A continuously running toilet is a common household issue that wastes water and increases utility bills, but most causes can be diagnosed and fixed without professional help. The problem typically stems from three main components: the flapper valve, fill valve, or water level adjustment. Simple adjustments like turning an adjustment screw on the fill valve or replacing a worn flapper often resolve the issue in minutes, while more complex problems may require replacing the entire fill valve assembly.

Key immediate actions to try:

• Adjust the water level by turning the fill valve’s adjustment screw counterclockwise to lower the water to the marked line ^[1]
• Check the flapper for wear or improper sealing, as this is the most common cause of intermittent running ^[4][5]
• Lift the float arm—if the water stops, the float needs adjustment via its attached screw ^[10]
• Shut off the water supply before any repairs to prevent flooding, using the valve behind the toilet ^[6][8]

Step-by-Step Fixes for a Running Toilet

Quick Adjustments: Water Level and Float

Most running toilets can be silenced by correcting the water level or float mechanism, which controls when the fill valve shuts off. These fixes require no tools and take less than a minute.

Start by removing the tank lid and observing the water level. If water is spilling into the overflow tube (the vertical pipe in the center), the fill valve is likely set too high. The adjustment screw—located on top of the fill valve (common in Kohler models) or along the float arm—controls this level. Turn the screw counterclockwise to lower the water until it sits ½ inch below the overflow tube or aligns with the water level line marked on the valve ^[1][5]. For older toilets with a float ball (a buoy on a metal arm), bend the arm downward slightly to lower the water cutoff point ^[8].

If lifting the float arm manually stops the water, the float is either stuck or misadjusted. Modern toilets often use a float cup that slides along a vertical rod—clean any debris blocking its movement, then adjust its height using the clip or screw ^[6]. Key signs the float is the issue:

• Water stops when you lift the float arm by hand ^[10]
• The tank fills to the overflow tube before shutting off ^[5]
• The fill valve hisses or sputters continuously ^[4]

For persistent float problems, replace the entire fill valve (e.g., Fluidmaster 400 model) if adjustments fail. This is a $15–$25 part available at hardware stores and requires shutting off the water supply first ^[7].

Flapper and Fill Valve Replacements

When adjustments don’t work, the flapper or fill valve is likely worn or faulty. These components degrade over time due to mineral buildup, chlorine, or physical wear, leading to leaks that trigger constant refilling.

Flapper Issues

The flapper—a rubber seal at the tank’s bottom—is the most common failure point. A worn, warped, or improperly seated flapper lets water leak into the bowl, prompting the fill valve to cycle repeatedly. Test it by adding a few drops of food coloring to the tank; if color appears in the bowl within 10 minutes without flushing, the flapper is leaking ^[4]. Common flapper problems:

• Chain too tight or loose: The chain connecting the flapper to the flush handle should have ½ inch of slack when closed. Too tight, and it won’t seal; too loose, and it won’t lift to flush ^[8].
• Mineral buildup: Clean the flapper seat (the rim it seals against) with vinegar and a sponge to remove deposits ^[6].
• Age: Replace flappers every 3–5 years, as rubber degrades. Universal flappers cost $5–$10 and snap into place without tools ^[9].

To replace the flapper:

1. Shut off the water supply and flush to empty the tank.
2. Disconnect the old flapper from the hinge pins and chain.
3. Attach the new flapper, ensuring the ears align with the hinge pins and the chain has proper slack ^[4].

Fill Valve Failure

If the flapper seals properly but the toilet still runs, the fill valve may be faulty. A hissing or trickling sound from the valve indicates it’s not shutting off completely. Try cleaning the valve stem and float with vinegar to remove mineral deposits—this resolves 30% of cases ^[7]. If cleaning fails:

• Replace the fill valve: Turn off the water, disconnect the supply line, and unscrew the old valve from the tank bottom. Install a new valve (e.g., Fluidmaster) by following the package instructions, typically involving a twist-lock mechanism ^[6].
• Check for cracks: Inspect the valve housing for hairline cracks, which require full replacement ^[5].

Persistent issues after these steps may indicate hidden leaks (e.g., at the tank-to-bowl gasket) or faulty plumbing vents, warranting a plumber’s inspection ^[3].