A running toilet is the single most common — and most expensive — silent leak in the average American home. The EPA estimates a faulty flapper alone can waste 200 gallons a day, adding $20–$30 to your monthly water bill. The good news: 9 out of 10 running toilets are fixed with a $5 part and 20 minutes of work. No plumber required.

Step 1: Confirm it's actually running

Open the tank lid and listen. If you hear a faint hiss or see water trickling into the bowl from the rim or the overflow tube, you have a leak. If the bowl water level keeps dropping and the tank refills every few minutes without anyone flushing, that's a "phantom flush" — same problem, different symptom.

Quick dye test: Drop 5 drops of food coloring into the tank. Wait 15 minutes without flushing. If color appears in the bowl, your flapper is leaking. This is the #1 cause and the easiest fix.

Step 2: Inspect the flapper (fixes ~60% of cases)

The flapper is the rubber disc at the bottom of the tank that lifts when you flush. Over time it warps, cracks, or gets coated with mineral deposits.

1. Turn off the water supply at the shut-off valve behind the toilet.
2. Flush to empty the tank.
3. Unhook the flapper chain and slide the flapper off the overflow tube ears.
4. Take the old flapper to the hardware store for an exact match — universal flappers are a coin flip.
5. Install the new one, reattach the chain with about ½ inch of slack, and turn the water back on.

Step 3: Adjust the chain length

Too tight and the flapper can't seat fully — water keeps trickling. Too loose and it slips under the flapper, holding it open. Aim for ½ inch of slack when the flapper is closed.

Step 4: Check the float height

The float controls when the fill valve shuts off. If it's set too high, water spills into the overflow tube continuously.

• Ball float: bend the metal arm gently downward.
• Cup float (most modern toilets): pinch the spring clip on the side of the fill valve and slide the cup down about ½ inch.

The water level should sit ~1 inch below the top of the overflow tube.

Step 5: Replace the fill valve

If the tank fills, then immediately starts hissing again, the fill valve is shot. A Fluidmaster 400A is the universal replacement, runs about $12, and installs in 15 minutes with a single adjustable wrench.

Step 6: Clean the flush valve seat

Mineral deposits on the rim where the flapper seats prevent a watertight seal. Shut off water, drain the tank, and gently scrub the seat with a Scotch-Brite pad — never sandpaper or steel wool, which will gouge the plastic.

Step 7: Check the flush handle and lever

A sticky handle or a bent lift arm can hold the flapper partially open. Wiggle the handle — if it feels gummy, unscrew the retaining nut inside the tank (it's reverse-threaded), clean the threads, and reinstall.

When you've done all 7 and it's still running

You're looking at a cracked overflow tube, a hairline crack in the tank itself, or a failed flush valve assembly. At that point, replacing the entire flush valve (or the whole toilet, if it's older than 15 years) is more cost-effective than another patch.