How to Replace a Toilet Fill Valve: The 15-Minute Job and the 4 Mistakes That Make It Fail

By Ariel, last updated June 30, 2026.

Replacing a toilet fill valve is a 15-minute job. The part costs $12. The four most common mistakes people make during the install (wrong water level, twisted refill tube, over-tight lock nut, leaky shank gasket) all look like the new part failed. Each looks like a defective valve. Each is actually an install error fixable in 30 seconds.

This guide walks the replacement step by step, then covers each install error with a fix. By the end you will have replaced a fill valve, set the water level correctly, and verified the bowl refills the way it should.

The fill valve swap in five steps

1. Shut off the water, drain the tank, disconnect the supply line.
2. Unscrew the lock nut from underneath the tank, pull the old fill valve out.
3. Install the new valve, hand-tighten the lock nut from underneath plus a quarter-turn with a wrench (not more).
4. Reconnect the supply line, set the height so the valve top is at least 1 inch above the overflow tube, route the refill tube to clip onto the overflow tube with the small angle adapter (do not push it inside the tube).
5. Turn water back on, set the water level 1 inch below the top of the overflow tube, test flush.

Total time: 15 minutes once the part is in hand. The part: a Fluidmaster 400A, a Korky QuietFILL 528, or a Korky QuietFILL Platinum 528MP. All universal, $10 to $14.

Is the fill valve actually the problem?

Run the dye test before you buy a part. Drop a few drops of food coloring into the tank, wait 10 minutes. If the bowl water turns blue, the flapper is leaking and the fill valve cycling is a downstream symptom; replace the flapper first.

If the bowl stays clear and you can hear the fill valve hissing, cycling on every 10 to 30 minutes, or running continuously into the overflow tube, the fill valve is the problem.

Three common fill-valve failure modes:

The fill valve hisses but does not refill. Internal diaphragm has failed; replace the valve.

A cycling valve that runs for a few seconds every 10 to 30 minutes is leaking internally back into the tank; replace the valve.

The fill valve will not shut off, water continuously runs into the overflow tube. The float is stuck or the valve is set too high. Try float adjustment first; replace if adjustment does not fix it.

Tools and parts

• Replacement fill valve, universal: Fluidmaster 400A, Korky QuietFILL 528, or Korky QuietFILL Platinum 528MP ($10 to $14)
• Adjustable wrench
• An old towel and a wide bowl or pan for catching water
• Sponge

That is the full kit. No plumber’s tape, no thread sealant, no special connectors.

Which fill valve to buy

Three universal options worth your money. We have installed every one of these in our test bathrooms.

The Fluidmaster 400A is the default, the most-installed fill valve in North America, and the one every Home Depot, Lowe’s, ACE, and hardware store stocks. $10. Easy to install, easy to adjust (cup-float style with an adjustment screw on top of the valve). Lasts 7 to 10 years in average residential service. The flow rate is the lowest of the three; refill can run noticeably slow under low supply pressure. Most households do not notice.

The Korky QuietFILL 528 (white cap) is Fluidmaster’s direct competitor at roughly the same price (about $11). Korky markets it as a faster-refilling valve; Fluidmaster does not publish comparable GPM figures. Adjustment is a tool-free twist-lock collar on the central column. Pick whichever your local store stocks; for practical purposes the two are interchangeable.

The Korky QuietFILL Platinum 528MP (silver cap) is the upgrade version of the 528. About $14. It adds a refill-amount adjuster and is designed to eliminate the refill-tube whistle that some fill valves produce in homes with hard piping or high water pressure. If you have ever heard a high-pitched whine from your tank during refill, the Platinum solves it. Otherwise unnecessary.

Skip: anything not branded Korky or Fluidmaster. The off-brand $5 fill valves have failure rates above 30% in the first year.

How to replace a toilet fill valve: the full procedure

The nine steps below are the same procedure a plumber uses on a service call. The part is a Fluidmaster 400A or Korky 528 universal fill valve, the tool is an adjustable wrench, and the total time is about 15 minutes once you have the part in hand.

Step-by-step replacement

Step 1: Shut off the water

Reach behind the toilet, find the chrome quarter-turn valve where the supply line enters the wall, and turn it clockwise until it stops. If your shutoff is the older oval-handle type, turn clockwise as far as it will go.

