Running toilets drive me nuts. The good news is this is usually a cheap, 20-minute fix.

Quick answer Most running toilets have a worn flapper or a water level set too high. Lift the tank lid: if water is spilling into the overflow tube, lower the float. If not, replace the flapper (bring the old one to the store so you match it).
How to Fix a Running Toilet
Photo: Tech. Sgt. Terrica Jones / Wikimedia Commons, Public domain
Tools & materials

  • Replacement flapper matched to your toilet (or a fill valve if that’s the problem)
  • Adjustable wrench
  • Sponge and towel
  • Pencil

Steps

  1. Take the tank lid off and set it somewhere it can’t fall. Watch what happens after a flush.
  2. Check the water level. When the tank finishes filling, the water should sit about 1/2 to 1 inch below the top of the overflow tube. If it’s running into the tube, lower the float: on a ball float, turn the adjusting screw on top of the fill valve (usually clockwise); on a cup float, squeeze the clip and slide the float down.
  3. Check the chain. It should have just a little slack when the flapper is closed. Too tight holds the flapper open; too long can get trapped under it.
  4. Test the flapper. Mark the water line with a pencil, shut off the supply valve behind the toilet, and wait 25 minutes or more. If the level drops, water is leaking past the flapper.
  5. Replace the flapper. Shut off the water, flush to drain the tank, unhook the chain and slip the old flapper off its ears. Wipe mineral grit off the valve seat, fit the new one, and reattach the chain with a little slack.
  6. Turn the water back on, let it fill, and flush a few times to check the seal and the level.
  7. Still filling nonstop? Lift the float by hand while it’s filling. If the water doesn’t stop, the fill valve is worn. Replace it following the instructions on the new valve’s package.
Safety: Turn the supply valve clockwise to close it. If that valve leaks or won’t turn, shut off the house main instead of forcing it.

Good to know

A flapper is a cheap part that wears out over time. Some water utilities suggest replacing it about every five years, before it starts leaking.