How to Fix a Running Toilet

A running toilet is almost always caused by one of three parts: the flapper, fill valve, or float. The flapper is the rubber seal at the tank bottom—when worn, water leaks into the bowl continuously. The fill valve refills the tank after flushing—when faulty, it runs constantly or cycles on and off. The float tells the fill valve when to stop—if misadjusted or stuck, the valve won't shut off. Diagnose by lifting the tank lid and watching what happens.

The flapper is the most common culprit and easiest fix. Turn off the water supply valve behind the toilet. Flush to empty the tank. Unhook the old flapper from the overflow tube and disconnect the chain from the flush lever. Take the old flapper to the hardware store to match the size. Install the new flapper by hooking it onto the overflow tube pegs and connecting the chain with about 1/2 inch of slack. Turn water back on and test. Total cost: $5-15.

If the flapper is fine but water keeps running, check the fill valve. First, try adjusting the float: for ball floats, bend the arm down slightly; for cup floats, squeeze the clip and slide the float down the shaft. The water level should be about 1 inch below the overflow tube top. If adjustment doesn't work, replace the fill valve: shut off water, flush, sponge out remaining water, disconnect the supply line, unscrew the old valve, and install the new one following package directions. Cost: $10-25.

After any repair, test thoroughly. Turn on the water supply and let the tank fill completely. Listen for running water—you should hear the fill valve shut off cleanly. Wait 5 minutes and check if the water level has dropped (indicating a leaky flapper). Try a flush and confirm the tank refills properly and stops. Add a few drops of food coloring to the tank; if color appears in the bowl without flushing, the flapper still leaks.

Call a plumber if: you've replaced parts but the toilet still runs, you see cracks in the tank or bowl, the toilet is very old and parts are hard to find, water is leaking onto the floor, or you're uncomfortable working with plumbing. A running toilet wastes 200+ gallons per day—if DIY fixes don't work quickly, professional repair pays for itself in water savings. Plumbers can also identify less common issues like a warped flush valve seat.