What a fill valve does and why it fails

The fill valve is the mechanism inside your toilet tank that refills the bowl after you flush. Water enters through the fill valve, and when the tank is full, a float arm or float cup shuts it off automatically. Over time, mineral deposits, debris, or wear cause the valve to stick, leak, or fail to shut off—which wastes water and can run your bill up noticeably.

You'll know the fill valve is failing if water continuously trickles into the bowl, the tank never fills completely, or water runs into the overflow tube (the hollow pipe in the center of the tank). Replacing it is a straightforward job that takes about 30 minutes and costs $15 to $40 for the valve itself. You do not need to turn off the water to the whole house—just the shutoff valve under the toilet.

Key Takeaways

  • Turn off the water at the shutoff valve under the toilet, flush to empty the tank, and remove the old fill valve by unscrewing the locknut underneath the tank.
  • Buy a replacement fill valve that matches your tank type—most modern toilets use a ballcock or float cup style, and hardware stores stock universal models that fit most tanks.
  • Install the new valve by sliding it through the hole, hand-tightening the locknut underneath, and adjusting the float so the water level stops about an inch below the overflow tube.
  • Test the valve by flushing and listening for the water to shut off cleanly; if it continues running, the float may need adjustment or the valve may not be seated properly.

Gather your tools and materials

You will need an adjustable wrench or a 1¼-inch wrench (the size of the locknut under most tanks), a bucket or towel to catch water, and the replacement fill valve itself. Most hardware stores and online retailers stock universal fill valves—brands like Fluidmaster and Korky make models that fit standard tanks. Bring a photo of your current valve or the tank model number to the store if you are unsure which type to buy.

Before you start, locate the shutoff valve on the water line under the toilet. It looks like a small knob or lever on the pipe coming from the wall. Turn it clockwise (or flip the lever perpendicular to the pipe) to stop water flow. If the valve is stuck or does not stop the water, you will need to shut off the main water supply to the house—usually a valve in the basement or near the water meter.

Remove the old fill valve

Flush the toilet to empty the tank. Use a sponge or towel to soak up any remaining water in the tank so you have a dry work area. Locate the locknut—a large nut on the underside of the tank where the fill valve enters. Place your bucket underneath to catch any drips, then use your wrench to turn the locknut counterclockwise. You may need to hold the fill valve steady from inside the tank with your other hand so it does not spin.

Once the locknut is loose, unscrew it by hand and set it aside. Lift the old fill valve out of the tank from the top. You may see mineral buildup or debris inside the tank around the hole—wipe this away with a damp cloth so the new valve seats cleanly.

Install the new fill valve

Before inserting the new valve, check that it comes with a rubber gasket or washer. Most universal valves include one; if yours does not, buy one separately—it prevents leaks between the tank and the valve. Slide the new fill valve down through the hole in the tank from the top, positioning it so the inlet (where water enters) points toward the back or side of the tank, away from the overflow tube.

From underneath the tank, hand-tighten the locknut onto the valve threads. Once it is snug, use your wrench to turn it another quarter turn—do not over-tighten, as this can crack the tank. The valve should sit firmly but not be forced. Turn the shutoff valve back on (counterclockwise or flip the lever parallel to the pipe) and let the tank fill.

Adjust the float and test the fill

As the tank fills, watch the float—a ball or cup that rises with the water level. The float should shut off the water when the tank is full, with the water level about one inch below the top of the overflow tube. If water continues running past the float or shuts off too early, the float needs adjustment.

Most modern fill valves have an adjustment clip or screw on the float mechanism. Consult the instructions that came with your valve—they show exactly where to adjust. Generally, moving the clip up raises the water level, and moving it down lowers it. Make small adjustments (about ¼ inch at a time), flush, and watch the tank refill until the water level is correct.

Check for leaks and listen for proper shutoff

After the tank is full, listen for the fill valve to stop running completely. You should hear a clear click or silence—not a continuous trickle. If water still runs into the bowl or the overflow tube after the tank is full, the float may not be positioned correctly, or the valve may not be seated properly in the tank.

Check underneath the tank for leaks around the locknut. If water drips, tighten the locknut slightly more with your wrench. If the leak persists, turn off the water, remove the valve, and check that the rubber gasket is in place and not damaged. Reinstall the valve and try again. Most fill valve problems resolve with a straightforward float adjustment or a snug locknut.

When to call a plumber

If you have adjusted the float multiple times and water still runs continuously, or if the tank cracks when you tighten the locknut, stop and call a plumber. Cracks in the tank cannot be repaired and require tank replacement. Similarly, if your shutoff valve does not work or the main water supply valve is stuck, a plumber can help you access the water supply safely.

If your toilet is very old or the fill valve hole is stripped (the locknut spins without tightening), a plumber may recommend replacing the entire tank rather than attempting a repair. This is rare, but it is worth knowing before you start.

Frequently Asked Questions

Can I replace the fill valve without turning off the water?

No. You must turn off the shutoff valve under the toilet first. If you try to work without shutting off the water, the tank will refill as you work and you will get soaked. If the shutoff valve does not work, turn off the main water supply to the house before proceeding.

What if the new fill valve is too tall for my tank?

Universal fill valves are adjustable in height. Most have a threaded rod or adjustable base that lets you shorten them to fit. Check the instructions for your specific valve—they explain how to cut or adjust the height. If the valve is still too tall after adjustment, return it and buy a different model designed for low-profile tanks.

How do I know which fill valve to buy?

Most modern toilets use a ballcock or float cup fill valve, and universal models fit the majority of tanks. Bring a photo of your current valve or your toilet's model number to the hardware store. Staff can match it for you. If you order online, read the product description to confirm it fits standard 2-inch tank openings.

Why does my new fill valve keep running after the tank is full?

The float is usually set too high or not positioned correctly. Check that the float moves freely and is not rubbing against the tank wall or overflow tube. Adjust the float clip down slightly, flush, and watch the tank refill. If the problem persists after several adjustments, the valve itself may be defective—return it for a replacement.

Is it normal to hear hissing or gurgling from the fill valve?

A brief hiss as the tank fills is normal. Continuous gurgling or hissing after the tank is full usually means the float is not shutting the valve off completely. Adjust the float up or down and test again. If the noise continues, the valve may have debris inside—turn off the water, remove the valve, and rinse it under running water before reinstalling.