How to Fix a Running Toilet

Identifying the Problem

A running toilet has water continuously flowing from the tank into the bowl. This can be caused by three components: a leaking flapper, a faulty fill valve, or an improperly adjusted float. To diagnose, remove the tank lid and observe the water level. If water is flowing over the top of the overflow tube, the float or fill valve needs adjustment. If the water level is normal but you hear running, the flapper is not sealing properly. A simple dye test (drop food coloring in the tank and wait 15 minutes) confirms a leaking flapper if color appears in the bowl.

Replacing the Flapper

The flapper is the rubber seal at the bottom of the tank that lifts when you flush and drops back down to seal the tank. Over time, flappers warp, crack, or develop mineral deposits that prevent a watertight seal. Turn off the water supply, flush to empty the tank, and unhook the old flapper from the overflow tube ears. Take the old flapper to the hardware store for matching — flappers are not universal. Install the new flapper by hooking it onto the overflow tube and connecting the chain to the flush lever with about 1/2 inch of slack.

Adjusting the Float

The float controls the water level in the tank. If the water level is too high (above the overflow tube), the fill valve never fully shuts off. For ball-and-arm floats, bend the arm downward slightly or turn the adjustment screw to lower the float. For column-style fill valves, squeeze the adjustment clip on the float and slide it down the column. The correct water level is about 1 inch below the top of the overflow tube — there is usually a mark on the inside of the tank or on the overflow tube indicating the proper level.

Replacing the Fill Valve

If adjusting the float does not stop the running, the fill valve itself may be worn out. Turn off the water, flush and sponge out remaining water, disconnect the supply line, and unscrew the mounting nut under the tank. Remove the old fill valve and install the new one, adjusting its height so the overflow tube on the fill valve is at least 1 inch above the tank's overflow tube. Tighten the mounting nut, reconnect the supply line, and adjust the float to set the correct water level. Universal fill valves fit most toilets and cost under ten dollars.