How a toilet tank works
When you flush, the flapper at the bottom of the tank lifts and lets water rush into the bowl. Then the flapper drops and seals, and the fill valve refills the tank until a float tells it to stop. A running toilet means one of those steps isn’t finishing.
Cause 1: The flapper doesn’t seal
Over time the rubber flapper warps, hardens or collects mineral buildup, so water slowly leaks into the bowl and the tank keeps refilling.
Tip: Try the food coloring test: put a few drops of food coloring in the tank and don’t flush. If color shows up in the bowl within 15 to 30 minutes, the flapper is leaking.
Cause 2: The water level is too high
If the water in the tank rises above the top of the overflow tube, it spills down the tube into the bowl nonstop. Lowering the float so the water stops about an inch below the top of the tube usually fixes it.
Cause 3: The fill valve is worn
If you hear hissing, or the tank keeps topping itself up, the fill valve may not be shutting off fully. Fill valves are inexpensive and quick to replace.
Cause 4: The chain is the wrong length
A chain that’s too short holds the flapper open slightly. One that’s too long can get caught under the flapper. There should be just a little slack when the flapper is closed.
When to call a plumber
- The tank or bowl is cracked.
- Water is leaking at the base of the toilet.
- You replaced the flapper and fill valve and it still runs.
- The shutoff valve behind the toilet won’t turn or leaks.
- The toilet is decades old and clogs often.
Why it’s worth fixing quickly
A constantly running toilet adds up on your water bill, and high water pressure in the house can be part of the cause. Most fixes are quick and inexpensive.