What You Need to Know Before Starting

A toilet water valve (also called a fill valve or ballcock) controls the water that refills your tank after you flush. When it fails, your toilet either runs constantly, fills slowly, or won't fill at all. Replacing it is a job you can do yourself in about 30 to 45 minutes with basic tools—no plumber needed.

The valve sits inside the tank, connected to the water supply line at the base. You'll need to turn off the water, drain the tank, disconnect the old valve, and install a new one. The hardest part is usually loosening the old connection under the tank, which can be tight or corroded. Once you understand the order of steps, the work is straightforward.

Key Takeaways

  • Turn off the water at the shutoff valve behind or below the toilet, then flush to empty the tank completely.
  • You will need an adjustable wrench, a flathead screwdriver, and a bucket to catch water that remains in the supply line.
  • The water valve bolts to the bottom of the tank from the inside, so you must reach under the tank with a wrench while holding the bolt steady from inside.
  • New fill valves come with instructions specific to the brand—read them before you start, because installation steps vary slightly between models.
  • After installation, turn the water back on slowly and let the tank fill completely before flushing to check for leaks.

Gather Your Tools and Materials

Before you touch anything, collect what you need. You will need an adjustable wrench (to fit various bolt sizes), a flathead screwdriver (to help disconnect the supply line or pry clips), a bucket (to catch water), and towels or rags (to soak up spills). Have your phone or a camera ready to photograph the old valve before you remove it—this helps you remember how parts connect.

Buy a replacement fill valve before you start. Common brands include Fluidmaster, Korky, and Kohler. Most hardware stores stock universal models that fit standard toilets. Bring the old valve with you if you can remove it easily, so the staff can confirm the new one will work. If you cannot remove it first, take a photo of the model number or describe the tank size (most are 2-inch or 1.5-inch connections).

Turn Off Water and Drain the Tank

Locate the shutoff valve on the water supply line behind or below the toilet. Turn it clockwise until it stops—you should feel resistance, but do not force it. The valve handle will point perpendicular to the pipe when it is fully closed. If you cannot find a shutoff valve or it will not turn, you may need to shut off water to the whole house at the main valve.

Flush the toilet to empty the tank. If water remains in the bowl, use a sponge or small cup to remove it and pour it into a bucket. Wipe the inside of the tank dry with a towel so you can see what you are doing. This step prevents water from splashing on you and makes it easier to spot leaks later.

Disconnect the Water Supply Line

The water supply line connects to the bottom of the fill valve with a compression fitting (a nut that screws on). Place your bucket underneath to catch any water still in the line. Use your adjustable wrench to turn the nut counterclockwise. Hold the valve body steady with your other hand so it does not spin—if it spins, you will damage the connection inside the tank.

Once the nut is loose enough, unscrew it by hand and pull the supply line away. A small amount of water will drip out; this is normal. Set the supply line aside where it will not get in your way. If the nut is stuck or corroded, spray it with penetrating oil (like WD-40) and wait 10 to 15 minutes before trying again.

Remove the Old Fill Valve

The fill valve bolts to the tank from underneath using a large nut. You will need to reach under the tank with your adjustable wrench while a helper (or you, if you can manage it) holds the valve body steady from inside the tank. If you are working alone, you can brace the valve with one hand while you work the wrench with the other, but it is easier with help.

Turn the nut counterclockwise under the tank. It will be tight—this is normal. Once it is loose enough, unscrew it by hand. The old valve will lift out from inside the tank. Some valves have a rubber gasket or seal attached; remove this and set it aside. You may be able to reuse it, but most new valves come with a fresh gasket, so you will likely replace it.

Install the New Fill Valve

Read the instructions that came with your new valve. Different brands have slightly different installation steps, and the instructions will tell you whether to install the gasket before or after inserting the valve. Most universal valves use a rubber gasket that sits on top of the bolt hole from inside the tank.

Insert the new valve through the hole from inside the tank, making sure the gasket is in place. From underneath the tank, thread the bolt and nut by hand first—do not use the wrench yet. Once it is hand-tight, use your adjustable wrench to tighten it another quarter turn. Do not over-tighten; you only need it snug enough that it will not leak. Over-tightening can crack the tank.

Reconnect the water supply line to the new valve. Hand-tighten the compression nut first, then use the wrench to snug it another quarter turn. Again, snug is enough; you do not need to crank it hard.

Refill the Tank and Check for Leaks

Turn the shutoff valve counterclockwise slowly to restore water pressure. Watch the tank fill. It should take 30 seconds to 2 minutes, depending on your water pressure. If water sprays or drips from the connections you just made, turn the shutoff valve back off, wait a minute, and tighten the nut another small amount with your wrench. Then turn the water back on and check again.

Once the tank is full, flush the toilet and watch it refill. The valve should shut off automatically when the tank reaches the correct level. If water continues to run into the bowl after the tank is full, the new valve may not be adjusted correctly—check the instructions for the adjustment procedure, which usually involves bending a float arm or turning a screw. If everything looks dry and the tank fills and empties normally, you are done.

Frequently Asked Questions

What if the old valve will not come loose?

Spray the nut with penetrating oil and wait 15 to 30 minutes. If it is still stuck, try tightening it slightly first—sometimes this breaks the corrosion seal—then loosen it. If the nut spins without loosening, the bolt inside the tank may be stripped. You may need to call a plumber, or you can try removing the entire tank from the bowl to access the bolt from a different angle.

Can I replace just the fill valve without replacing the whole tank?

Yes. The fill valve is a separate part that bolts into the tank. You do not need to replace the tank unless it is cracked or damaged. A new fill valve costs $15 to $40 and solves most running-toilet problems.

Why is water still leaking after I tightened everything?

Check that the gasket is seated properly—it should sit flat against the tank with no twists or folds. If the gasket is damaged, remove the valve and install a new gasket. If the leak is from the supply line connection, turn off the water and tighten the nut one more quarter turn. If it still leaks, the compression fitting may be damaged and need replacement.

Do I need to turn off water to the whole house?

No, if your toilet has a shutoff valve on its supply line. Use that valve. If your toilet has no shutoff valve, you should install one before you need it again—they cost $10 to $20 and take 10 minutes to install. For now, you will need to shut off the main water valve to the house.

How long does a fill valve usually last?

Most fill valves last 5 to 10 years depending on water quality and use. Hard water or sediment in the water supply can shorten the life. If your valve fails again within a year or two, the problem may be sediment in your water line—ask your plumber about installing a sediment filter.