioritize since it doesn't cause visible damage the way a leak or clog does, but the ongoing water waste adds up steadily every day it continues. Fixing it promptly, rather than letting it run for months, is one of the more straightforward ways to lower a water bill without any change in household habits.

Older toilets versus newer low-flow models

An older toilet with a standard-volume tank tends to waste more water per running episode than a modern low-flow model, simply because there's more water available to leak past a failing flapper. That said, low-flow toilets aren't immune to the same flapper and fill valve issues, and some early low-flow designs have their own reputation for flapper seal problems, so a running toilet is worth diagnosing the same way regardless of the toilet's age or efficiency rating.

A quick maintenance habit that prevents most running-toilet calls

Periodically lifting the tank lid to visually check that the flapper looks intact, sits flat against the valve seat, and that the fill valve shuts off cleanly once the tank reaches its fill line takes less than a minute and can catch early wear before it turns into a constantly running toilet. It's a simple habit, but one most homeowners only start after dealing with a running toilet at least once.