Why does my toilet keep running — and how do I fix it?

Evidence grade: B1 — Trade practice

A running toilet is almost always one of three things: a flapper that no longer seals, a chain that is too tight or too tangled, or a fill valve that will not shut off. All three are fixable with basic tools — and the shutoff valve behind the toilet is your safety net. Learn where it is before you touch anything else.

Effort: Easy · under 1 hr · screwdriver or pliers, replacement flapper or fill valve, sponge

The 60-second diagnosis (lid off, water on)

Lift the tank lid and watch:

1. Water flowing into the overflow tube? The water level is set too high. Adjust the float so the water fills to about 1 inch below the top of the overflow tube — on most valves, slide the float clip down or turn the adjustment screw about a quarter turn counterclockwise.
2. Chain holding the flapper open? The chain should have about ½ inch of slack — enough that the flapper can close fully, not so much that excess chain falls under the flapper and wedges it open.
3. Flapper warped, cracked, or slimy? If the chain is fine, clean the flapper and the rim of the flush-valve seat where it rests. If the flapper is visibly deteriorated, it is done — replace it.
4. Tank refilling in fits, or not stopping at all? Suspect the fill valve. Sediment can keep it from closing; many valves let you pop the cap, hold an inverted cup over the opening, and pulse the supply on and off to flush debris. If that fails, the valve needs replacing.

Replacing the flapper

1. Turn off the shutoff valve behind the toilet (clockwise), then flush and hold the handle down until the tank is nearly empty.
2. Unhook the chain from the flush-lever arm and unclip the flapper ears from the overflow tube.
3. Clip the new flapper on and reattach the chain with about ½ inch of slack.
4. Turn the water back on, let the tank fill, and test-flush. The hissing should stop once the water reaches the fill line.

Stop rules — call a plumber NOW if

• Water is on the floor around the toilet's base. That is a base seal or supply problem, not a tank problem — different repair, different stakes.
• The tank itself is cracked or the supply line drips when you touch it. Ceramic under pressure and flexing braided lines are not DIY.
• The shutoff valve behind the toilet will not turn, or will not stop the water. Do not disassemble a toilet with live water. Close the house main instead, and if you do not know where your main shutoff is, that is itself the moment to get a plumber — a running tank wastes water, but a burst connection floods.
• The toilet is overflowing. Close the shutoff (or push the flapper down and hold it) and call — do not flush again.

A running toilet can waste an enormous amount of water quietly and inflate a water bill [1] — but every fix above starts at the same place: know where the shutoff is.

PPE and ventilation

No special PPE is required beyond what you would want for comfort. The tank holds clean supply water, not bowl water [1]; rubber gloves are optional. Ventilation is not a factor.

Honest limits

• This article covers standard flapper toilets. Dual-flush and tower/canister toilets use a gasket at the base of the tower rather than a flapper — the same diagnosis ladder applies, but the parts differ.
• Flapper designs vary widely by brand; if nothing in the store matches yours, a universal flapper or a call to the manufacturer usually resolves it.
• If a brand-new flapper and a clean valve seat still do not stop the running, the flush valve itself may be cracked or corroded — that repair requires detaching the tank and is usually worth handing to a plumber.

Sources

1. Southern Living — "How To Fix A Running Toilet Before It Wastes Water And Raises Your Bill" — https://www.southernliving.com/how-to-stop-a-toilet-from-running-12057112

Repair Lab · Release 1 · Evidence last checked September 30, 2026 · A project of AOJEL