Toilet Fill Valve Diagram: How It Works and Parts Explained

Remove the lid from your toilet tank and look inside. You will see two main assemblies: the tall vertical component on the left is the fill valve, and the assembly in the center covering the drain opening at the bottom of the tank is the flush valve and flapper. The fill valve is the part responsible for refilling the tank after a flush and then stopping the water flow at exactly the right level.

Modern fill valves -- including those found on TOTO Drake, TOTO Drake II, Kohler Highline, Kohler Cimarron, American Standard Champion 4, and American Standard Cadet 3 -- use a float-cup design. The float cup rides up and down a central shaft. Older toilets use the ballcock design, where a rubber or plastic float ball sits at the end of a horizontal arm. Both types perform the same function, but float-cup valves are adjustable without tools, quieter, and less prone to the annoying hissing and chattering that plagues aging ballcock designs.

For context on how the fill valve fits into the overall tank assembly, see our toilet parts diagram and our guide on how a toilet fill valve works.

The diagram below maps every part of a standard float-cup fill valve. Each labeled component is explained in full detail beneath the diagram. Understanding each part helps you diagnose problems precisely and order the correct replacement component without guessing.

1. Cap and Seal Assembly

The cap sits at the very top of the fill valve body and contains the primary shut-off mechanism. In modern float-cup valves, a rubber diaphragm or piston inside the cap compresses to block water flow when the float rises to the set position. In older ballcock valves, the cap assembly houses a plunger that seals against a brass seat. The cap is the most common source of seal failure. When the diaphragm cracks or becomes stiff with age, the valve cannot fully close and water continues trickling into the tank or over the overflow tube.

On Fluidmaster 400A valves (the most widely sold fill valve in North America), the cap unscrews counter-clockwise to reveal the diaphragm seal. Replacing that seal -- sold as a repair kit for under $5 -- restores like-new shutoff without replacing the entire valve. TOTO's fill valves on models like the Aquia IV and UltraMax II use a proprietary cap design, so always check the model number before ordering a replacement seal.

2. Float Cup

The float cup is a hollow plastic cylinder that slides up and down the fill valve shaft. As the tank empties during a flush, the float cup drops, which opens the valve and allows water in. As the tank refills, the rising water lifts the float cup. When the float cup reaches the preset height, a lever arm attached to the cap assembly closes the diaphragm and shuts off water flow. The entire process takes between 60 and 90 seconds on a properly functioning residential toilet.

The float cup design replaced the old float-ball-and-arm ballcock because it takes up less space, it is not affected by water splashing during the flush cycle, and it is adjustable without bending a metal arm. To raise or lower the water level on most float-cup valves, simply pinch the clip that attaches the float to the shaft and slide it up or down, or rotate the adjustment screw on top of the cap.

3. Adjustment Clip or Screw

Every float-cup fill valve includes a water-level adjustment mechanism. On Fluidmaster valves, a screw on top of the cap rotates to raise or lower the cutoff point. On some valves, a spring clip on the side of the float cup slides to reposition the float. The adjustment range is typically 7 to 13 inches of height, which covers the depth range of nearly all residential toilet tanks from the TOTO Drake (tank depth approximately 12 inches) to the Woodbridge T-0001 and Swiss Madison St. Tropez, which have slimmer tanks around 8 to 9 inches.

The correct water level inside the tank is 1 inch below the top of the overflow tube. Measure with a ruler before adjusting. Setting the water level too high causes constant running as water trickles into the overflow tube. Setting it too low reduces the force of each flush and can result in incomplete bowl clearance and poor MaP performance.

4. Main Body and Shaft Column

The main body is the rigid vertical tube that runs from the lock nut at the bottom of the tank to the cap assembly at the top. Inside this column, water travels upward from the supply line inlet to exit through ports near the top of the valve, filling the tank. The float cup travels along the outside of this column. Some valves have a telescoping column that adjusts to different tank heights; others are a fixed height with a range of adjustment built into the float clip.

Hard water is the primary enemy of the main body. Calcium and magnesium deposits accumulate inside the column over time, restricting flow and causing slow fill times. If your tank takes more than 3 to 4 minutes to refill fully, sediment inside the main body column -- or a clogged inlet screen -- is the most common cause before the valve itself has mechanically failed.

5. Inlet Screen and Filter

At the bottom of the main body, just above or inside the shank, sits a small mesh screen. Its job is to catch sediment, pipe scale, and debris from the house water supply before those particles can reach the delicate diaphragm seal inside the cap assembly. Debris that bypasses this screen can lodge in the diaphragm and hold the valve partially open, causing the toilet to run constantly.

