Quick answer
A toilet fill valve, also called a ballcock, is the valve inside a toilet tank that opens after a flush to let in fresh water, refills both the tank and the bowl, and closes when a float tells it the tank has reached the set level.
Plumbing codes require it to be anti-siphon, so tank water can never be pulled back into the home’s drinking water, and set so its critical level mark sits at least 1 inch above the overflow pipe.
Quick facts
The key facts about Toilet Fill Valve, with sources:
Key takeaways
- The fill valve is the inlet side of the tank: it lets water in. The flapper and flush valve are the outlet side.
- A float, either a cup that slides on the shaft or an older ball on an arm, closes the valve at the set level.
- Code requires an anti-siphon valve with its C.L. mark at least 1 inch above the overflow pipe.
- Hissing, a tank that refills on its own, or water running into the overflow are the classic failure signs.
- A stuck fill valve can waste far more water than a dripping faucet, so fix it promptly.
What a toilet fill valve does inside the tank
Lift the tank lid on most toilets and you will see two upright parts: the wide flush valve and overflow pipe in the middle, with a rubber flapper at its base, and a narrower post, usually on the left and fed by the supply line under the tank, which is the fill valve.
Its whole job is to refill. When you flush, the tank empties, the float drops and the valve opens, sending fresh water two ways: into the tank, and through a small refill tube clipped to the overflow pipe so the bowl refills as well 2.
Once the float rises to its set height, the valve closes and the tank sits quiet until the next flush.
The codes still call it a ballcock, after the old brass valve with a floating ball on a long arm 1. Most replacements sold today use a float cup that rides up and down the valve body instead.
Fill valve vs. flush valve
The fill valve lets water into the tank. The flush valve lets water out of the tank into the bowl, sealed by the flapper. A tank that keeps running can be either one, so test before buying parts.
How one refill cycle works
- Flush: The flapper lifts and the tank drains into the bowl.
- Float drops: As the level falls, the float falls with it and opens the valve.
- Tank fills: Supply water enters through the valve and refills the tank.
- Bowl refills: Part of the flow goes down the refill tube into the overflow pipe and on to the bowl, restoring the trap seal 2.
- Float rises and shuts off: At the set level, the float closes the valve. One maker recommends a level about 1/2 inch below the top of the overflow pipe 2.
- FlushTank empties
- Float dropsValve opens
- Tank and bowl refillRefill tube feeds the bowl 2
- Float risesValve closes at set level 2
- Back to step 1
Source: [2]
Which fill valve types you may find
| Type | How the float works | Notes |
|---|---|---|
| Float-cup valve | A cup slides up and down the valve body | The common modern replacement; height adjusts on the shank 2 |
| Ballcock with float ball | A ball on a horizontal arm | The older style; the code term still used for all fill valves 1 |
| Valve without a hush tube | Varies | Code then measures the 1-inch rule from the bottom of the water inlet 1 |
| Flushometer (no tank) | No float; a pressurized valve | Not a fill valve; protected by a vacuum breaker set at least 6 inches above the bowl rim 1 |
Example: setting the three heights that must line up
One maker’s installation sheet lists three heights to check after installing its fill valve, and California’s code sets the middle one; the cutaway below shows how they stack 21. This table covers what you actually turn or move to get each height right:
| Height to fix | What you adjust | How (one maker’s valve) |
|---|---|---|
| C.L. mark too low | Valve body height | Twist the shank to raise the valve until the mark sits at least 1 inch above the overflow top 21 |
| Water level too high or low | Float position | Turn the level screw; 8 turns move the float about 1/2 inch 2 |
| Bowl overfills and siphons | Bowl refill amount | Set the refill so the bowl stops at a level line marked after adding a gallon 2 |
| Refill tube in the overflow | Tube position | Clip it to the overflow pipe above the water; never push it down inside 2 |
If your old valve leaves the C.L. mark below the overflow top, the replacement must be raised, because that 1-inch air gap is what keeps tank water out of your drinking water 3.
Where the fill valve connects
Under the tank, the threaded shank of the fill valve passes through the tank bottom and is held by a locknut. The supply line from the wall stop valve screws onto that shank. Inside, the refill tube runs from a nipple on the valve to a clip on the overflow pipe 2.
On a parts list or plumber’s invoice it may appear as a fill valve, ballcock, ballcock assembly or anti-siphon fill valve. The supply connector is often replaced at the same time; one maker advises replacing it if it is worn or over 5 years old to prevent flooding 2.
What a good fill valve gets right
When it works
When it fails
- Hissing or trickling around the clock.
- Phantom refills when the tank slowly drains.
- Slow refills from a clogged inlet.
- Constant flow into the overflow, which EPA counts among common leaks 4.
