How to Replace a Toilet Fill Valve (Step-by-Step Guide)
Replace a toilet fill valve yourself in 20 minutes for $12. The Fluidmaster 400A fits most toilets. Covers removal, installation, and water level adjustment.
Quick answer: To replace a toilet fill valve, shut off the supply valve behind the toilet, flush to empty the tank, unscrew the locknut under the tank, lift the old valve out, drop in a new Fluidmaster 400A ($12, fits most toilets), hand-tighten the locknut, reconnect the supply line, and adjust the float so water stops an inch below the overflow tube. Total job: 20 to 30 minutes, no special tools.
A toilet fill valve is the tall plastic tower on one side of the tank. It opens after each flush to refill the tank from the supply line, then closes when the float rises to the correct level. When it starts to fail, the toilet either won’t refill, refills very slowly, or runs with a continuous hissing sound even though nobody flushed. If you’re not sure what’s actually wrong, diagnosing a toilet that won’t fill up covers the full list before you start pulling parts.
This is one of the easier plumbing jobs in the house. No soldering, no drain work, nothing structural. The part costs $12. The job takes under thirty minutes. Plumbers bill $125 to $175 to do exactly what you’re about to do.
What You’ll Need
- Fluidmaster 400A fill valve ($12-15 at any hardware store; this is the standard)
- Adjustable pliers or slip-joint pliers
- Old towel or sponge
- Small bucket or shallow basin
- Adjustable wrench (optional, for a stubborn supply line nut)
One note on the Fluidmaster 400A: it fits most toilets built in the last 30 years and adjusts from 9 to 14 inches tall, which covers nearly all residential tank depths. If your toilet is older than the mid-1980s or uses a non-standard tank, the Korky 528 is a solid alternative at about the same price. Both work fine; I’ve used both.
Step 1: Shut Off Water and Empty the Tank
Find the supply valve behind the toilet near the floor. It’s a small oval knob or a straight slot handle. Turn it clockwise until it stops. If the valve is corroded and won’t close fully, you’ll need to shut off water to the house at the main before proceeding.
Flush the toilet to drain the tank. Most of the water drains in one flush; the last inch or so at the bottom doesn’t go anywhere. Use a sponge or an old towel to soak up what’s left. You don’t need a perfectly dry tank, just dry enough to avoid a sheet of water dripping when you pull the supply line.
Set the tank lid on a folded bath towel on the floor. Porcelain is heavier than it looks and cracks on tile without much warning.
Step 2: Disconnect the Supply Line and Remove the Old Fill Valve
There are two connections to undo.
Under the tank: The supply line attaches to the bottom of the fill valve with a threaded coupling. Unscrew it counterclockwise by hand or with adjustable pliers. Have your bucket ready; a little residual water will drip when you break this joint.
Also under the tank, you’ll find the locknut holding the fill valve in place. It’s usually plastic, occasionally brass on older toilets. Unscrew it counterclockwise by hand. It should come loose without much effort; if not, channel-lock pliers give you the grip.
Inside the tank: Lift the old fill valve straight out. Note where the refill tube connects to the overflow tube (the tall center pipe). Unhook it. Toss the old valve.
Step 3: Set the Height on the New Fill Valve
Before installing the Fluidmaster 400A, set the height. Grasp the top of the valve with one hand and the lower body with the other, then turn counterclockwise and pull to extend or push to compress. The goal: the critical level mark (a small arrow on the shaft, labeled “CL”) should sit at least 1 inch above the top of the overflow tube.
If the CL mark ends up below the overflow tube after installation, you’ll get a siphoning problem: water slowly drains back into the bowl even with the flapper seated. Takes 10 seconds to check before the valve goes in. Check it.
Step 4: Install the New Fill Valve
Lower the fill valve into the tank hole. The rubber washer at the base of the shaft goes inside the tank, seated against the opening. Feed the threaded shank down through the hole.
Under the tank, thread the locknut clockwise onto the shank. Hand-tighten until snug, then a quarter-turn with pliers. That’s all it needs. The rubber washer does the sealing; over-tightening cracks the plastic tank floor. If you hear the porcelain starting to flex, back off.
Reattach the supply line to the threaded connection at the bottom of the shank. Hand-tight plus a quarter-turn with pliers. Same logic applies.
Step 5: Attach the Refill Tube
The refill tube comes with the Fluidmaster kit: the small flexible tube that runs from a nipple on the fill valve to the overflow tube. Clip the black refill clip onto the rim of the overflow tube, then press the tube into the clip. The tube should hang over the overflow tube but not extend down inside it.
This matters. If the refill tube runs down into the overflow tube, it creates a continuous siphon: water trickles out of the tank, the fill valve runs to compensate, and the toilet hisses around the clock. A hissing toilet right after a fill valve replacement is almost always this.