Cleaning the inlet screen is a straightforward maintenance task. Shut off the supply valve, flush to drain the tank, then turn the cap of the fill valve counter-clockwise and lift it off. Use needle-nose pliers to pull out the screen, rinse it under running water, reinstall it, and replace the cap. This 5-minute job often resolves slow fill and continuous running issues without any replacement parts needed. Homes with well water or older galvanized supply lines benefit most from periodic screen cleaning.

6. Shank

The shank is the threaded section at the bottom of the fill valve that passes through the pre-drilled hole in the floor of the toilet tank. The shank has a flat or star-shaped anti-rotation grip just above the tank bottom that prevents the valve from spinning when you tighten or loosen the lock nut below. Standard fill valves use a 7/8-inch outside-diameter shank that fits the industry-standard 1-inch tank opening used by TOTO, Kohler, American Standard, Gerber, and virtually all residential brands.

7. Lock Nut

The plastic or brass nut that threads onto the shank from below the tank is the lock nut. Hand-tighten this nut, then add no more than one quarter turn with pliers. Overtightening the lock nut is a common DIY mistake: too much torque cracks plastic tank bases, particularly on one-piece designs like the TOTO UltraMax II or Woodbridge T-0001, where the tank and bowl are a single molded unit. The lock nut only needs to be snug enough to prevent the shank washer from leaking.

8. Shank Washer and Seal

A rubber cone washer or flat washer sits between the lock nut and the bottom exterior of the tank. This washer compresses when the lock nut is tightened, creating the watertight seal that prevents water from leaking around the shank where it passes through the tank. Shank washers harden and shrink with age, especially in homes with hot-water-fed toilet supply lines, and a failed shank washer is a common cause of slow drips from the base of the tank. Most fill valve replacement kits include a new shank washer as standard.

9. Refill Tube

The refill tube is the small-diameter flexible tube that runs from a port near the top of the fill valve body over to the overflow tube, where it clips in place and directs a thin stream of water down into the bowl during the refill cycle. This bowl-refill stream replenishes the water that sits in the toilet bowl trap -- the curved section at the base of the bowl that maintains a water seal preventing sewer gases from entering the home.

A missing, disconnected, or kinked refill tube causes the bowl water level to be too low after every flush. This low bowl level exposes the P-trap and allows sewer gas odors to pass through. If your toilet bowl has less than 2 to 3 inches of visible water after a complete refill cycle, check whether the refill tube is properly seated in the overflow tube and producing a steady trickle during the fill cycle.

10. Supply Line Connector

At the very base of the shank, below the tank, the supply line connects via a slip-joint or threaded coupling. Most modern installations use a 3/8-inch compression-fitting supply line that connects to a 1/2-inch IPS or 7/8-inch slip joint on the shank. The supply line itself is not a fill valve part, but it is the last link in the water delivery chain. A kinked, corroded, or improperly seated supply line reduces flow rate to the fill valve and causes slow tank refill independent of any valve malfunction.

The ballcock design dates to the mid-1800s and remained the standard fill valve style in American homes until the 1990s. It is identifiable by the large rubber or plastic sphere on the end of a horizontal brass or plastic arm. As tank water drops, the ball falls, pulling the arm down and opening a plunger valve at the pivot point. As water rises, the buoyant ball lifts the arm until it closes the plunger. Adjusting the water level on a ballcock requires physically bending the arm upward (to raise the level) or downward (to lower it), a process that is imprecise and requires retesting after each bend.

Float-cup valves became the dominant design after the Energy Policy Act of 1992 mandated that new toilets use no more than 1.6 gallons per flush (GPF). Achieving consistent 1.6 GPF performance requires a precise water level cutoff that the ballcock could not reliably provide. Float-cup valves deliver that precision via tool-free adjustment and are the standard fill valve on every toilet from major brands today, including all TOTO Tornado Flush and Double Cyclone models, the Kohler Class Five lineup, and all American Standard ActiClean and VorMax designs.

A third type -- the floatless fill valve -- is used in some pressure-assist and commercial toilets. These valves sense water pressure at the supply inlet rather than using a buoyant float, allowing them to operate at very precise cutoff pressures. The Flushmate pressure-assist system used in some American Standard and Gerber commercial units relies on a floatless design integrated into the pressure vessel.

