A toilet runs when water streams or leaks from the toilet tank into the bowl. A constantly running toilet is not only annoying but it also wastes many gallons of water every day.

A running toilet can be the result of water leaking through a poorly fitting or defective flapper or tank ball that fails to seal the flush valve between the tank and bowl, or it can be cause by water leaking into the overflow tube. For a clear understanding of what these parts are, see How a Toilet Works & Toilet Plumbing Diagrams.

The simplest and most common remedy for a toilet that runs is to jiggle the flush handle. This is often enough to free up a flapper or float ball to properly stop the movement of water from tank to bowl.

When water refills the toilet tank after a flush, it should stop before it reaches the top of the overflow tube. Otherwise, it will continually spill down inside the tube to the bowl. To stop this type of running, you must make an adjustment that prevents the water from rising that high.

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If you have a somewhat old float ball flushing mechanism, try lifting the float above water level. If water stops running, bend the float arm (the wire that holds the float ball) slightly downward so that the float ball will reach its shut-off level sooner as the water in the tank rises. The water should shut off automatically when its level is about 1/2 inch below the top of the overflow tube.

Note that the float ball may have leaked and filled up with water so it can’t float properly. To determine whether this has happened, turn off the water supply valve and flush the toilet to empty the tank. Then unscrew the ball from the float arm. You should be able to tell if it contains water by feeling its weight and shaking it. If it has water inside, replace it with a new one. Or, better still, replace the entire valve mechanism with a newer type of valve that doesn’t rely on a float ball.

Whether your toilet utilizes a rubber flapper or a tank ball to seal the flush valve between the tank and the bowl will depend on the age and type of toilet you have; tank balls used to be the standard, but flappers are more common in newer toilets.

If you have the older type of toilet with a tank ball, the mechanism that holds the tank ball may have become misaligned. Reach in and wiggle the tank ball to make sure it is free to move up and down directly over the flush valve. If it doesn’t move freely through its holder, try polishing it with some steel wool. Be sure the lift wires are properly connected. If this doesn’t do the trick, replace the tank ball mechanism with a flapper.

Replacement flappers made for all types of toilets are available for under $10. Flappers are particularly simple to install after you remove the old flapper or tank ball—you just slide the collar down the overflow tube, centering the flapper over the valve seat, and hook up the lift chain to the trip lever so that it has a little slack. Instructions are given on most flapper product labels.

If a lift chain connects the flapper to the trip lever, check it. This often becomes kinked or snagged so that it doesn’t allow the flapper or tank ball to seat properly. If this has happened, adjust it. Also check the two rubber ears on the collar of the flapper to make sure they hinge properly on the overflow tube.

Before you actually replace a flapper or make other types of repairs inside the tank, turn off the water to the toilet at the shut-off valve located behind the toilet. Then flush the toilet once to drain the water from the tank.

After installing a new flapper, test it to make sure it flops up and down easily. Then turn the water supply shut-off valve back on to refill the toilet tank and do a test flush. If the flapper closes too quickly, check the lift chain’s length—pull it tight and then back it off one link and reattach it.

In some cases, replacing the flapper won’t fix the problem. Some toilets have a corroded flush valve seat that will continue to leak. You can try scouring the flush valve seat with steel wool or a special tool made for this purpose. Or you can replace the entire assembly, which is a considerably bigger job. Follow the instructions that come with the valve seat replacement kit.

Can you always tell if your toilet is running and wasting water? Not necessarily. Sometimes it’s obvious—the toilet makes a hissing sound or you can hear the ball cock repeatedly refilling the tank because the water level in the tank slowly drops. If you’re not sure whether your toilet is running, squeeze a few drops of food coloring into the tank to color the water. After a few minutes, if water is sneaking into the bowl from the tank, the water in the bowl will begin to turn color.