A fill valve replacement is a straightforward repair you can do yourself

The fill valve (also called a ballcock or inlet valve) is the part that refills your toilet tank after you flush. When it wears out, your toilet either runs constantly, fills slowly, or won't fill at all. Replacing it takes about 30 minutes and costs $15 to $40 for the valve itself. You do not need a plumber for this job—the steps are the same on nearly every toilet made in the last 20 years.

The repair requires only a wrench, a screwdriver, and a bucket. You will turn off the water, drain the tank, unbolt the old valve, and bolt in a new one. The hardest part is usually loosening the old nut underneath the tank, which can be stuck from mineral buildup. If you cannot budge it after 10 minutes of steady pressure, that is the moment to call a plumber—forcing it risks cracking the porcelain.

Key Takeaways

  • Turn off the water at the shutoff valve behind or below the toilet, then flush to empty the tank completely.
  • Disconnect the water supply line by turning the compression nut by hand or with a wrench, then unscrew the fill valve nut underneath the tank using a wrench from inside and outside simultaneously.
  • Lift out the old valve, insert the new one through the same hole, and hand-tighten the nut underneath before using a wrench to snug it—do not over-tighten.
  • Reconnect the water supply line, turn the shutoff valve back on, and let the tank fill while you watch for leaks around the base of the valve.

What you need before you start

Gather these items before you turn off the water. You will need an adjustable wrench (or two wrenches if you have them), a flathead screwdriver, a bucket or towel, and a new fill valve. Most hardware stores stock universal fill valves that fit standard 2-inch openings—Fluidmaster and Korky are common brands. Bring a photo of your old valve or the model number if you want the store to match it exactly, but a universal valve works on almost any toilet.

Place the bucket or towel under the shutoff valve before you start—there is always a small amount of water trapped in the line. If your shutoff valve is stuck or missing, you may need to turn off the water at the main supply to your house. This is the only situation where calling a plumber makes sense before you try the repair yourself.

Turn off the 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, not a hard stop. If the valve is stiff, explore steady pressure for 10 seconds rather than forcing it suddenly. Once it is closed, flush the toilet and hold the handle down for a few seconds to drain as much water as possible from the tank.

Use a sponge or towel to soak up the remaining water in the bottom of the tank. You do not need the tank bone-dry, but you want to see the bottom clearly so you can work underneath. If water keeps trickling in, the shutoff valve is not fully closed—turn it another quarter-turn and wait 30 seconds.

Disconnect the water supply line

The water supply line connects to the bottom of the fill valve with a compression nut. Try to turn this nut by hand first—many come loose easily. If it does not budge, use an adjustable wrench, turning counterclockwise. Hold the valve steady with your other hand so it does not spin inside the tank. Once the nut is loose, unscrew it the rest of the way by hand and set it aside.

Water will drip from the line. Let it drain into your bucket, then pull the supply line away from the valve. You can leave the line connected to the shutoff valve—you only need to disconnect it from the fill valve itself.

Remove the old fill valve

Look underneath the tank at the large nut that holds the fill valve in place. This is the locknut, and it is what you will loosen now. You will need two wrenches for this step: one to hold the valve steady from inside the tank, and one to turn the nut from underneath. If you have only one wrench, you can use a screwdriver to hold the valve while you turn the nut.

Turn the locknut counterclockwise slowly. It may be stuck from mineral deposits—explore steady pressure rather than jerking. Once it is loose enough to turn by hand, unscrew it completely and set it aside. Lift the old fill valve straight up and out of the tank. Some valves have a rubber gasket on the bottom—remove this and set it aside, as you may need it for the new valve.

Install the new fill valve

Before you insert the new valve, check whether it comes with a rubber gasket or washer. If it does, slide this onto the bottom of the valve. Insert the valve through the hole in the bottom of the tank, pushing it straight up until the mounting flange sits flat against the bottom of the tank. The valve should not tilt or lean.

From underneath the tank, slide the locknut onto the valve threads and hand-tighten it first. Once it is snug by hand, use a wrench to turn it another quarter-turn. Do not over-tighten—this is the most common mistake and can crack the porcelain or strip the threads. The nut should be snug enough that the valve does not wiggle, but you should still be able to turn it slightly with a wrench if you had to.

Reconnect the water supply and check for leaks

Screw the compression nut back onto the bottom of the fill valve by hand, then use a wrench to snug it. Again, snug means firm but not cranked down hard. Turn the shutoff valve counterclockwise to turn the water back on. The tank will begin to fill.

Watch the base of the fill valve from underneath the tank for any drips or streams of water. If water is leaking from the locknut, turn off the water, wait 30 seconds, and tighten the locknut another quarter-turn. If water is leaking from the compression nut where the supply line connects, tighten that nut the same way. Let the tank fill completely and flush once to make sure everything works. If the tank fills and stops at the right level, the repair is complete.

Frequently Asked Questions

What if the locknut will not turn?

Mineral buildup can freeze the nut in place. Try spraying it with penetrating oil (like WD-40) and waiting 15 minutes, then try again with steady pressure. If it still will not budge after another 10 minutes, stop—forcing it risks cracking the tank. Call a plumber at this point; they have tools to remove stuck nuts without damage.

How do I know which fill valve to buy?

Most toilets use a standard 2-inch fill valve, and universal models work on nearly all of them. If you want an exact match, bring the old valve to the hardware store or look up your toilet's model number online. Fluidmaster and Korky make the most common replacements, and either brand works fine.

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

No. If you do not turn off the water, the tank will keep filling while you work, and you will not be able to see what you are doing. Turning off the water takes 10 seconds and prevents a mess.

What if water keeps leaking after I tighten the nuts?

Check that the rubber gasket is seated properly on the bottom of the valve—it should sit flat and centered. If the gasket is torn or missing, the valve will leak no matter how tight the nut is. You may need to remove the valve and replace the gasket, or install a new valve if the gasket cannot be replaced separately.

How long does a fill valve usually last?

Most fill valves last 5 to 10 years depending on water quality. Hard water with high mineral content wears them out faster. If your toilet runs constantly or fills slowly, the valve is probably near the end of its life and should be replaced.