Why Does My Toilet Keep Running and How Do I Fix It?

Root cause

The flapper is the rubber seal at the bottom of the tank. Over time it warps, hardens, or gets coated with mineral buildup and no longer creates a watertight seal — so water leaks continuously from the tank into the bowl.

30-second check

Add a few drops of food coloring to the tank. Wait 10–15 minutes without flushing. If color appears in the bowl, the flapper is leaking.

Tools & materials

Replacement flapper matching your toilet brand ($5–10), sponge or towel.

Step-by-step

1. 1 Shut off the water at the valve behind the toilet and flush to empty the tank.
2. 2 Unhook the old flapper from the pegs on the overflow tube and detach the chain from the flush lever.
3. 3 Snap the new flapper onto the same pegs and reconnect the chain with a little slack.
4. 4 Turn the water back on, let the tank fill, and repeat the food-coloring test to confirm.

Root cause

The chain connects the flush lever to the flapper. Too long and it slips under the flapper and blocks the seal; too short and it keeps the flapper propped open. Either way, water keeps running.

30-second check

Lift the tank lid and watch a flush. If the chain droops under the closed flapper, it's too long. If the flapper doesn't fully drop, it's too short.

Tools & materials

Needle-nose pliers (optional).

Step-by-step

1. 1 Unhook the chain and re-hook it at a link that leaves about a half-inch of slack when the flapper is closed.
2. 2 Trim excess chain with pliers if it's long enough to catch under the flapper.
3. 3 Flush a few times to confirm the flapper opens fully and closes cleanly.

Root cause

The fill valve refills the tank after a flush and shuts off when the float reaches the set level. When it wears out or gets clogged with sediment, it keeps running past the shut-off point.

30-second check

Lift the float arm manually while the tank fills. If the water stops, the float needs adjusting. If it keeps running, the fill valve itself is failing.

Tools & materials

Universal fill valve kit ($10–20), adjustable wrench, sponge.

Step-by-step

1. 1 Shut off the water and flush to empty the tank, then sponge out remaining water.
2. 2 Disconnect the water supply line and unscrew the fill valve lock nut under the tank.
3. 3 Lift out the old valve, drop in the new one, and tighten the lock nut hand-tight plus a quarter turn.
4. 4 Reconnect the supply line, turn the water on, and adjust the float so water stops about an inch below the overflow tube.

Root cause

If the float is set too high, water rises above the overflow tube and drains continuously into the bowl — sounding like the toilet is running.

30-second check

Look inside the tank. If water is trickling into the top of the overflow tube (the tall open pipe in the middle), the float is set too high.

Tools & materials

Screwdriver (for older ball-float assemblies).

Step-by-step

1. 1 For a cup-style float, pinch the clip on the float rod and slide the float down about a half-inch.
2. 2 For a ball-and-arm float, turn the adjustment screw on top of the fill valve clockwise, or gently bend the float arm downward.
3. 3 Flush and watch — water should stop about an inch below the top of the overflow tube.