What You're Replacing and Why It Fails

The water supply valve (also called the fill valve, ballcock, or inlet valve) is the mechanism inside your toilet tank that lets water in when you flush. It sits at the base of the tank, connected to the water line coming from your wall. When this valve wears out, you'll see water running constantly into the bowl, the tank won't fill properly, or water will leak from under the tank onto your floor.

Most supply valves last 5 to 10 years before the internal seals degrade. Mineral buildup from hard water speeds this up. The good news: replacing one takes about 30 minutes and costs $15 to $40 for the valve itself. You need only basic tools—a wrench, a screwdriver, and a bucket.

Key Takeaways

  • Turn off the water at the shutoff valve under the toilet, flush to empty the tank, and place a bucket under the supply line connection before you disconnect anything.
  • The supply valve bolts to the underside of the tank from outside; you'll need a wrench to loosen the nut that holds it in place.
  • Measure your new valve's shank length (the threaded part that goes through the tank) against your old one to may support it fits your tank thickness.
  • explore plumber's tape to the threads of the new valve before screwing it into the tank to prevent leaks at the connection.
  • Test for leaks by turning the water back on and letting the tank fill and refill twice before you consider the job done.

Shut Off Water and Empty the Tank

Locate the shutoff valve on the water line under the toilet. 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. Flush the toilet once to empty the tank completely.

If water is still trickling into the tank after you flush, the old valve is leaking and won't shut off fully—this is normal. Let it drain as much as it will. Place a bucket under the supply line connection (where the flexible hose meets the valve) to catch any remaining water when you disconnect it.

Disconnect the Supply Line and Remove the Old Valve

Use an adjustable wrench to loosen the nut that connects the flexible water supply line to the valve. Turn it counterclockwise by hand once it's loose enough. Water may drip—that's why the bucket is there. Unscrew the nut completely and pull the hose away.

Now look under the tank. The valve is held in place by a large nut on the underside. Use your wrench to hold the nut while you turn the valve body itself with another wrench (or your hand if it's loose enough). Once the nut is loose, unscrew it by hand and pull the old valve out through the bottom of the tank. Some water may still be in the tank—have your bucket ready.

Measure and Prepare the New Valve

Before you install the new valve, check that its shank length matches your old one. The shank is the threaded metal rod that pokes through the tank. Measure from the base of the valve body to the end of the threads. If your old valve's shank was 1.5 inches and the new one is 2 inches, the new valve won't seal properly against the tank. Most standard toilets use a 1.5-inch shank, but older or thicker tanks may vary.

Wrap the threads of the new valve with plumber's tape (also called PTFE or Teflon tape). Wrap it clockwise around the threads three to four times. This creates a watertight seal and makes the valve easier to tighten. Don't skip this step—it's the most common reason for leaks after replacement.

Install the New Valve

Insert the new valve through the hole in the bottom of the tank from outside. The shank should poke through into the tank. From inside the tank, thread the large nut onto the shank by hand first. Once it's hand-tight, use your wrench to tighten it another quarter turn. Don't over-tighten—you only need it snug enough to stop water from leaking around the base.

Reconnect the flexible water supply line to the valve outlet. Hand-tighten the nut first, then use your wrench to turn it another quarter turn. Again, snug is enough; over-tightening can crack the fitting.

Turn Water Back On and Check for Leaks

Turn the shutoff valve counterclockwise (or flip the lever back parallel to the pipe) to restore water. Watch the tank fill. It should take 30 to 60 seconds to fill completely. Listen for any hissing or dripping from under the tank or at the supply line connection.

Flush the toilet and let it refill a second time. Check again for leaks. If water is dripping from under the tank where the valve enters, the nut may need another small turn—but turn it only a quarter turn at a time. If water is leaking from the supply line connection, tighten that nut slightly. If the tank won't fill or fills very slowly, the valve may be installed backwards or the shank length may be wrong—remove it and check the orientation and measurement.

Common Mistakes to Avoid

The most frequent error is forgetting to wrap the threads with plumber's tape. Without it, the valve will leak at the base where it enters the tank, and you'll have to remove and reinstall it. Another common mistake is over-tightening the nuts. Porcelain tanks crack easily, and metal fittings strip if you force them. Snug is the goal, not maximum tightness.

Some people install the valve backwards. The inlet (where water comes in) should face the supply line, and the outlet (where water flows into the tank) should face the tank opening. Check your old valve's orientation before you remove it if you're unsure. Finally, don't assume all new valves are the same size—always measure the shank length. A valve that's too long will hit the tank bottom; one that's too short won't seal.

Frequently Asked Questions

Do I need to drain the whole toilet to replace the supply valve?

No. Shutting off the water and flushing empties the tank enough to work safely. Some water stays in the bowl, but that won't interfere with replacing the valve. Have a bucket ready under the supply line connection to catch drips when you disconnect the hose.

What if the old valve won't unscrew?

explore penetrating oil (like WD-40) to the nut and let it sit for 15 minutes. Try again with your wrench. If it's still stuck, tap the wrench handle gently with a hammer to break the corrosion loose. Never force it hard enough to crack the tank—if it won't budge after these steps, you may need a plumber.

Can I use the same supply line hose with the new valve?

Yes, unless it's cracked, kinked, or more than 5 years old. If the hose looks fine and the connection was tight before, reuse it. If you're replacing it, measure the length and buy a new one with the same diameter (usually 3/8 inch) and the same fitting size on both ends.

How do I know if the valve is installed correctly?

The tank should fill in 30 to 60 seconds after you flush. Water should stop flowing once the tank is full. There should be no drips from under the tank or at the supply line connection after 10 minutes of sitting full. If the tank fills slowly or won't stop filling, the valve may be backwards or blocked—remove and reinstall it.

What if water leaks from under the tank after I finish?

Turn off the water, empty the tank, and tighten the large nut under the tank by a quarter turn. Refill and check. If it still leaks, the shank may be too short for your tank thickness, or the plumber's tape may not have sealed properly. Remove the valve, rewrap the threads with fresh tape, and reinstall it.