What a fill valve cannot fix
A new fill valve will not stop a leak from the tank into the bowl. That is a flapper or flush valve seal problem.
It also will not fix a weak flush caused by a low water level set on purpose, a clogged trap or rim jets.
Changing the tank level changes how much water the toilet uses per flush, so do not raise it above the maker’s line.
Federal and WaterSense limits are why the level matters. EPA says older toilets can use as much as 6 gallons per flush, federal standards cap new ones at 1.6 gallons, and WaterSense models use 1.28 gallons or less 5.
A fill valve set above the maker’s line pushes a low-flow toilet back toward the old numbers.
Chemistry matters too. One maker warns that drop-in tank cleaners containing bleach or chlorine damage tank parts, may cause flooding and void its warranty 2.
What changes the cost of a fill valve repair
We do not print prices, because fair numbers need dated local quotes. A fill valve itself is a small part.
What changes the bill is whether you replace it yourself or pay for a service call, whether the supply line and wall stop valve also need replacing, and whether the tank bolts or flush valve are worn at the same time.
The bigger cost is often the water you are not seeing. EPA says a stuck fill valve can waste 4,300 gallons a day if nobody notices 4. Our leak cost calculator shows what a running toilet adds up to.
For lifespan, go by the warranty: one maker’s fill valve carries a limited seven-year warranty 2. Water quality and cleaners shorten that.
How a fill valve is replaced
A summary of one maker’s installation sheet 2. Many homeowners do this job; call a plumber if the stop valve will not close or the tank is cracked.
- Close the stop valve under the tank by turning it clockwise, flush, and sponge out the remaining water 2.
- Disconnect the supply line, remove the locknut under the tank and lift out the old valve 2.
- Set the new valve height so the C.L. mark sits at least 1 inch above the overflow pipe top 21.
- Install with the shank washer inside, hand-tighten the locknut, and do not overtighten, which can crack the tank 2.
- Clip the refill tube to the overflow pipe; do not push it down into the pipe, which wastes water 2.
- Reconnect the supply, open the valve, check for leaks, then set the water level below the overflow top 2.
How to keep a fill valve working
- Listen for hissing after the tank should be full.
- Look into the tank now and then: water should stop below the overflow pipe top 2.
- Skip bleach or chlorine drop-in tablets in the tank 2.
- Replace a worn or older supply connector when you service the valve 2.
- Do an occasional dye test to rule out flapper leaks that make the fill valve cycle 6.
Warning signs and when to call a plumber
- Water running into the overflow pipe after the tank fills.
- A refill every few minutes with nobody flushing (often the flapper, sometimes the valve).
- A hiss or whine during or after refill.
- A tank that fills very slowly.
- Water pooling under the tank at the shank or supply nut.
Do not overtighten
Tank and valve plastic crack easily. One maker warns that overtightening can crack the valve or tank and cause flooding; hand-tighten nuts only 2. If water is spreading toward outlets or a ceiling below, close the stop valve or the main shutoff first.
Call a plumber if the stop valve will not shut, the tank is cracked, or water is leaking onto the floor. See toilet repair and installation for the service side.
Code rules for fill valves
Required and anti-siphon (2025 California Plumbing Code §603.5.2, UPC-based): water closet and urinal tanks must have a ballcock, installed with its critical level at least 1 inch above the full opening of the overflow pipe.
If the valve has no hush tube, the bottom of the water inlet must sit 1 inch above that opening 1.
Product standard: the same code lists anti-siphon fill valves for gravity tanks under ASSE 1002 / ASME A112.1002 / CSA B125.12 1. Look for that marking on the package.
A city handout (Bloomington, Minnesota): requires an approved anti-siphon ballcock marked “C-L”, delivering water through an air gap or vacuum breaker; if there is no C-L mark, the bottom of the vacuum breaker counts as the critical level, and the overflow tube must not be cut off 3.
IPC states number these rules differently, and local amendments vary. Rules checked 2026-10-13.
How the fill valve differs from other tank parts
| Part | What it does | Typical failure |
|---|---|---|
| Fill valve | Lets water in and stops at the set level | Runs into the overflow; hisses |
| Flush valve | Lets water out to the bowl | Cracked seat or seal leaks |
| Flapper | Seals the flush valve between flushes | Worn rubber; most toilet leaks, per EPA 5 |
| Overflow pipe | Sends excess water to the bowl | Cut too short, causing poor flushes 3 |
| Wax ring | Seals the toilet to the drain | Leaks at the base |
Fill valve myths that waste water
- Myth A running toilet always means a bad flapper.
- Reality If water spills into the overflow, the fill valve or its level setting is the cause.
