What a fill valve does and why it fails

The fill valve (also called a ballcock or inlet valve) is the part that refills your toilet tank after you flush. Water enters through this valve, and a float mechanism shuts it off once the tank is full. When it wears out, you'll notice water running constantly into the bowl, the tank taking forever to refill, or water leaking inside the tank.

Fill valves typically last 5 to 10 years before internal seals degrade. Mineral buildup from hard water and sediment in the supply line can speed up failure. The good news: replacing one is a straightforward job that takes about 30 minutes and requires only basic tools you likely already own.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet, then flush to empty the tank completely before starting work.
  • The fill valve sits on the left side of the tank (when facing the toilet) and connects to the water supply line with a compression nut underneath.
  • You'll need an adjustable wrench, a flathead screwdriver, and a bucket to catch water; no special tools or plumbing experience required.
  • New fill valves cost between $15 and $40 depending on the type, and most hardware stores stock universal replacement models that fit standard tanks.
  • After installation, adjust the float arm so water stops filling when the tank is about an inch below the overflow tube opening.

Gather your tools and materials

Before you start, collect everything you'll need so you don't have to hunt for tools mid-job. You need an adjustable wrench (or two wrenches if you have them), a flathead screwdriver, a bucket or towel to catch water, and a new fill valve. Most hardware stores carry universal fill valves that work in standard tanks—ask a staff member to point you to the toilet repair section if you're unsure.

Take a photo of your current fill valve before you remove it, or write down the brand name if it's visible. This helps you match the replacement if you run into questions. Also check whether your tank has a float ball (a hollow sphere on an arm) or a float cup (a cylinder that slides up and down the valve stem)—this affects which replacement model you buy, though universal valves work with both.

Turn off water and empty the tank

Locate the shutoff valve on the water supply line under the toilet. It's usually 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. Then flush the toilet to drain the tank completely. If water is still trickling in after the flush, the fill valve is failing—that's exactly what you're fixing.

If water continues to run after you've shut off the supply, the shutoff valve itself may be faulty. In that case, you can still proceed, but work quickly and have your bucket ready. Some water will spill when you disconnect the supply line, so place the bucket under the compression nut where the water line meets the fill valve.

Disconnect the water supply line

Use your adjustable wrench to loosen the compression nut where the water supply line connects to the fill valve. This nut sits underneath the tank. Turn it counterclockwise until it's loose enough to unscrew by hand, then finish unscrewing it. Water will drip—that's why the bucket is there. Gently pull the supply line away from the valve.

If the nut is stuck or corroded, spray it with penetrating oil (like WD-40) and wait 10 minutes before trying again. Do not force it or you risk cracking the fitting. Once the supply line is free, set it aside where it won't drip on your floor.

Remove the old fill valve from the tank

Inside the tank, you'll see the fill valve stem sticking up through the bottom. On the underside of the tank, there's a large locknut that holds the valve in place. Use your adjustable wrench to turn this locknut counterclockwise. You may need to hold the valve stem steady with your other hand so it doesn't spin. Once the locknut is loose, unscrew it the rest of the way by hand.

Lift the old fill valve straight up and out of the tank. Some water may still be inside the tank—tilt the valve slightly as you remove it so any remaining water drains back into the tank rather than onto your floor. Set the old valve aside. Wipe the area around the hole in the tank bottom with a dry cloth to remove any sediment or mineral buildup.

Install the new fill valve

Before inserting the new valve, check that the rubber washer on the bottom of the valve is in place and not cracked. If it looks worn, replace it with the spare washer that usually comes with the new valve. Slide the new fill valve down through the hole in the tank, positioning it so the outlet (where water flows into the tank) points toward the center of the tank.

From underneath the tank, thread the locknut onto the valve stem by hand first, then tighten it with your wrench. Turn it clockwise until it's snug—firm enough that the valve won't wiggle, but not so tight that you strain the plastic threads. Hand-tight plus a quarter turn with the wrench is usually right. Reconnect the water supply line by threading the compression nut back onto the valve and tightening it with your wrench until it's snug.

Adjust the float and test the fill

Turn the shutoff valve back on by turning the knob counterclockwise (or flipping the lever parallel to the pipe). Water will begin filling the tank. Watch the float mechanism—it should rise as the tank fills and shut off the water when the tank is nearly full. The water level should stop about an inch below the top of the overflow tube (the hollow pipe in the center of the tank).

If water keeps running after the tank is full, the float is set too high. Most new fill valves have an adjustment clip or screw on the float mechanism. Consult the instructions that came with your valve—usually you lower the float by moving a clip down the stem or turning a small screw counterclockwise. Make small adjustments (a quarter turn or one notch at a time) and test after each one. If water is shutting off too early and the tank isn't filling completely, raise the float instead.

Flush the toilet a few times and listen for the fill valve to shut off cleanly each time. Check under the tank for any drips around the compression nut or locknut. A small drip where the supply line connects usually means the nut needs another quarter turn—tighten it slightly and test again. If the leak persists, turn off the water, disconnect the line, and inspect the ferrule (the metal ring inside the nut) for damage.

Frequently Asked Questions

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

No. The shutoff valve under the toilet controls water only to that tank. If you can't find it or it doesn't work, you can turn off the main water supply to the house, but the toilet shutoff is all you need for this job.

What if the locknut under the tank won't budge?

Spray it with penetrating oil and wait 15 minutes. If it's still stuck, try tightening it slightly first (a quarter turn), then loosening it—this can break corrosion loose. If it remains frozen, you may need a pipe wrench for better grip, or call a plumber rather than risk cracking the tank.

Can I use the old float from my broken valve on the new one?

Not reliably. Float mechanisms are designed to work with specific valve models, and mixing them can cause the tank to overfill or underfill. Use the float that comes with your new valve, or buy a replacement float designed for that valve model.

How do I know which fill valve to buy?

Universal fill valves fit most standard tanks and are the safest choice if you're unsure. Bring a photo of your old valve or the tank model number (usually on a sticker inside the tank) to the hardware store, and staff can confirm the replacement will fit.

Is it normal for water to drip from under the tank after I finish?

A few drops are normal while the system settles. If water drips steadily 10 minutes after you've finished, tighten the compression nut another quarter turn. If it still leaks, the ferrule may be damaged and you may need to replace the supply line.