Stop a Running Toilet

Materials

• 1 Universal flapper, fits most 2-inch flush valves (Take the old flapper to the store if you want an exact match instead of universal) — $5-$8
• 1 Fill valve assembly, adjustable height (Only needed if cleaning doesn't fix a stuck-open valve—buy Fluidmaster 400A or equivalent) — $12-$20
• 1 Supply line, braided stainless 12-inch (Replace if the old one is kinked or corroded—measure from shut-off valve to tank inlet) — $6-$10

Step by step

1. Diagnosis · 2 minutes

   Watch the Leak Reveal Itself

   Lift the tank lid and set it somewhere safe—porcelain breaks easily. Flush once and watch what happens. The flapper should drop and seal completely. The water should refill to about an inch below the overflow tube. The fill valve should shut off with a definite click. If water keeps running into the overflow tube, the fill valve isn't stopping. If the flapper doesn't seal, you'll see ripples in the bowl.

   Tip: Add two drops of food coloring to the tank and wait ten minutes without flushing—if color appears in the bowl, the flapper is leaking.
2. Repair · 5 minutes

   Replace the Worn Seal

   The flapper is the rubber seal at the bottom of the tank. Feel it—if it's slimy, pitted, or warped, it's done. Unhook it from the posts on either side of the flush valve and take it to the hardware store for a match. Universal flappers fit most toilets, but exact replacements seal better. Snap the new one in place and reconnect the chain with about half an inch of slack when the flapper is closed.

   Tip: Chlorine tablets in the tank destroy flappers in six months—use in-bowl cleaners instead.
3. Fine-tuning · 2 minutes

   Get the Slack Right

   Too much slack and the flapper closes before the tank empties, causing weak flushes and constant topping off. Too tight and the flapper can't seal. The chain should have just enough slack to let the flapper rest flat but lift immediately when you press the handle. Move the chain clip up or down on the lift arm until you get it right. Cut off excess chain so it doesn't catch under the flapper.

   Tip: If the chain keeps tangling, twist it a few times before attaching—it hangs straighter.
4. Adjustment · 3 minutes

   Lower the Water Line

   If water runs into the overflow tube, the fill valve is set too high. Look for the adjustment mechanism—usually a screw on top or a clip on the side. Turn the screw counterclockwise or pinch the clip and slide the float down. The water line should stop about an inch below the top of the overflow tube. Flush and verify the fill valve shuts off cleanly.

   Tip: Some fill valves have a thin refill tube clipped to the overflow—make sure it's positioned inside the tube, not outside.
5. Deep clean · 8 minutes

   Clear the Jammed Valve

   Sediment can jam the fill valve open. Turn off the water at the shut-off valve behind the toilet and flush to empty the tank. Unscrew the cap on top of the fill valve and lift out the float assembly. You'll see a small rubber seal or diaphragm—rinse it under the sink and wipe out any grit from the valve body. Reassemble, turn the water back on, and test.

   Tip: If you see black rubber crumbs, the valve internals are degrading and the whole unit needs replacement.
6. Replacement · 15 minutes

   Swap Out the Bad Valve

   Fill valves cost $12 to $20 and install in fifteen minutes. Turn off the water and flush. Disconnect the supply line under the tank with an adjustable wrench. Unscrew the locknut holding the old valve and lift it out. Drop in the new valve, tighten the locknut hand-tight plus a quarter turn, and reconnect the supply line. Adjust the height, turn the water on, and set the fill level.

   Tip: Fluidmaster and Korky valves are quiet and reliable—avoid the cheapest option, which uses louder mechanisms.
7. Verification · 5 minutes

   Confirm the Fix Holds

   Flush three times in a row and watch the entire cycle each time. The flapper should seal immediately, the fill should stop at the right level, and there should be no hissing after the tank refills. Check again in an hour. If everything stays quiet, the job is done. If the problem returns, you missed something—go back to step one.

   Tip: Put a piece of tape on the water line in the tank—if it moves overnight, you still have a slow leak.