How to Replace a Toilet Fill Valve

A failing fill valve—also called a ballcock or inlet valve—is one of the most common toilet problems, and it's completely fixable in under an hour. You'll know it's the culprit when the tank won't fill properly after flushing, runs constantly, or hisses and refills on its own. The good news is that modern fill valves are reliable, inexpensive, and straightforward to swap out. This is a repair that doesn't require a plumber and doesn't demand advanced skills; just patience and a wrench.

1. Cut the Water Supply First. Locate the shutoff valve on the water supply line (usually on the wall or floor behind the toilet) and turn it clockwise until tight. Flush the toilet to drain the tank completely. If water remains, use a sponge or small cup to remove the last inch or so from the bottom.
2. Separate Tank from Supply. Use an adjustable wrench to hold the nut on the underside of the toilet tank (the one connecting the supply line) while you turn the nut at the fill valve with a second wrench. Turn counterclockwise until hand-tight, then finish unscrewing by hand. Pull the supply line away from the tank.
3. Extract the Old Valve. From underneath the tank, use a large adjustable wrench or fill-valve wrench to grip the locknut (the wide hexagonal nut holding the valve in place). Turn counterclockwise while steadying the valve from inside the tank with your other hand so it doesn't spin. Once loose, unscrew by hand and lift out the old valve.
4. Clear Debris and Corrosion. Wipe away any mineral deposits, corrosion, or debris from the opening in the bottom of the tank using a dry cloth or soft brush. This ensures a clean seal with the new valve and prevents leaks.
5. Seat the New Valve Firmly. Insert the new fill valve through the tank opening from the inside. From underneath, hand-thread the locknut onto the valve threads, then use your wrench to tighten it firmly—snug but not brutally tight. You'll feel resistance when the rubber washer seats properly.
6. Reattach the Supply Line. Thread the supply line nut onto the fill valve by hand first, then tighten it with a wrench using the same two-wrench method you used to disconnect it. The connection should be snug but not over-tightened, which can crack plastic fittings.
7. Calibrate the Float Level. Turn the shutoff valve back on and let the tank fill. Most fill valves have a float clip or adjustment screw that sets the water level—aim for about an inch below the overflow tube. Once adjusted, flush the toilet twice to confirm it refills smoothly and shuts off cleanly.
8. Verify No Leaks Present. Watch under the tank for drips around the locknut and supply line connection for a full two minutes. If you see water, snug the offending fitting slightly with a wrench. Let the toilet sit for an hour, then check again—slow weeping sometimes stops as washers settle.