Short definition

A running toilet is one whose tank water keeps flowing into the bowl after the flush completes — either continuously or in short refill cycles every few minutes. The fix is almost always inside the tank: a worn flapper, a chain that’s caught on something, or a fill valve that won’t shut off.

What it is

Lift the tank lid and you’ll see the diagnostic in plain sight. The cause is one of these, in order of frequency:

  1. Worn flapper. The rubber seal at the bottom of the tank no longer seats fully. Water seeps from tank to bowl; the fill valve refills the tank to make up for it; cycle repeats. Top cause across every plumbing book. Drop a few drops of food coloring into the tank, wait 15 to 30 minutes, and check the bowl — colored water in the bowl confirms a flapper leak. (Saving Water Partnership offers free dye tablets if you want the official kit.)
  2. Chain problem. Chain too tight: flapper can’t close. Chain too loose or kinked: flapper drops too soon or hangs up.
  3. Float set too high. Water reaches the overflow tube and trickles down constantly. Bend the float arm down or twist the float-cup adjustment.
  4. Worn fill valve. Valve won’t shut off cleanly; constant low-flow refill. Replace the fill valve assembly.
  5. Cracked or corroded flush-valve seat. Even a perfect flapper can’t seal a damaged seat. Less common; requires flush-valve replacement.

Why it matters to a homeowner

A running toilet is the single biggest household water-waster. Severe cases waste 200-plus gallons a day; typical ones, 20 to 50. WA water-and-sewer rates being what they are, that’s $50 to $300 a year — sometimes more — silently leaving your bill.

DIY parts run $5 to $30: $5 for a flapper, $10 to $20 for a fill valve, $15 to $40 for a full flush-valve assembly. A pro toilet repair runs $90 to $250. The repair itself is usually 5 to 15 minutes once you’ve identified the part — most homeowners can handle it with a sponge, a bucket, and an angle stop you can reach behind the toilet.

The fastest path: do the dye test first to confirm flapper vs. fill side, then go to the part that diagnostic points you to. Don’t replace everything at once.