Overview

A running toilet is one where water keeps flowing into the tank or bowl long after a flush should have finished, usually because a worn or misaligned part inside the tank isn't sealing properly. It's one of the most common minor plumbing complaints and is a great starter project for a beginner, since the fix almost always means adjusting or swapping an inexpensive tank part rather than doing any cutting or soldering.

Tools & materials

  • Adjustable wrench
  • Screwdriver (flathead and/or Phillips)
  • Rubber or nitrile gloves
  • Small bucket or towel (to catch drips)
  • Sponge or old towel for mopping up tank water
  • Replacement flapper (universal or model-matched)
  • Replacement fill valve, if needed
  • Replacement flush valve seal, if needed

Steps

  1. Remove the tank lid and set it somewhere padded so it doesn't chip; don't flush while it's off.
  2. Watch and listen for a minute: note whether water is trickling into the overflow tube, whether the flapper looks warped or isn't dropping flush against the seat, and whether the fill valve keeps hissing after the tank should be full.
  3. If water is spilling into the overflow tube, that usually means the water level is set too high or the float is stuck — locate the fill valve's float (a ball on an arm, or a cup that slides on the valve shaft) and lower it slightly, or turn the adjustment screw, so the water shuts off about an inch below the top of the overflow tube.
  4. Check the flapper (the rubber or plastic flap at the bottom of the tank that lifts when you flush): press it down gently while the water is running — if the running stops, the flapper is the culprit. Look for cracks, warping, or mineral buildup on its sealing edge.
  5. If the flapper is worn, note its size, turn off the water supply valve at the wall behind the toilet, flush to drain most of the tank, then unhook the flapper from the flush valve pegs and disconnect its chain from the flush lever. Attach a new flapper of the same size and reconnect the chain with a small amount of slack — too much or too little slack can also cause running.
  6. If the flapper looked fine but water was still hissing continuously, the fill valve itself may be worn out. With the water supply off and the tank drained, disconnect the supply line and the refill tube, unscrew the retaining nut underneath the tank, and lift out the old valve.
  7. Set the new fill valve to the correct height for your tank, drop it in, hand-tighten the locknut underneath, reconnect the water supply line, and clip the small refill tube to the overflow tube.
  8. Turn the water supply back on slowly, let the tank fill, and flush a couple of times while watching for leaks at every connection and confirming the water shuts off well below the overflow tube.
  9. If everything still runs after replacing the flapper and/or fill valve, double-check the flush lever's chain isn't snagged and that the tank-to-bowl gasket isn't leaking underneath.

Safety warnings

  • Turn off the shutoff valve behind or below the toilet before disconnecting any water line, and have a towel or small bucket ready to catch residual water.
  • Wear gloves when working inside the tank; tank water is used only for flushing, not clean, and edges of old plastic parts can be sharp.
  • Handle the porcelain tank lid carefully and set it on a padded surface — it's heavy, chips easily, and is often not sold separately if it cracks.
  • Don't overtighten plastic locknuts or fittings; plastic threads strip or crack easily and can cause a bigger leak than the one you started with.
  • If you shut off the toilet's local valve and it still leaks or won't fully stop the flow, use the home's main water shutoff instead rather than working with water still running.

When to call a pro

  • The tank or bowl itself is cracked, or the seal between the tank and bowl is leaking — this usually means replacing the toilet, not just an inside part.
  • The shutoff valve behind the toilet is corroded, won't turn, or leaks when you try to close it.
  • Water is leaking from the base of the toilet onto the floor, which can point to a failed wax ring or floor damage beyond a simple parts swap.
  • You've replaced the flapper and fill valve and the toilet still runs or refills erratically.
  • You notice water stains, soft flooring, or signs of a leak into the floor below the bathroom, which may involve subfloor or structural repair outside minor plumbing.

Cost estimate

$10–$30 for a replacement flapper and/or fill valve

Sources