Step 6: Turn On Water and Set the Float Level
Open the supply valve slowly. The fill valve will activate and start filling the tank. Watch the water rise.
On the Fluidmaster 400A, the float slides on the fill valve shaft. Pinch the release tab and slide the float up to raise the water level, down to lower it. The target: water stops about 1 inch below the top of the overflow tube. There’s usually a waterline mark on the inside tank wall labeled “WL.” Use that as your reference.
When the tank fills completely and the fill valve shuts off, flush once. The valve should kick on immediately and refill the tank in 60 to 90 seconds for a standard 1.6-gallon toilet. Significantly longer than that suggests the supply valve isn’t fully open.
Put the tank lid on only after two full flush cycles confirm everything is working. Skipping this test and sealing the lid is how a 25-minute job turns into an hour.
Common Mistakes
Over-tightening the locknut. Quarter-turn after hand-tight. People who work on metal fixtures instinctively torque harder; that habit cracks porcelain tanks.
Refill tube running into the overflow tube. It looks tidier. It causes the toilet to hiss indefinitely. Clip it at the top of the tube, not inside.
Float set too high. If the float is set so the waterline reaches the rim of the overflow tube, water continuously trickles over into it and the fill valve never fully shuts off. A quick slide of the float down the shaft fixes it.
Not checking the supply line washer. The supply line has a rubber washer inside the coupling nut. If it’s cracked or deformed, the connection weeps even when tightened correctly. Takes two seconds to inspect. Replacement washers are 50 cents.
Assuming the fill valve is the problem when the flapper is the actual cause. Before swapping the fill valve, do the finger test: press down on the flapper with one finger while the toilet is running. If the running stops, the flapper is leaking, not the fill valve. A toilet flapper replacement is a $5 fix and takes 10 minutes.
FAQ
How do I know my toilet fill valve needs replacing?
Three signs: the toilet hisses continuously even after confirming the flapper seals (press down on the flapper; if the running doesn’t stop, the fill valve is suspect), the tank takes 3 to 5 minutes to refill instead of 60 to 90 seconds, or the toilet refills correctly but then starts running again 20 to 30 minutes later. That last one usually means the fill valve’s internal diaphragm has worn through. The toilet repair guide covers the full diagnostic flow.
What fill valve fits my toilet?
The Fluidmaster 400A fits the large majority of toilets built after 1985. It adjusts from 9 to 14 inches and fits standard 2-inch tank holes. If your toilet has a 7/8-inch diameter shank hole (some older low-profile tanks), the Fluidmaster 400AH is the right model. When in doubt, take the old fill valve to the hardware store and set it next to the options. The visual match is immediate.
How tight should the locknut be?
Hand-tight plus a quarter-turn with pliers. The rubber washer at the base of the shank does the sealing work. More clamping force doesn’t improve the seal; it risks cracking the tank floor or stripping the plastic threads. If you see the tank bottom deforming as you tighten, you’ve already gone too far.
Why is my toilet still running after I replaced the fill valve?
Two most common causes: the refill tube is inserted down into the overflow tube instead of clipping at the top (pull it out and reclip), or the flapper has been the problem all along and the fill valve swap didn’t fix it. Press down on the flapper with a finger while the toilet is running. If the sound stops, the flapper is leaking. The running toilet repair guide covers both scenarios with the diagnostic order.
How long does a fill valve last?
Fluidmaster rates the 400A at 5 to 7 years under normal use. In practice, most people don’t replace them until the valve fails, which tends to happen around the 7 to 12 year mark in areas with average water quality. Hard water and chloramines (common in municipal water systems) shorten that range. If you’re replacing a fill valve on a toilet that’s more than 10 years old, spending another $5 on a new flapper at the same time makes sense; the tank is already empty, and flappers on older toilets are often near the end of their service life.
What’s the difference between a fill valve and a flush valve?
The fill valve is the vertical tower that brings fresh water into the tank. The flush valve is the seat and overflow tube assembly in the center of the tank that the flapper seals against. They’re separate parts. A leaking flapper is a flush valve problem; a hissing or slow-refilling tank is usually a fill valve problem. If you’re replacing a flapper rather than the fill valve, the toilet flapper replacement guide covers that job separately.
How much does it cost to replace a toilet fill valve?
The Fluidmaster 400A runs $12 to $15 at hardware stores. A plumber’s service call to do the same swap runs $125 to $175 minimum before parts. The EPA WaterSense program puts a continuously running toilet at up to 200 gallons per day in wasted water. At typical US water rates, that’s $20 to $70 extra per month on the water bill. The fill valve is one of the better uses of $12 and 30 minutes in the house.