- Myth Pushing the refill tube into the overflow helps refill.
- Reality One maker warns it wastes water; clip it above instead 2.
- Myth Higher water in the tank means a better flush.
- Reality The level belongs below the overflow top, about 1/2 inch per one maker 2.
- Myth Any valve that fits is fine.
- Reality Code requires an anti-siphon valve listed to ASSE 1002 or its equivalents 1.
- Myth Bleach tablets keep the tank clean and safe.
- Reality One maker says bleach and chlorine damage parts and void the warranty 2.
Codes and water quality where you live
The 1-inch critical level rule appears in UPC-based codes such as California’s 1 and in local handouts in IPC states such as Minnesota’s 3; check what your city enforces.
Hard water is the other regional factor: scale and grit can foul fill valve seals and slow refills.
Check your area’s water with our hardness tool, and watch for refill problems after any water main work, which can stir up sediment.
When to adjust, replace or call a plumber
If water runs into the overflow, lower the float first. If the valve still hisses or will not shut off, replace it. If the tank refills on its own but nothing runs into the overflow, test the flapper before blaming the fill valve.
Next step: lift the tank lid, flush, and watch one full cycle. Note where the water stops.
Questions about Toilet Fill Valve
Why does my toilet keep running after I replaced the fill valve?
The usual causes are a water level set at or above the overflow pipe, a refill tube pushed down inside the overflow, or a flapper that still leaks.
Fluidmaster’s instructions say to set the level below the overflow top and clip the tube above it.
Then do a dye test: color in the bowl without a flush means the flapper, not the new valve, is the problem.
How do I know if my toilet fill valve is bad?
Common signs are water running into the overflow pipe after the tank is full, a constant hiss, a very slow refill, or a valve that never fully shuts off. If the tank refills on its own but nothing spills into the overflow, the flapper is the more likely cause.
What is the difference between a fill valve and a flush valve?
The fill valve lets water into the tank and shuts off at the right level. The flush valve is the large opening at the bottom of the tank that lets water out into the bowl when you flush. The flapper seals the flush valve between flushes.
How high should the water be in a toilet tank?
Below the top of the overflow pipe. One maker recommends about 1/2 inch below. Many tanks also have a water line marked inside. If water reaches the overflow top, it runs into the bowl constantly and wastes water.
What is the critical level mark on a fill valve?
The C.L. mark shows the height of the valve’s anti-siphon point. California’s plumbing code and fill valve makers require it to sit at least 1 inch above the overflow pipe opening. That gap keeps tank water from being siphoned back into your drinking water.
Is a ballcock the same as a fill valve?
Yes. Ballcock is the older name, from valves that used a floating ball on an arm, and plumbing codes still use it. Most modern fill valves use a float cup that slides on the valve body, but they do the same job and must meet the same anti-siphon rules.
Can I replace a toilet fill valve myself?
Many homeowners do. You shut off the stop valve, drain the tank, swap the valve, set its height and water level, and check for leaks. Call a plumber if the stop valve will not close, the tank is cracked, or water leaks under the tank after you finish.
How much water does a running toilet waste?
It can be a lot. EPA says a stuck fill valve can waste 4,300 gallons of water a day if it goes unnoticed, and about 20 percent of toilets leak. Most toilet leaks come from worn flappers, which waste less but run around the clock.
Why does my toilet fill valve make a hissing noise?
Hissing usually means water is still passing through the valve after it should be closed, often from a worn seal, debris or a float set too high. Lowering the float or cleaning the valve may help. If not, replacing the valve is a common fix.
Sources
- 2025 California Plumbing Code (Title 24, Part 5) §603.5 Specific Requirements, incl. §603.5.2 Ballcock, as published by GoCodeBook, retrieved .
- Fluidmaster, 400ARHR PerforMAX Fill Valve Installation Instructions (Part 4-2028), retrieved .
- City of Bloomington, Minnesota, Building and Inspection, Toilet Tank Elements Information Sheet, retrieved .
- U.S. EPA WaterSense, Know Your Flow and Curb Water Waste (EPA 832-F-26-001, February 2026), retrieved .
- U.S. EPA WaterSense, Residential Toilets, retrieved .
- U.S. EPA WaterSense, Fix a Leak Week, retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Code requirement, 1-inch critical level, hush-tube rule, ASSE 1002 listing and flushometer vacuum breaker height from 2025 California Plumbing Code §603.5 (third-party copy).
Installation steps, height checks, water level, refill tube, bleach warning, supply connector advice and warranty are from one Fluidmaster instruction sheet (single maker, labeled). Bloomington, MN handout for the C-L and overflow rules. Stuck-valve waste and leak share from EPA WaterSense. No prices.
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