If the shutoff leaks when you turn it or will not shut off, that is a separate repair. Do not proceed without a working shutoff.

Step 2: Drain the tank

Take the tank lid off. Hold the flush lever down until the tank empties. Sponge or towel out the remaining water at the bottom; you want the tank as dry as possible before the next step.

Step 3: Disconnect the supply line

Underneath the tank, find the supply-line connection. Place your towel and pan under the connection (residual water will drip). Use the adjustable wrench to loosen the supply-line nut counter-clockwise. About a quarter-cup of water will come out. Catch it.

Set the supply line aside.

Step 4: Remove the old fill valve

Underneath the tank, find the plastic lock nut at the base of the fill valve. Turn it counter-clockwise by hand. If it is stiff, use the adjustable wrench (carefully; plastic strips if you over-torque). The lock nut comes off in your hand.

From inside the tank, lift the old fill valve straight up and out.

Take a moment with the old valve. If the housing is mineral-encrusted, the internal seal is the failure mode. If the housing looks clean and the float is sticking, you probably could have salvaged it with adjustment. Either way, the new valve is going in.

Step 5: Adjust the new valve’s height

Most universal fill valves have a height-adjustment collar at the base. The CL (critical level) mark on the valve must sit at least 1 inch above the top of the overflow tube. This is a code requirement (ASSE 1002 / ASME A112.1002, referenced via UPC Chapter 4 anti-siphon ballcock requirements) and a safety feature; it prevents the tank water from siphoning back into the supply line during a pressure drop.

To adjust: hold the valve next to your tank’s overflow tube. Turn the collar to extend or shorten the valve. Lock it at a length that puts the top of the valve 1 inch above the overflow tube top, with the valve seated at the bottom of the tank.

This is the single step most installation guides skip. Get the height wrong and either the valve fails inspection or it does not refill properly.

Step 6: Install the new fill valve

Drop the valve through the hole in the tank where the old one came out. The valve’s rubber shank washer should be at the bottom of the valve, seated against the inside of the tank.

From underneath the tank, slide the lock nut up the valve threads. Hand-tighten. Then use the adjustable wrench for one quarter-turn additional. Not half a turn, not full turn. Over-tightening the plastic lock nut cracks the tank around the hole (which is the next “leaking new fill valve” failure mode); do not over-tighten.

Step 7: Connect the supply line

Reconnect the supply line to the bottom of the fill valve. Hand-tighten the supply nut, then a quarter-turn with the wrench. Same warning: do not over-tighten.

Step 8: Route the refill tube

This is the install error every fill-valve replacement guide gets wrong. The new fill valve includes a thin refill tube (about 4 inches long, clear or black) that connects the valve cap to the overflow tube. Its job is to send a small stream of water into the bowl during refill to restore the bowl water level.

Three positions for the refill tube:

Correct: clip the refill tube onto the side of the overflow tube using the small angle adapter included in the box. The refill tube hangs above the overflow tube opening but does not enter it.

Wrong, type 1: push the refill tube inside the overflow tube. The water flow continues into the overflow tube even after the tank is full, which means the bowl water level is too high, and the fill valve never shuts off. This is the install error that mimics a “new fill valve does not work” complaint.

Wrong, type 2: leave the refill tube disconnected. The bowl does not refill after a flush; the bowl water level is too low; the next flush is weaker. Toilet still works, just badly.

Use the angle adapter. Clip the refill tube onto the overflow tube wall with the tube ending just above the overflow tube opening, not inside it.

Step 9: Test

Turn the shutoff valve counter-clockwise to restore water. Listen for the tank refilling. The valve should shut off cleanly when the water reaches the water-line mark on the tank wall (or 1 inch below the top of the overflow tube).

Press the flush lever. The tank empties, the fill valve starts refilling, you hear a small additional stream into the bowl (the refill tube doing its job), the tank reaches the water line, and the valve shuts off. Bowl water level returns to normal.

If anything in that sequence is off, see the next section.

The four install errors that make a new fill valve fail

1. Water level set too high

The valve is working but the water level keeps rising into the overflow tube. The fix is the float adjustment.

