How do I fix a running toilet?

A toilet runs when water keeps moving from the tank into the bowl after the flush should be done. Inside the tank, two parts do the work. The flapper seals the drain at the bottom. The fill valve refills the tank and shuts off when the float rises to its set level. When either part fails, the toilet either runs without stopping or cycles on and off on its own.

To find the cause, take the tank lid off and watch one full cycle. Press the handle, let the tank empty and refill, then look. Water spilling into the overflow tube points to a float set too high or a stuck fill valve. Water trickling down into the bowl with the tank already full points to the flapper. A hissing sound that never stops usually means the fill valve never reaches its shutoff point.

Two quick checks narrow it fast. First, press the flapper down with a finger or a stick. If the running stops, the flapper or its seat is the problem. Second, look at the water line. It should sit about one inch below the top of the overflow tube. Water at or above the tube top means the level is too high and is draining off through the overflow.

This page covers the diagnosis and the common fixes. For the full step-by-step on swapping out a flapper or fill valve, see our guide on how to replace a toilet flapper and fill valve.

Most running toilets trace back to the flapper. Fluidmaster, a leading maker of toilet repair parts, states plainly that "flappers are the leading cause of leaking or running toilets." The flapper is a flexible rubber or silicone disc. Each flush lifts it; gravity and water pressure should drop it back into a watertight seal. Over a few years the rubber hardens, warps, or grows a film of mineral scale that holds it open a crack.

That crack is all it takes. Water seeps past the seal into the bowl, the tank level falls, and the fill valve clicks on to top it back up. Sometimes the leak is so slow you never hear the bowl fill, only the periodic refill. That is the phantom flush, also called a ghost flush, where the toilet seems to flush itself with no one in the room.

Hard water makes this worse, and Phoenix water is hard. City of Phoenix water runs roughly 10 to 17 grains per gallon, near the top of the "hard" range. Calcium and lime build up on the flapper and on the flush valve seat it presses against. The scale keeps the seal from closing fully and chews through cheap flappers faster. Drop-in bleach tablets speed the damage even more, so skip them.

To test a suspect flapper, shut off the supply valve, flush to drain the tank, and feel the rubber. If it is stiff, cracked, or coated with gritty scale, replace it. Smear a little black residue from the flapper onto a paper towel; a flapper that sheds black marks has broken down and is leaking. Wipe the flush valve seat clean so the new flapper meets smooth rubber. In hard-water areas a chlorine-resistant flapper, such as Fluidmaster's hard-water model, holds up longer than a standard one. When you buy a replacement, match the flush valve size, since flappers come in two-inch and three-inch versions and the wrong size will not seal.

When a new flapper does not fix it, work through the rest of the tank.

The fill valve refills the tank and is supposed to shut off when the float reaches its set point. A worn fill valve never fully closes, so it keeps feeding water that runs out the overflow tube. You hear a steady hiss or trickle. Fill valves are not repaired; they are swapped for a new unit, a job most homeowners can do in under half an hour.

The float sets how high the tank fills. Set it too high and water rises above the overflow tube and pours down the drain forever, since the fill valve never sees the tank as "full." On a column-style valve, pinch the clip and slide the float down to lower the level. On an older ball-and-arm float, turn the adjustment screw or gently bend the arm down. Aim for a water line about one inch below the top of the overflow tube.

The chain links the handle to the flapper. Too short or tangled, it holds the flapper open a hair so water leaks past. Too long, and it slips under the flapper and props it open. Set the chain with about a half inch of slack when the flapper is seated, enough to lift cleanly but not so much that it snags. Clear any kink and make sure the chain is not caught beneath the flapper.

If you swap the flapper and fill valve and the toilet still runs oddly, or several fixtures act up together, the problem may sit in the drain venting rather than the tank. Plumbing vents keep trap pressure within one inch of water column under the International Plumbing Code, and a blocked vent can cause strange flushing behavior. That is a job for a plumber.

A toilet that refills on its own with no one touching it is leaking through the flapper. The seal lets water dribble into the bowl until the tank drops enough to trigger the fill valve, which runs for a few seconds and shuts off. The cycle repeats every several minutes to a few hours. The leak is often too slow to see or hear, which is why people think the toilet is flushing itself.

The dye test from EPA WaterSense confirms it. Put a few drops of food coloring or a dye tablet into the tank, not the bowl. Wait 10 minutes without flushing. If color shows up in the bowl, water is leaking past the flapper and you have a confirmed leak. EPA puts the threshold plainly: color in the bowl after 10 minutes means a leak. Flush right after so the dye does not stain.

The dye test is the fastest way to catch a silent leak before it runs up a water bill. It also confirms a repair worked. After you replace the flapper, run the test again. A clean bowl after 10 minutes means the seal is good. If color still bleeds through, the flush valve seat itself may be pitted or scaled, and the whole valve, not just the flapper, may need attention.

These leaks waste real water. EPA WaterSense reports that household leaks waste nearly one trillion gallons across the country every year, and the average home loses more than 9,300 gallons annually. About 9 percent of homes leak 50 gallons a day or more. Toilets account for about 30 percent of a home's indoor water use, so a running toilet is one of the costliest leaks to ignore, especially where water is scarce. A federal toilet uses 1.6 gallons per flush and a WaterSense model 1.28, so a constant trickle can quietly waste many times a single flush every hour.

Most running-toilet repairs are within reach for a homeowner with basic tools. Work in this order.

• Confirm the leak. Run the EPA dye test. Color in the bowl after 10 minutes means a leak you can chase down.
• Check the float and water level. If water spills into the overflow tube, lower the float so the line sits about an inch below the tube top. This costs nothing and fixes many cases.
• Inspect the chain. Free any tangle and set about a half inch of slack so the flapper seats flat.
• Replace the flapper. Shut the supply valve, drain the tank, swap the flapper, and wipe the valve seat. Use a hard-water flapper in Phoenix.
• Replace the fill valve. If a hiss continues after a new flapper, change the fill valve. Set the new valve cap about three inches above the overflow tube.

Retest with dye after each step. Many toilets are fixed by the float and flapper alone.

Call a plumber when the basics do not hold. Signs you are past a DIY fix: the toilet still runs after a new flapper and fill valve, the supply or shutoff valve leaks or will not turn, the tank or bowl is cracked, or several fixtures gurgle and drain poorly together, which points to a vent or drain-line problem rather than the tank. In Phoenix, where hard-water scale wears parts fast, a plumber can also clean a scaled flush valve seat and check whether repeated failures point to a larger water-quality issue.