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 the valve, and a float arm or float cup rises as the tank fills, shutting off the water when it reaches the right level. When the fill valve wears out, you'll see 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 depending on water quality and how often the toilet is used. Hard water, sediment, and mineral buildup corrode the internal seals. Once it starts leaking or running, replacement is usually cheaper and faster than repair kits.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet before you start, then flush to empty the tank.
  • You'll need an adjustable wrench, a flathead screwdriver, and a new fill valve that matches your toilet's tank opening size (most are 1.25 inches).
  • The fill valve bolts to the bottom of the tank from the inside, so you'll work from inside the tank and underneath the toilet at the same time.
  • After installation, adjust the float so the water stops filling when the tank is about an inch below the overflow tube.
  • If water still leaks after you finish, the rubber washers may not be seated correctly—remove and reinstall the valve to reseat them.

Gather your tools and the right replacement valve

Before you start, turn off the water at the shutoff valve under the toilet (turn the handle clockwise). Flush once to empty the tank completely. If there's no shutoff valve under your toilet, you'll need to turn off the main water supply to the house.

You'll need an adjustable wrench, a flathead screwdriver, and a bucket or towel to catch water. Buy a replacement fill valve that matches your tank opening—most modern toilets use a 1.25-inch opening, but older tanks may use 1.5 inches. Bring a photo of your current valve to the hardware store, or measure the opening with a ruler. Universal fill valves work in most tanks, but some brands (Kohler, Toto, American Standard) make tank-specific versions that fit more securely.

Remove the old fill valve from underneath the tank

Reach under the toilet tank and locate the fill valve nut—a large hex nut where the valve bolts through the bottom of the tank. Place your bucket underneath to catch residual water. Use your adjustable wrench to hold the nut steady while you turn the locknut (the smaller nut on top of it) counterclockwise with a second wrench or your fingers. Once the locknut is loose, unscrew it by hand.

Reach inside the tank and gently pull the old valve upward and out. Water may still drip from the opening—that's normal. Wipe the bottom of the tank dry with a towel. Look at the rubber washer and cone washer that came out with the valve; if they're cracked, hardened, or flattened, that's why the old valve leaked.

Install the new fill valve and tighten from below

Most new fill valves come with fresh rubber washers and a cone washer already attached or in the box. If yours didn't, use the old washers only if they're still flexible and undamaged. Insert the new valve through the opening from inside the tank, positioning it so the fill tube (the small pipe) points toward the overflow tube in the center of the tank.

From underneath, slide the cone washer onto the valve stem, then the rubber washer, then hand-thread the locknut clockwise until it's snug. Use your wrench to tighten it another quarter turn—do not over-tighten, as this can crack the tank. The valve should sit firmly but not be forced. Once the locknut is tight, turn the water back on at the shutoff valve and watch for leaks around the base of the valve.

Adjust the float and test the fill level

As the tank fills, watch the float arm or float cup rise. The water should stop filling when the tank is about one inch below the top of the overflow tube (the hollow pipe in the center). If water is still running into the overflow tube, the float is set too high and needs to come down.

For a float arm (a horizontal rod with a ball on the end), bend the arm gently downward to lower the float. For a float cup (a cylinder that slides on the valve stem), pinch the clip on the side and slide the cup down. Make small adjustments—a quarter inch at a time—and flush to test. Once the water shuts off at the right level, flush again to confirm the tank refills properly and the valve closes completely.

Check for leaks and reseat if water still drips

Let the tank sit for 10 minutes without flushing. If water is trickling into the bowl, the fill valve is leaking. First, check that the float isn't stuck against the tank wall or the overflow tube—move it gently and flush again. If it still leaks, the rubber washers may not be seated evenly against the tank opening.

Turn off the water, flush to empty the tank, and reach under the toilet. Loosen the locknut a quarter turn, then tighten it again firmly. This reseats the washers. Turn the water back on and test. If leaking continues, the tank opening may be cracked or the new valve may be defective—in that case, contact the valve manufacturer or call a plumber to inspect the tank.

When to call a plumber instead

If you don't have a shutoff valve under the toilet and turning off the main water supply is difficult or unclear, a plumber can handle this safely. If the tank is cracked around the valve opening, or if you strip the locknut threads while tightening, the tank itself may need replacement—that's a job for a professional.

If you're uncomfortable working under the toilet or the valve won't stop leaking after you've reseated it twice, a plumber can diagnose whether the problem is the valve, the tank, or the water supply line. A professional can also install a shutoff valve if your toilet doesn't have one, which makes future repairs much easier.

Frequently Asked Questions

Do I need to drain the whole tank before I start?

Yes. Flush once after turning off the water to empty most of the tank. A small amount of water will remain in the bowl and around the valve opening—that's why you need a bucket underneath. You don't need to siphon or bail out every drop.

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

No. Water will spray out as soon as you loosen the valve nut. Always turn off the shutoff valve under the toilet first. If there is no shutoff valve, you must turn off the main water supply to the house before you start.

What if the new valve leaks right after I install it?

Check that the float isn't blocking the valve from closing fully. If the float is clear, turn off the water, loosen the locknut underneath by a quarter turn, and tighten it again to reseat the washers. This fixes most leaks. If it still leaks after reseating, the tank opening may be damaged or the valve may be defective.

How do I know what size fill valve to buy?

Most modern toilets use a 1.25-inch valve opening. Measure the hole at the bottom of the tank with a ruler, or bring a photo of your current valve to the hardware store. The package will say what tank sizes the valve fits—universal valves work in most tanks, but measure to be sure.

Can I reuse the old washers from my fill valve?

Only if they're still soft and flexible with no cracks or flat spots. Most new fill valves come with fresh washers, which is why they seal better than old ones. If the old washers look hard or compressed, throw them away and use the new ones.