What a fill valve does and why you might need to replace it
A fill valve is the mechanism inside your toilet tank that refills the bowl with water after you flush. When you press the handle, water drains from the tank, the float drops, and the fill valve opens to let fresh water back in. When the tank is full, the float rises and shuts the valve off. If your fill valve is broken, your toilet will either run constantly, refill slowly, or not refill at all.
Replacing a fill valve takes about 30 to 45 minutes and requires only basic tools you likely have at home. The hardest part is usually removing the old valve, which can be stuck or corroded. You do not need to be a plumber to do this job, and the part itself costs between $15 and $40 depending on the brand and model.
Key Takeaways
- Turn off the water supply at the shutoff valve under the toilet, then flush to empty the tank completely before you start.
- The fill valve is held in place by a large nut underneath the tank; you will need an adjustable wrench or fill valve wrench to loosen it.
- Measure your old valve or bring it to the hardware store to match the replacement, because fill valves come in different heights and connection types.
- The new valve slides in from the top of the tank and is secured from underneath with the same nut; hand-tighten first, then snug with a wrench.
- After installation, adjust the float so the water level sits about one inch below the overflow tube to prevent running and wasting water.
Shut off the water and empty the tank
Locate the shutoff valve on the water line that feeds your toilet. It is usually a small knob or lever on the pipe under the tank, near the wall. Turn it clockwise (or pull the lever toward the tank) until it stops. The water supply is now off.
Flush the toilet once to empty the tank completely. If water is still in the tank after flushing, use a sponge or small cup to remove it. You need the tank to be dry so you can see what you are doing and so water does not spill all over the floor when you remove the old valve.
Remove the old fill valve from underneath the tank
Look under the tank where the water line connects. You will see a large nut (usually brass or plastic) that holds the fill valve in place from below. This nut is often stuck tight because mineral deposits build up over years. Spray it with penetrating oil like WD-40 and let it sit for 5 to 10 minutes.
Use an adjustable wrench or a fill valve wrench (a specialized wrench designed for this job, available at any hardware store for about $5) to turn the nut counterclockwise. Turn slowly and steadily. If it does not budge after a minute of firm pressure, spray it again and wait another 5 minutes. Do not force it hard enough to crack the tank.
Once the nut is loose, unscrew it by hand. The old fill valve will now lift out from the top of the tank. You may need to wiggle it gently side to side. Disconnect the water line from the bottom of the old valve by turning the compression nut where the line meets the valve.
Measure and buy the correct replacement valve
Fill valves come in different heights and with different connection types. The height matters because it must fit inside your specific tank without hitting the overflow tube (the hollow pipe in the center of the tank). The connection type refers to how the water line attaches—some use a compression fitting, others use a threaded connection.
Measure the height of your old valve from the bottom of the mounting nut to the top of the valve body. Bring the old valve or a photo of it to your hardware store and match it to a replacement. Common brands include Fluidmaster, Korky, and American Standard. If you are unsure, describe your toilet model to the store staff—they can usually identify the right part.
Install the new fill valve
Slide the new fill valve down into the tank opening from the top. The rubber washer on the bottom of the valve should sit flat against the inside bottom of the tank. From underneath, thread the large mounting nut onto the valve stem by hand first. This prevents you from cross-threading it.
Once the nut is hand-tight, use your wrench to snug it another quarter turn. Do not over-tighten—you only need it tight enough that it does not leak. Reconnect the water line to the bottom of the new valve using the compression nut, turning it clockwise by hand until it is snug, then a half turn more with a wrench.
Turn the shutoff valve back on by turning the knob counterclockwise (or pulling the lever away from the tank). Water will flow into the tank. Watch for leaks around the mounting nut underneath and around the water line connection. If you see drips, tighten the nut a quarter turn more.
Adjust the float to the correct water level
As the tank fills, watch the float—a ball or cup that rides on the water surface. The float controls when the fill valve shuts off. Most new fill valves come with the float set too high, which means the tank overfills and water runs into the overflow tube (the hollow pipe in the center of the tank).
The correct water level is about one inch below the top of the overflow tube. If the water level is too high, locate the float adjustment clip or screw on the valve. On Fluidmaster valves, there is usually a small clip that slides along a rod—slide it down to lower the float and the water level. On other brands, you may turn a small screw counterclockwise to lower the float. Make small adjustments and let the tank refill after each one.
Once the water shuts off at the right level, flush the toilet and watch the refill cycle. The tank should fill in 30 to 60 seconds and stop on its own. If it keeps running, lower the float a bit more. If the bowl does not refill completely, raise the float slightly.
Test for leaks and proper operation
Flush the toilet three times and watch for leaks under the tank. Check the connection where the water line meets the valve and the large mounting nut. A slow drip usually means the nut needs another quarter turn with the wrench. If water is spraying or leaking fast, turn off the water, remove the valve, and check that the rubber washer is seated correctly.
Listen to the tank after it fills. A properly working toilet should be silent once the water level reaches the float setting. If you hear a hissing sound that continues for more than a minute after flushing, the fill valve may not be closing all the way. This sometimes happens if mineral deposits are stuck on the valve seat. Turn off the water, remove the valve again, and rinse the rubber seal and the valve opening with clean water before reinstalling.
Frequently Asked Questions
What if the old fill valve nut will not budge?
Spray it with penetrating oil and wait 10 to 15 minutes, then try again. If it still will not turn, try tapping the wrench gently with a hammer to break the corrosion loose. As a last resort, you can carefully use a hacksaw to cut a slot in the top of the nut and use a flat-head screwdriver to turn it, but this risks damaging the tank if you slip.
Can I replace just the fill valve seal instead of the whole valve?
Some fill valves have replaceable seals, but most modern ones do not. If your valve is less than 10 years old and only leaks slightly, you can try turning off the water, removing the valve, and rinsing the rubber seal and valve seat with clean water. For older valves or if rinsing does not work, replacement is usually the better choice.
How do I know if my fill valve is the problem and not something else?
If your toilet runs constantly or refills slowly after flushing, the fill valve is usually the culprit. If the bowl does not refill at all, it could be the fill valve or a closed shutoff valve. Check that the shutoff valve under the tank is fully open first. If the bowl refills but water runs into the overflow tube, the float is set too high or the fill valve is not closing properly.
Do I need to turn off the water at the main shutoff if the toilet shutoff valve does not work?
Yes. If the shutoff valve under the toilet will not turn or leaks when you try to close it, turn off the main water supply to your house instead. The main shutoff is usually in the basement, crawlspace, or outside near the meter. Once you replace the toilet shutoff valve, you can turn the main water back on.
What should I do with the old fill valve?
Most hardware stores accept old plumbing fixtures for recycling. You can also throw it in the trash. Do not pour any water that was in the tank down the drain unless you emptied it completely—old tank water can contain mineral deposits or rust that may clog your pipes.