The fill valve cycle has six distinct phases, each tied to a specific component working in sequence:

1. Flush initiation. Pressing the handle rotates the trip lever inside the tank, lifting the flapper chain. The flapper lifts off the flush valve seat, and approximately 1.28 to 1.6 gallons of stored water rapidly exits through the flush valve into the bowl, forcing waste through the trapway and into the drain stack.
2. Float drop. As tank water drains away in 3 to 5 seconds, the fill valve float cup or ball drops with the water level. This drop mechanically opens the diaphragm or plunger inside the cap assembly, allowing pressurized water from the supply line to enter the valve body.
3. Water entry and screening. Water from the household supply line (typically 20 to 80 PSI in residential systems) enters through the shank at the bottom of the valve. It passes through the inlet mesh screen, which catches any particulate material, then travels up the inside of the main shaft column.
4. Tank fill and bowl refill. Water exits the top of the fill valve shaft through ports that direct flow into the tank. Simultaneously, a separate small stream exits through the refill tube port and runs down the overflow tube into the bowl, replenishing the water seal in the P-trap.
5. Float rise and approach to cutoff. As the tank fills, the float cup rises along the shaft. The float's position continuously reduces the opening of the diaphragm, gradually reducing flow rate as the float nears the target level.
6. Shutoff. When the float reaches the preset water level (set by the adjustment clip or screw), the diaphragm closes completely. Water flow stops, and the tank is ready for the next flush. Total refill time on a properly functioning fill valve with adequate supply pressure is 60 to 90 seconds for a 1.28 GPF tank and 90 to 120 seconds for a 1.6 GPF tank.

Constant Running

A toilet that runs continuously almost always traces to one of two fill valve issues: the float is set too high, causing water to constantly trickle into the overflow tube, or the cap diaphragm has failed and the valve cannot fully close. To distinguish between them, remove the tank lid. If you can see or hear water draining into the overflow tube, lower the float adjustment. If the water level is correct but the toilet still runs, the diaphragm seal is the cause. On Fluidmaster 400A valves, a $3 to $5 seal kit is the repair. On other brands, full valve replacement at $10 to $25 is often easier than sourcing proprietary parts.

For a step-by-step repair walk-through, see our guide on how to fix a running toilet.

Slow Tank Fill

If the tank takes more than 3 minutes to refill after a flush, the fill rate is restricted. The most common causes in order of likelihood are: clogged inlet screen (solved by cleaning, no parts needed), kinked supply line, supply shut-off valve not fully open, low household water pressure (check with a gauge at an outdoor hose bib), or internal scale buildup inside the fill valve body. Check the inlet screen first before assuming valve replacement is needed. Many slow-fill complaints resolve with a 5-minute screen cleaning.

Hissing or Whistling Noise

A hissing sound during the fill cycle is normal if it is brief and quiet. A loud hiss or whistle that lasts throughout the fill cycle points to a diaphragm that is failing to open fully (restricting flow and creating turbulence), high supply pressure above 80 PSI (causing turbulence inside the valve), or debris partially blocking the inlet port. A persistent post-fill hiss (after the tank appears full) signals that the diaphragm is not sealing completely. See our dedicated article on toilet fill valve noise causes and fixes for specific solutions by noise type.

Leaking at Tank Base

Water puddling beneath the tank on the left side typically traces to the fill valve shank washer -- the rubber cone or flat seal between the lock nut and the exterior bottom of the tank. This washer hardening with age or cracking from chemical cleaning products allows water to seep along the shank threads. Shut off the supply valve, drain the tank, and replace the shank washer. Do not overtighten the lock nut: firm hand pressure plus a quarter turn with pliers is sufficient. Also check the supply line connection at the shank for a loose fitting, which produces a similar drip in the same location.

Tank Not Filling at All

If the tank produces no water after a flush, check the supply shut-off valve (the oval handle or football-shaped valve behind and below the toilet) first. It may have been bumped closed or failed internally. Turn it counter-clockwise to fully open. If the supply valve is open but no water flows, the inlet to the fill valve may be fully blocked by debris, or the fill valve may have a catastrophic mechanical failure. For a complete diagnostic tree, see our guide on toilet not filling with water.

Water level adjustment is the most common fill valve maintenance task homeowners perform themselves, and it requires no tools on most modern valves. The target is always 1 inch below the top of the overflow tube -- a manufacturer-specified clearance that appears in the installation documentation for TOTO, Kohler, American Standard, Gerber, and Fluidmaster fill valves. This 1-inch clearance provides a safety margin so water cannot drain continuously into the overflow tube while still keeping enough water in the tank for a full, effective flush.

To adjust a clip-style float-cup valve:

1. Locate the spring clip on the side of the float cup where it contacts the adjustment shaft.
2. Pinch the clip inward and slide the float cup up (to raise the water level) or down (to lower it).
3. Release the clip to lock the float at the new position.
4. Flush and observe the refill. The water should stop 1 inch below the overflow tube top.
5. Repeat the adjustment until the level is correct.

To adjust a Fluidmaster 400A screw-top valve:

1. Locate the adjustment screw on the very top of the valve cap (a flat-head slot visible from above).
2. Turn clockwise to raise the water level or counter-clockwise to lower it.
3. Each quarter turn moves the cutoff point approximately 1/4 inch.
4. Flush and recheck. The refill should stop with water 1 inch below the overflow tube top.

