Why Is My Toilet Running? 4 Common Causes Explained

A toilet that runs constantly or intermittently may seem like a minor annoyance, but make no mistake — it is one of the most wasteful plumbing problems a homeowner can have. A running toilet can silently waste between 200 and 700 gallons of water per day, depending on the severity of the leak. For a Cobb County or Fulton County homeowner, that translates to an extra $50–$200 added to a single water bill. Understanding exactly why a toilet runs helps you either fix it yourself or know what to explain to a plumber.

How a Toilet Actually Works

Before diagnosing a running toilet, it helps to understand the simple mechanism inside that tank behind you. Every gravity-flush toilet operates on the same basic principle:

1. You flush. The flush handle raises a flapper (a rubber or plastic seal at the bottom of the tank) via a chain.
2. With the flapper raised, the stored water rushes down through the flush valve opening into the bowl, creating the flush.
3. As the tank empties, a float (either a ball float on an arm or a cup float on the fill valve) drops with the water level.
4. The dropping float opens the fill valve, which refills the tank with fresh water.
5. As the water rises, the float rises with it. When it reaches the set level, it signals the fill valve to shut off — and the cycle is complete.

A running toilet almost always means one of three things: the flapper isn’t sealing, the float is set too high, or the fill valve is failing.

Cause 1: A Worn or Warped Flapper

The flapper is the most common cause of a running toilet by far. Flappers are made of rubber, and rubber degrades over time — especially in Metro Atlanta where municipal water contains chloramines (a disinfectant combination of chlorine and ammonia used by Atlanta’s water treatment plants). Chloramines aggressively attack rubber components. Most plumbers recommend replacing toilet flappers every 3–5 years in Atlanta simply due to water chemistry, regardless of whether the toilet is running.

A simple test: lift the tank lid and press down firmly on the flapper with your finger while the toilet is running. If the running sound stops immediately, the flapper is the culprit and has a bad seal. Replacement flappers cost about $5 at any hardware store and take less than five minutes to swap out.

Cause 2: The Float Is Set Too High

If the float is adjusted to a level that causes water to fill above the open top of the overflow tube (the tall open tube in the center of the tank), water will constantly drain down that tube into the bowl — creating a continuous running sound. You’ll know this is the issue if you look in the tank and see water trickling down into the overflow tube.

The fix is adjusting the float height. On older ball-float systems (with a ball on a metal arm), you can bend the arm slightly downward. On modern cup-float systems, there’s an adjustment screw or clip on the fill valve stem. The water level in the tank should sit about an inch below the top of the overflow tube.

Cause 3: A Failing Fill Valve

Fill valves wear out over time. When they do, they may allow a slow, continuous trickle of water into the tank even when the float is at the correct height. You can confirm this by putting a few drops of food coloring in the tank and waiting 15–20 minutes without flushing. If colored water appears in the bowl without any flushing, water is getting through somewhere — usually a bad flapper or fill valve. Fill valves are inexpensive ($10–$20) and relatively straightforward to replace.

Cause 4: A Cracked Flush Valve Seat

The flush valve seat is the smooth opening at the bottom of the tank that the flapper presses against to seal. If the seat develops a crack, warp, or mineral deposit along its edge, the flapper cannot form a complete seal no matter how new it is. Running your finger around the rim of the flush valve seat and feeling for roughness or deposits can diagnose this. Mineral deposits can sometimes be sanded smooth; a cracked seat requires replacing the entire flush valve assembly.

The “Ghost Flush” — Intermittent Running

Has your toilet ever started running on its own with nobody in the room? This phenomenon is called a ghost flush or phantom flush. It happens when a slow flapper leak gradually causes the tank water level to drop below the trip point of the fill valve, which then kicks on briefly to refill. It can happen every 10 minutes or every few hours. It’s almost always caused by a flapper issue and is very common in older Marietta and Kennesaw homes.