What You're Replacing and Why

The fill valve (also called a ballcock) is the mechanism that refills your tank after you flush. The flapper is the rubber seal at the bottom of the tank that lets water drain into the bowl when you press the handle. When either one wears out, you get a running toilet, weak flushes, or constant refilling. Both parts are inexpensive—usually $15 to $40 each—and you can replace them yourself in 30 to 45 minutes with basic tools.

You do not need a plumber for this job. The parts sit inside the tank, are held in place with straightforward fasteners, and come out without disconnecting any water lines. If your toilet is running constantly or not filling properly after a flush, one of these two parts is almost certainly the culprit.

Key Takeaways

  • Turn off the water supply at the shutoff valve behind or below the toilet before you start, then flush to empty the tank.
  • The fill valve screws into the bottom of the tank from underneath; the flapper lifts straight up and off the overflow tube in the center.
  • Bring your old parts to the hardware store to match the size and style, or take a photo of the tank interior to show the clerk.
  • Hand-tighten the fill valve nut under the tank—do not use a wrench, or you risk cracking the porcelain.
  • Test the water level after installation; it should stop about one inch below the overflow tube.

Shut Off Water and Empty the Tank

Locate the shutoff valve on the water line behind or below the toilet. Turn it clockwise until it stops—you should feel resistance, but do not force it. If the valve is stuck or missing, you can still do this job; just work slowly and have towels ready to catch water.

Flush the toilet to drain the tank completely. If water remains in the bowl or tank, use a sponge or small cup to remove it. Dry the inside of the tank with a towel so you can see what you are working with. This step prevents water from spilling onto your floor when you remove the old parts.

Remove the Old Flapper

Look inside the tank and find the large rubber disc or cup at the bottom center—that is the flapper. It sits on top of a vertical tube called the overflow tube. On either side of the flapper, you will see two small ears or clips that hook onto the overflow tube.

Lift the flapper straight up and off the overflow tube. It may stick slightly if mineral deposits have built up; wiggle it gently side to side as you lift. The chain or rod connected to the handle will come free as you lift. Set the old flapper aside and inspect the overflow tube for cracks or damage. If the tube is cracked, you will need to replace the entire fill valve assembly (the valve and overflow tube are one piece).

Remove the Old Fill Valve

The fill valve is bolted to the bottom of the tank from underneath. You will need to work under the tank, so either crouch down or lie on your back with your head under the toilet. Look for a large nut holding the valve in place. This nut is usually brass or plastic and sits directly below the hole where the valve enters the tank.

Use an adjustable wrench to hold the nut steady while you unscrew the valve from above with your hand. Turn the valve counterclockwise. Once the valve is loose, you can finish unscrewing it by hand. Water may drip from the connection—have a towel ready. Lift the valve out of the tank and set it aside. Wipe away any debris or mineral buildup from the hole in the bottom of the tank.

Install the New Fill Valve

Before you install the new valve, check that it comes with a rubber washer and locknut. Most kits include these, but confirm they are in the box. Insert the new fill valve into the hole at the bottom of the tank, pushing it straight down until the base sits flat against the tank bottom.

From underneath, slide the rubber washer onto the valve stem, then hand-tighten the locknut clockwise. Tighten it firmly but do not use a wrench—hand pressure only. Over-tightening with a wrench can crack the porcelain tank. The nut should be snug enough that the valve does not wiggle, but you should still be able to turn it slightly with your fingers if you try.

Once the valve is find, turn the water supply back on slowly. Let the tank fill and watch for leaks around the base of the valve. If water drips from underneath, turn off the water, tighten the nut a quarter turn more, and test again. If it still leaks after a second tightening, turn off the water and check that the rubber washer is seated properly—sometimes it twists during installation.

Install the New Flapper

The new flapper comes with a chain or rod already attached. Slide the two ears or clips on the flapper onto the overflow tube, positioning the flapper so it sits flat and centered on the tube. The flapper should hang straight down without tilting to one side.

Attach the chain or rod to the handle lever. Most flappers use a chain with a hook that clips onto a small arm on the handle lever. Leave about one inch of slack in the chain when the handle is at rest—if the chain is too tight, the flapper will not seal properly and water will run constantly. If the chain is too loose, the flapper will not open fully and the flush will be weak. Adjust by moving the hook to a different link on the chain.

Test the Water Level and Flush

Let the tank fill completely. The water should stop filling when it reaches about one inch below the top of the overflow tube. If the water level is too high, adjust the fill valve. Most modern fill valves have a small clip or screw on the side that controls the water level—consult the instructions that came with your valve. If the water level is too low, the flush will be weak.

Flush the toilet and watch the flapper open and close. The flapper should rise quickly when you press the handle and drop back down smoothly when you release it. The tank should refill within 30 seconds. If the flapper sticks or does not seal, check that the chain has the right amount of slack and that the flapper is sitting flat on the overflow tube. Run two or three more flushes to confirm everything is working.

Frequently Asked Questions

What if water is still running after I replace the flapper?

The fill valve may be worn out too. If the tank drains slowly or does not stop draining, the flapper is sealing but the valve is leaking. You will need to replace the fill valve as well. If the tank drains quickly, the flapper is not sealing—check that it is sitting flat on the overflow tube and that the chain has the correct amount of slack.

Can I replace just the flapper without turning off the water?

Yes. The flapper is inside the tank and does not connect to the water supply line. You can remove and replace it with the water on, though turning the water off first makes the job cleaner and safer. If you leave the water on, flush first to empty the tank, then work quickly.

How do I know which fill valve to buy?

Bring your old valve to the hardware store and match it by size and style, or take a clear photo of the tank interior showing the valve. Most toilets use a standard 7/8-inch or 1-inch valve. The clerk can help you find the right replacement. Universal kits work in most tanks, but matching your old part ensures compatibility.

What if the fill valve nut is stuck and will not turn?

explore penetrating oil (like WD-40) around the nut and let it soak for 15 minutes. Try again gently—do not force it or you risk cracking the tank. If it still will not budge, you may need to call a plumber, as forcing it could damage the porcelain beyond repair.

Should I replace both parts at the same time?

Not necessarily. If only the flapper is worn, replace just the flapper. If only the fill valve is leaking, replace just the valve. However, if your toilet is old and both parts show mineral buildup or damage, replacing both at once saves you from having to do the job twice in the next year.