What a fill valve does and when to replace it

The fill valve is the mechanism inside your toilet tank that refills the bowl after you flush. Water enters through the fill valve, and a float arm rises with the water level until it shuts off automatically. When a fill valve fails, you'll see water running constantly into the tank, the water level dropping on its own, or the tank taking much longer than usual to refill after a flush.

Replacing a fill valve is a straightforward repair that takes 20 to 45 minutes and costs $15 to $40 for the valve itself. You don't need a plumber for this job—the valve sits inside the tank where you can reach it, and the connections are hand-tight. The hardest part is usually getting the old valve out, which sometimes sticks after years of mineral buildup.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet before you start, then flush to empty the tank.
  • The fill valve connects to the water supply line with a compression nut under the tank—loosen it by hand or with an adjustable wrench, then pull the valve straight up and out.
  • Slide the new fill valve into the opening, hand-tighten the compression nut underneath, then turn the water back on and adjust the float arm so the water stops at the fill line marked inside the tank.
  • If the old valve won't budge, soak the nut with penetrating oil for 15 minutes, or use a basin wrench if your space is tight.

Turn off water and empty the tank

Locate the shutoff valve on the water supply line below the toilet—it's a small knob or lever on the pipe that runs from the wall to the bottom of the tank. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. Then flush the toilet once to drain the tank completely.

If you can't find the shutoff valve or it won't turn, you can still do this repair—just work slowly and have towels ready. The tank will leak when you disconnect the supply line, but the amount is manageable if you catch it in a bucket or towel.

Disconnect the water supply line

Look under the tank where the water supply line connects. You'll see a compression nut (a hexagonal nut) where the line screws onto the fill valve. Use an adjustable wrench to hold the fill valve body steady with one hand while you turn the nut counterclockwise with the other hand. Loosen it until water stops dripping, then unscrew it the rest of the way by hand.

If the nut is stuck, spray it with penetrating oil (like WD-40) and wait 15 minutes before trying again. If your space is very tight and a wrench won't fit, a basin wrench—a specialized wrench with a long handle and a jaw that grips from the side—can reach into cramped spots. You can rent one from most hardware stores for $5 to $10.

Remove the old fill valve

Once the supply line is loose, look inside the tank at the fill valve. It has a large nut on the underside of the tank (called the locknut) that holds it in place. Reach under the tank and turn this nut counterclockwise by hand or with an adjustable wrench. As you loosen it, the valve will start to lift out of the tank opening.

Pull the valve straight up and out of the tank. If it's stuck, don't force it—spray penetrating oil around the base where it enters the tank, wait 15 minutes, and try again. Once it's out, empty any remaining water from the tank into a bucket or use a sponge to soak it up so you have a dry workspace.

Install the new fill valve

Most new fill valves come with a rubber gasket (a flat seal) already attached to the bottom. If yours doesn't, slide the gasket onto the valve stem before you insert it. Lower the new valve into the tank opening, making sure it sits straight and the gasket seals against the tank bottom.

Reach underneath and hand-tighten the locknut clockwise until it's snug. Don't over-tighten—you want it firm enough that the valve doesn't wiggle, but not so tight that you crack the tank. Then reconnect the water supply line by hand-tightening the compression nut, and use the wrench to tighten it another quarter turn.

Adjust the float and test

Turn the shutoff valve back on by turning it counterclockwise (or flipping the lever parallel to the pipe). Water will flow into the tank. Watch the float arm rise as the tank fills—it should stop when the water reaches the fill line marked inside the tank (usually about an inch below the overflow tube).

If the water level is too high or too low, adjust the float. Most modern fill valves have a small clip or adjustment screw on the float arm—check the instructions that came with your valve. Turning the screw clockwise lowers the water level; counterclockwise raises it. Make small adjustments (a quarter turn at a time) and let the tank refill between each change. Once the level is correct, flush the toilet a few times to confirm the valve shuts off cleanly and the tank refills properly.

What to do if the valve leaks after replacement

If water drips from under the tank after you've installed the new valve, the compression nut is usually loose. Turn off the water, empty the tank, and use the wrench to tighten the nut another quarter turn. Reconnect the water and check again.

If water leaks from inside the tank (running into the bowl), the float may be set too high, or the new valve itself may be defective. Lower the float first and test. If that doesn't stop it, the valve may have a manufacturing flaw—most hardware stores will exchange it without a receipt if you bring it back within 30 days.

Frequently Asked Questions

Do I need to turn off the water before I start?

Yes. Turning off the shutoff valve prevents water from spraying when you disconnect the supply line. If you can't locate or operate the shutoff valve, you can still proceed, but have towels and a bucket ready because water will leak when you loosen the compression nut.

What's the difference between a fill valve and a ballcock?

Ballcock is an older term for the same part—a fill valve with a float ball attached. Modern fill valves use a float cup or clip instead of a ball. Both work the same way and are replaced using the same steps. The new valve you buy will likely be a float-cup style, which is more reliable and easier to adjust.

Can I replace just the float arm instead of the whole valve?

Sometimes. If the valve itself isn't leaking and only the float arm is bent or broken, you can buy a replacement arm for $5 to $10. However, if the valve is running constantly or leaking, replacing the whole valve is more reliable and only costs a few dollars more.

What if the new valve doesn't fit the opening in my tank?

Fill valves come in standard sizes, and the opening in your tank is designed for one of them. If the valve doesn't slide in easily, check that the rubber gasket is seated correctly on the valve stem and that you're inserting it straight down. If it still doesn't fit, the valve may be the wrong type—take a photo of your old valve to the hardware store and ask for the correct replacement.

How long does a fill valve usually last?

Most fill valves last 5 to 10 years depending on water hardness and mineral content. Hard water (high in calcium and magnesium) causes mineral buildup that clogs the valve sooner. If you notice the tank running constantly or refilling slowly, the valve is likely near the end of its life and should be replaced.