A fill valve replacement takes 30 minutes and costs $15 to $40

The fill valve (also called a ballcock or inlet valve) is the mechanism that refills your tank after you flush. When it fails, your toilet either runs constantly, refills slowly, or won't fill at all. Replacing it is a straightforward job that requires no special skills—just a wrench, a screwdriver, and the ability to turn off the water.

The entire process takes about 30 minutes. You'll spend $15 to $40 on the valve itself, depending on the type. The hardest part is usually removing the old valve, which can be stuck or corroded. Everything else is assembly.

Key Takeaways

  • Turn off the water at the shutoff valve under the tank, then flush to empty it completely before you start.
  • The fill valve bolts to the bottom of the tank from inside; you'll need a wrench to loosen the nut underneath and remove it.
  • Buy a replacement that matches your tank opening size (most are 1¼ inch) and bring the old valve to the store if you're unsure.
  • Install the new valve by reversing the removal steps, hand-tighten first, then use a wrench to snug the connection without over-tightening.
  • Adjust the float or fill height after installation so the tank stops filling when it reaches the line marked on the inside of the tank.

Turn off the water and empty the tank

Locate the shutoff valve on the water line beneath the toilet tank. It's a small knob or lever on the pipe coming from the wall. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. You should feel resistance, but don't force it.

Flush the toilet once. The tank will empty and the fill valve won't refill it because the water is off. If water is still trickling in, the shutoff valve may be faulty—turn it a bit more, or if it won't seal, you may need to replace that too. For now, proceed with the tank as empty as it will get.

Disconnect the water supply line

Look under the tank where the water line enters. You'll see a nut connecting the flexible supply line to the fill valve. Use an adjustable wrench to hold the nut and turn it counterclockwise. The line will come free. Some water may drip out—have a towel ready.

If the nut is stuck or corroded, spray it with penetrating oil (like WD-40) and wait 10 minutes before trying again. Don't use excessive force; if it won't budge after two attempts, you may need a plumber's help to avoid cracking the tank.

Remove the old fill valve from the tank

Inside the tank, you'll see the fill valve assembly. At the bottom, where it passes through the tank wall, there's a large nut holding it in place. Reach under the tank from outside and hold this nut with a wrench while you turn the valve itself counterclockwise from inside the tank. The valve will unscrew and come out.

If the nut won't turn, the valve is likely corroded. Spray penetrating oil around the nut, wait 15 minutes, and try again. If it's still stuck, gently tap the wrench with a hammer to break the corrosion loose. Once the valve is free, remove any rubber washers or gaskets that come with it—you'll replace these with new ones on the new valve.

Install the new fill valve

Before you buy a replacement, check the opening size on your tank. Most toilets use a 1¼-inch valve, but some older tanks use 1½ inch. Bring the old valve to the hardware store if you're unsure, or measure the hole at the bottom of the tank with a ruler. Buy a fill valve that matches.

Slide the new valve through the opening from inside the tank. From underneath, thread the mounting nut onto the valve by hand first—don't use a wrench yet. Once it's hand-tight, use a wrench to snug it another quarter-turn. Over-tightening can crack the tank, so stop as soon as you feel solid resistance.

Reconnect the water supply line

Reattach the flexible supply line to the fill valve by hand-threading the nut first. Once it's snug by hand, use a wrench to tighten it one more quarter-turn. Don't over-tighten—you're aiming for a seal, not maximum force. Turn the shutoff valve back on by rotating it counterclockwise (or flipping the lever parallel to the pipe).

Water will flow into the tank. Watch for leaks around the connection you just made and under the tank where the valve enters. If you see a drip, turn the shutoff valve off again and tighten the connection slightly more. Small drips often stop once the rubber washer settles.

Adjust the float and test the fill height

As the tank fills, you'll see the float (a ball or cup attached to the valve) rise with the water level. The tank should stop filling when the water reaches the line marked inside the tank, usually about an inch below the overflow tube. If it fills too high or too low, 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 new valve—they'll show you exactly where it is. Typically, moving the clip down lowers the fill level, and moving it up raises it. Make small adjustments (¼ inch at a time) and flush to test. Once the tank stops at the right level, you're done.

Frequently Asked Questions

What if water leaks from under the tank after I install the new valve?

A small drip often stops once the rubber washer compresses. Turn off the water, wait an hour, then turn it back on. If it still leaks, turn off the water again and tighten the mounting nut under the tank by another quarter-turn with a wrench. Retighten the supply line connection as well. Leaks that don't stop after two tightening attempts usually mean the washer is damaged or the tank is cracked.

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

You can, but you'll get soaked. Water will pour out the moment you disconnect the supply line. If your shutoff valve is broken and won't turn off, you can still work around it by having towels and a bucket ready, but it's messy and wastes water. A plumber can replace a faulty shutoff valve for $100 to $200 if yours won't seal.

How do I know if my fill valve is actually broken?

If the tank runs constantly, refills slowly after flushing, or won't fill at all, the fill valve is usually the culprit. You can also listen—a hissing sound coming from inside the tank often means the valve is leaking internally. If you're unsure, turn off the water and wait 30 minutes. If the tank level drops, the fill valve is leaking and needs replacement.

Do I need a plumber for this job?

Most homeowners can do this themselves if the old valve isn't severely corroded. If the mounting nut won't budge after penetrating oil and gentle tapping, or if you crack the tank during removal, call a plumber. A professional can usually replace a fill valve in under an hour for $150 to $300 in labor.