On a Fluidmaster 400A: turn the small plastic adjustment screw on top of the valve counter-clockwise to lower the water level (clockwise raises it). Per Fluidmaster’s install instructions, 8 turns moves the float about 1/2 inch, so each full turn changes the level by roughly 1/16 inch.

On a Korky QuietFILL 528 or QuietFILL Platinum 528MP: twist the locking collar to unlock it, slide it down the central column to lower the water level, then twist to lock. No tools needed.

Set the water 1 inch below the top of the overflow tube. Flush once to confirm the valve shuts off at the new level.

2. Refill tube pushed inside the overflow tube

This is the most common reason a new fill valve “does not work.” Lift the refill tube out of the overflow tube and clip it to the side of the overflow tube with the small angle adapter.

3. Over-tightened lock nut

The plastic lock nut at the tank-to-valve connection cracks the porcelain around the hole if you over-torque it with a wrench. Visible symptom: water dripping from underneath the tank around the fill valve.

Fix is unfortunately full: a cracked tank cannot be repaired reliably. You need a new tank (which means a new toilet for most one-piece designs). Avoid this by hand-tightening the lock nut and adding only a quarter-turn with a wrench.

4. Shank gasket twisted or missing

When the fill valve was installed, the rubber shank gasket between the valve and the inside of the tank either was not in place or got twisted. Visible symptom: water dripping from underneath the tank around the fill valve, but the tank is not cracked.

Fix: shut off water, drain tank, lift the valve out, verify the gasket is seated flat against the inside of the tank, re-install. Hand-tighten the lock nut, quarter-turn with the wrench. No more.

When to call a plumber

The fill-valve replacement above is doable by anyone who can use a wrench. Three cases warrant a plumber:

The shutoff valve does not work. Replacing the shutoff requires cutting and re-soldering or re-PEXing the supply line.

The tank cracks during install (from over-tightening or from previous stress). Toilet must be replaced.

The tank-to-bowl gasket between the tank and the bowl is the actual problem (water dripping from where tank meets bowl, not from the fill-valve area). That is a tank-removal job; doable as DIY but call a plumber if you have never lifted a tank off a bowl.

FAQ

How often should I replace my fill valve?

The Fluidmaster 400A and Korky QuietFILL 528 typically run 7 to 10 years before the internal diaphragm or seal degrades. Chlorinated municipal water and hard water both shorten the lifespan. If you can hear the fill valve cycling on by itself more than once an hour with the toilet not in use, replace it.

Why is my new fill valve still hissing?

Three possibilities. The most likely is the refill tube is pushed inside the overflow tube; pull it out and use the angle adapter. The second is the float is set too high; lower it. The third is supply pressure is too high (above 80 psi); install a pressure regulator at the supply line.

Can I install a fill valve myself?

If you can use an adjustable wrench and read a step-by-step, yes. The Fluidmaster 400A and Korky QuietFILL 528 both include diagrams. Budget 25 minutes for a first-time install (15 for the replacement, 10 for the setup details).

What is the lifespan of a toilet fill valve?

7 to 10 years on universal valves (Fluidmaster, Korky) in average residential service. Brand-specific fill valves built into premium toilets (TOTO, Kohler Class Five, Geberit) typically last 10 to 15 years. Off-brand fill valves often fail within 18 months.

Do I need to turn off the water for the whole house?

No. The chrome shutoff valve behind the toilet isolates this fixture. Turn off only the toilet’s shutoff. The rest of your house keeps water.

Sources & references

• Fluidmaster: 400A specification, installation, and adjustment documentation
• Korky: QuietFILL 528 and QuietFILL Platinum 528MP installation guide
• IAPMO: Uniform Plumbing Code Chapter 4 anti-siphon ballcock requirements (ASSE 1002 / ASME A112.1002)
• EPA WaterSense: fill valve and leak diagnostic resources

See also on My Best Toilet

• How to replace a toilet flapper for the other half of running-toilet repairs
• How to fix a running toilet for the symptom-based flowchart that points to fill valve vs flapper
• How to adjust a toilet float for the adjustment-only solution before replacement
• How to fix a wobbly toilet for base movement, which can dislodge fill-valve connections during the install