Toilet Keeps Running: How to Find the Cause in Five Minutes and Fix It Yourself

A toilet that keeps running, or that refills by itself every so often with nobody near it, is wasting water around the clock. The EPA's WaterSense program calls toilets one of the most common sources of household leaks, and a running toilet can waste hundreds of gallons a day, which shows up on the water bill before anything else does. The good news is that the cause is almost always one of three inexpensive parts inside the tank, none of them require a plumber, and the whole diagnosis takes five minutes with the tank lid off.

This page covers how to tell which part is at fault, the fixes in order from free to twenty dollars, and the handful of situations where the problem is outside the tank and the plumber gets the call. Before you start, know where the toilet's shutoff valve is, the small oval handle on the wall or floor behind the bowl; if it is stuck or missing, our main shutoff guide is the backup.

Find the Cause

1. Take the lid off and watch. Set it somewhere it cannot fall; they crack. Look at the water level in the tank. There is a tall open tube in the middle, the overflow tube. The water should sit about an inch below its top. If water is trickling into the overflow tube, the level is too high, which is cause two below. If the level is fine, move on.
2. Do the dye test. Put a few drops of food coloring or a dye tablet in the tank water and do not flush. Wait 15 to 30 minutes. Color in the bowl means water is leaking past the flapper, the rubber flap at the bottom of the tank, which is cause one. A toilet that runs for a few seconds on its own every so often, the "phantom flush," is the same leak: the tank slowly drains past the flapper until the float drops far enough to trigger a refill.
3. Watch a flush. Push the handle and watch the flapper lift and drop. If it does not seal because the chain is too short and holds it up, or too long and gets trapped under it, that is cause three. If the fill valve, the tall assembly on the left, keeps hissing or running long after the tank is full, that is cause four.

Most Likely Causes and Fixes

1. A worn flapper (the dye test shows color). Rubber flappers warp, harden, and grow mineral deposits, and after a few years they stop sealing against the flush valve seat. Fix: turn off the shutoff valve, flush to empty the tank, unhook the old flapper from the two pegs on the overflow tube and from the chain, and hook on a new one. Take the old flapper to the store or check the toilet model; most take a standard 2-inch flapper, newer high-efficiency toilets often take a 3-inch. Wipe the seat it rests on with a cloth first; grit on the seat defeats a new flapper. Ten minutes.
2. The water level is too high (water running into the overflow tube). The float is set so the fill valve never fully closes, and water spills into the overflow tube and down into the bowl continuously. Fix: lower the float. On a modern fill valve, there is a screw or a clip on the float that adjusts its height; on an old ballcock with a ball on an arm, bend the arm down slightly. Aim for about an inch below the top of the overflow tube, or the fill line marked inside the tank. Flush and check. Five minutes.
3. The chain is the wrong length or tangled. A chain with no slack holds the flapper slightly open; a chain with too much slack falls under the flapper and props it up. Fix: adjust the chain so it has about a half inch of slack when the flapper is closed, and clip off the excess so it cannot get trapped. Two minutes.
4. The fill valve is failing (hissing or running with the level correct). The valve that refills the tank wears out, gets clogged with grit, or loses its seal, and either runs constantly or refills erratically. Fix: a new fill valve. Turn off the shutoff, flush and sponge the tank dry, disconnect the supply line under the tank, unscrew the locknut, lift out the old valve, set the new one to the right height, tighten, reconnect, and set the float. Twenty minutes with an adjustable wrench. The pick below is the one most plumbers reach for.
5. The refill tube is pushed down into the overflow tube. The small rubber tube from the fill valve should clip to the top of the overflow tube and spray in from above. If it is shoved down inside it, it can siphon water out of the tank and trigger refills. Fix: pull it up and clip it at the rim. One minute.
6. A corroded or cracked flush valve seat. If a new flapper still leaks and the seat it rests on feels rough or pitted, the flush valve itself needs replacing, which means removing the tank from the bowl. Doable, but a bigger job; this is where many owners call the plumber.

When to Call a Plumber

• The shutoff valve behind the toilet will not turn or leaks when you turn it. Do not force it; close the main and have the valve replaced.
• Water on the floor around the base of the toilet, or a toilet that rocks. That is the wax ring or the flange, not the tank, and it needs the toilet pulled.
• A crack in the tank or the bowl. Replacement, not repair.
• The tank-to-bowl bolts or gasket leak when you replace the flush valve, and tightening does not stop it.
• The toilet still runs after a new flapper, a correct water level, and a new fill valve. Something unusual is going on, and the plumber will find it faster.

Prevent It

Do the dye test once a year, or whenever the water bill jumps. Skip in-tank drop-in cleaner tablets, which eat rubber flappers and are the most common reason a flapper fails early. Replace the flapper every few years as routine. If you have several toilets, a leak sensor behind each one catches the failure that is worse than a running toilet: a supply line that lets go. The first-30-days guide has you find every toilet shutoff and test it the week you move in.

Frequently Asked Questions

How much water does a running toilet waste?

A silent flapper leak wastes dozens of gallons a day; a toilet running continuously can waste hundreds. WaterSense estimates household leaks overall average nearly 10,000 gallons a year per home, with toilets among the leading sources. It is usually the most expensive leak in the house that nobody can see.

Why does my toilet run for a few seconds every hour?

That is the phantom flush: a slow flapper leak lets the tank level drop until the fill valve kicks on to top it up. Do the dye test; the answer is almost always a new flapper.

Can I use any flapper?

Flappers come in 2-inch and 3-inch sizes and several shapes. Match the size of the flush valve opening, and when in doubt take the old one to the store or look up the toilet model number, which is usually stamped inside the tank on the back wall.

Do I need to turn off the water to replace a flapper?

Turn off the shutoff and flush so you are not working in a full tank. If the shutoff is stuck, you can hold the flapper closed by hand while you unhook it, but a stuck shutoff is its own problem worth fixing while the plumber is in the house.

The Bottom Line

Lid off, check the water level against the overflow tube, dye test for the flapper, watch a flush for the chain and the fill valve. New flapper, lower the float, fix the chain, or swap the fill valve, in that order of likelihood, and every one is a same-day job with parts that cost less than one month of the wasted water. Water on the floor, a stuck shutoff, or a crack is the plumber's.

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