What a fill valve does and why it fails

The fill valve is the mechanism inside your toilet tank that refills the bowl after you flush. Water enters through the valve, and a float arm rises as the tank fills, shutting off the water when it reaches the right level. When the fill valve wears out, you get a running toilet, weak refills, or water that trickles constantly into the overflow tube.

Fill valves fail because of mineral buildup in hard water, debris caught in the valve seat, or straightforward age — most last 5 to 10 years. Replacing one is a straightforward job that takes about 30 minutes and costs $15 to $40 for the valve itself. You do not need a plumber.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the tank, then flush to empty the tank completely.
  • The fill valve sits in the center of the tank bottom and is held in place by a large nut underneath — you will need an adjustable wrench to remove it.
  • Measure your tank opening or bring the old valve to the store to buy the correct replacement size, as fill valves come in different diameters.
  • Install the new valve in reverse order: nut underneath first, then tighten from below while holding the valve steady from above.
  • Turn the water back on slowly and adjust the float arm so the water stops when the tank is three-quarters full.

Shut off water and drain the tank

Locate the shutoff valve on the water supply line beneath the toilet tank — it looks like 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 continues to trickle in after you flush, the fill valve itself is stuck open — this is normal and does not affect the replacement. If no water comes out when you flush, the shutoff valve was already closed or the tank is empty from a leak.

Remove the old fill valve from below the tank

Look under the toilet tank where the water supply line enters. You will see a large nut (usually brass or plastic) holding the fill valve in place from underneath. Place a bucket or towel underneath to catch any remaining water in the line.

Use an adjustable wrench to turn that nut counterclockwise while you hold the fill valve steady from inside the tank with your other hand — this prevents the valve from spinning and damaging the tank. Once the nut is loose, unscrew it by hand and pull the old valve out through the top of the tank. The rubber gasket (a flat ring) will come out with it or stay stuck to the tank bottom — remove it if it stays.

Buy and prepare the replacement valve

Fill valve sizes are measured by the diameter of the opening they fit into. Standard tanks use either a 1.25-inch or 1.5-inch valve. Measure the hole in your tank with a ruler, or bring the old valve to a hardware store and hold it up to the replacement options on the shelf — they will look identical in size.

Common replacement brands include Fluidmaster, Korky, and Kohler. All work the same way. Buy a valve that comes with a new gasket and locknut. Before installation, hold the new valve up to the light and look through the inlet opening at the top — it should be clean and free of debris. If you see sediment, rinse it under running water.

Install the new fill valve

Insert the new fill valve through the hole in the tank from the top, with the inlet (where water enters) pointing toward the back of the tank. Slide the rubber gasket onto the threaded part of the valve from underneath, then hand-tighten the locknut onto the threads.

Now use your adjustable wrench to tighten the nut from below while you hold the valve steady from above — tighten until snug, but do not over-tighten, as this can crack the tank. The valve should not spin or move when you push on it. Reconnect the water supply line to the inlet at the top of the valve, hand-tightening the connection first, then using a wrench to snug it another quarter-turn.

Turn water back on and adjust the float

Turn the shutoff valve counterclockwise (or flip the lever parallel to the pipe) to restore water. Open it slowly — you may hear air hissing out of the fill valve, which is normal. Let the tank fill completely, then flush and watch it refill.

The water should stop filling when the tank is about three-quarters full. If it fills past that point or does not fill high enough, adjust the float arm. Most modern fill valves have a clip on the arm that you can slide up or down the valve body to change the shutoff point — slide it down to lower the water level, up to raise it. Flush again and check. Once the level is correct, you are done.

What to do if the new valve leaks

If water drips from under the tank after installation, the locknut is not tight enough. Turn off the water, empty the tank, and tighten the nut another quarter-turn with your wrench. If water leaks from the connection where the supply line meets the valve inlet, turn off the water and tighten that connection with a wrench — do not over-tighten.

If water still leaks after tightening, the gasket may be pinched or damaged. Turn off the water, remove the nut, pull out the valve, and check that the gasket is seated flat and not torn. If it looks damaged, replace it with a new one from the hardware store. Reinstall the valve and tighten again.

Frequently Asked Questions

Do I need to turn off the water to replace the fill valve?

Yes. Always turn off the shutoff valve under the tank before you start. If you do not, water will spray out when you remove the old valve and you will have a mess to clean up.

What if the locknut is stuck and will not turn?

explore penetrating oil (like WD-40) around the nut and let it sit for 15 minutes. Try again with your wrench. If it still will not budge, tap the wrench handle gently with a hammer to break the corrosion loose, then turn slowly. Do not force it — you risk cracking the tank.

Can I replace the fill valve without removing the tank?

Yes — you work from inside the tank and underneath it without lifting it off. The only exception is if the tank is cracked or the fill valve hole is damaged, in which case the tank itself needs replacement.

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

If your toilet runs constantly, refills slowly, or water trickles into the overflow tube (the hollow pipe in the center of the tank), the fill valve is likely worn. Try turning off the shutoff valve — if the running stops, the fill valve is the problem. If water still runs, the issue is the flapper or flush valve instead.

What if I accidentally crack the tank during removal?

Stop when ready and turn off the water. A cracked tank cannot be repaired and must be replaced. This is rare if you hold the valve steady while turning the nut, but it can happen if you force the nut or explore pressure to the tank walls.