Why Your Toilet Keeps Running

That constant hissing or trickling sound from your toilet isn't just annoying — a running toilet can waste a significant amount of water continuously. The good news: in most cases, the fix takes less than 30 minutes and costs under $20 in parts. You don't need a plumber for this.

There are three main culprits behind a running toilet:

  1. A faulty or worn flapper
  2. A misadjusted or broken float
  3. A faulty fill valve

What You'll Need

  • Rubber gloves
  • Replacement flapper (match to your toilet model if possible)
  • Adjustable pliers
  • Food coloring (for diagnosis)
  • Paper towels

Step 1: Diagnose the Problem

Remove the toilet tank lid and set it aside carefully — it's ceramic and fragile. Look inside. The water level should sit about 1 inch below the top of the overflow tube (the tall tube in the center). Now observe:

  • Water spilling into the overflow tube? Your float is set too high or the fill valve is faulty.
  • Water level normal but toilet still runs? The flapper is likely leaking.

Flapper leak test: Add a few drops of food coloring to the tank. Wait 10 minutes without flushing. If color appears in the bowl, your flapper isn't sealing properly.

Step 2: Replacing the Flapper

The flapper is the rubber seal at the bottom of the tank. It's the most common cause of a running toilet and the easiest fix.

  1. Turn off the water supply valve (the oval knob on the wall behind the toilet — turn clockwise).
  2. Flush the toilet to empty the tank.
  3. Unhook the flapper from the two pegs on the overflow tube and disconnect the chain from the flush handle arm.
  4. Take the old flapper to a hardware store to match the size, or buy a universal flapper.
  5. Hook the new flapper onto the overflow tube pegs, then attach the chain to the handle arm — leave about ½ inch of slack in the chain.
  6. Turn the water supply back on and let the tank fill.
  7. Flush and observe — the running should stop.

Step 3: Adjusting the Float

If water is running into the overflow tube, your float is positioned too high, causing the tank to overfill.

  • Ball-and-arm float (older style): Bend the metal or plastic arm downward slightly, or turn the adjustment screw at the fill valve to lower the water level.
  • Cup float (cylinder-style, more modern): Pinch the clip on the side of the fill valve and slide the float down until the water sits 1 inch below the overflow tube.

Step 4: Replacing the Fill Valve (If Needed)

If adjusting the float doesn't help and the toilet continues to run, the fill valve itself may be worn. Replacement fill valves are inexpensive and come with instructions. The process involves:

  1. Turning off the water supply and emptying the tank.
  2. Disconnecting the water supply line from the bottom of the tank.
  3. Removing the locknut holding the fill valve in place.
  4. Installing the new valve and adjusting its height per the instructions.
  5. Reconnecting the supply line and turning the water back on.

When to Call a Plumber

If you've replaced the flapper and fill valve and the toilet still runs, there may be a crack in the overflow tube or a problem with the flush valve seat — the part the flapper rests on. These repairs are still doable as a DIY project, but if you're uncomfortable proceeding, a plumber can typically resolve it in one short visit.

The Takeaway

A running toilet is one of the most fixable household problems. Replace the flapper first — it solves the problem the majority of the time. Keep the old parts until you've confirmed the fix is working, and always turn off the water supply before starting any work inside the tank.