To adjust an older ballcock fill valve:

1. Locate the metal or plastic arm connecting the float ball to the valve body.
2. Grip the arm near the center and gently bend it upward to raise the water level or downward to lower it. Make small, incremental bends.
3. Alternatively, some ballcocks have an adjustment screw at the pivot point that moves the ball position without bending the arm.
4. Flush and recheck. The imprecision of the ballcock often requires two or three adjustment attempts.

Full fill valve replacement is a straightforward DIY repair that most homeowners can complete in under 30 minutes. The tools you need are an adjustable wrench or channel-lock pliers, a sponge or wet/dry vacuum, and a new fill valve compatible with your tank depth. For a complete walkthrough with photos, see our dedicated guide on how to replace a toilet fill valve. Here is the process in summary:

1. Shut off the water supply. Turn the oval shut-off valve behind the toilet clockwise until it stops. Flush to drain most of the water from the tank.
2. Remove remaining water. Use a sponge or wet/dry vacuum to remove the last inch of water in the tank. This keeps the work area dry.
3. Disconnect the supply line. Hold the fill valve shank steady with one hand and unthread the supply line nut with the other. Have a towel ready for the small amount of water still in the line.
4. Remove the refill tube. Pull the refill tube off the overflow tube and then off the port on the fill valve body.
5. Unthread the lock nut. Below the tank, unthread the plastic or brass lock nut from the shank. Turn counter-clockwise. No tools are needed for plastic nuts; use pliers for brass. Lift the old fill valve out of the tank from above.
6. Prepare the new fill valve. Adjust the new valve height to match your tank depth before installing it. The shank should protrude at least 1/2 inch below the tank bottom; the top of the valve should be at least 1 inch above the water line you plan to set.
7. Install the new valve. Insert the shank through the tank hole. Position the anti-rotation tab if present. Thread the lock nut on from below and tighten hand-tight plus a quarter turn. Do not overtighten.
8. Reconnect the supply line and refill tube. Thread the supply line nut onto the shank. Attach the refill tube from the valve port to the inside of the overflow tube, ensuring the tube does not sit below the water line (this causes siphoning).
9. Restore water and adjust. Open the shut-off valve slowly. Let the tank fill. Check for leaks at the shank washer and supply line connection. Adjust the float to set the water level 1 inch below the overflow tube top.
10. Test and verify. Flush three times and observe the refill cycle. Tank should refill in 60 to 90 seconds and stop cleanly at the set water level without any continued running or hissing.

For a look at the best fill valve replacements by tank depth and brand compatibility, see our best toilet fill valves guide, which covers Fluidmaster, Korky, TOTO OEM, and Kohler OEM options with performance data.

Not every fill valve fits every toilet. Tank depth, shank length, and float design all affect compatibility. The table below covers the most common residential toilets and their OEM and recommended aftermarket fill valve options.

The universal measurement to confirm before purchasing any aftermarket fill valve is the distance from the bottom of the tank floor to the bottom of the overflow tube. The top of the fill valve body must sit below the top of the overflow tube by at least 1 inch. Telescoping fill valves like the Fluidmaster 400AH (adjusts from 7 to 13 inches) cover most residential applications in a single part number.

For a broader comparison of toilet options across brands, see our guide to the best flushing toilets which covers MaP scores, GPF ratings, and value across every price tier.

EPA WaterSense-certified toilets must use no more than 1.28 gallons per flush (GPF) and pass performance testing demonstrating they can effectively remove at least 350 grams of simulated waste per flush, with many certified models achieving far higher scores. MaP flush testing (Maximum Performance Testing, conducted independently at map-testing.com) rates toilets on a scale up to 1,000 grams. Top performers including the TOTO Drake II, TOTO UltraMax II, Kohler Cimarron, and American Standard Champion 4 achieve the maximum 1,000-gram MaP score at 1.28 GPF, proving that a properly functioning fill valve and a precision-calibrated water level are sufficient for complete waste removal at reduced water usage.

A malfunctioning fill valve undermines all of this engineering. A fill valve that runs water continuously into the overflow tube at a rate visible only as a thread-thin stream can waste 30 gallons per hour, or 720 gallons per day -- more water than most households use in an entire week. Replacing a $15 fill valve in a household of two people can realistically save thousands of gallons per year, which at average U.S. water rates of $0.005 per gallon represents a meaningful annual reduction in utility costs.

This is why EPA WaterSense and plumbing codes have emphasized fill valve quality alongside flush volume. The International Plumbing Code (IPC) Section 425 specifies requirements for fill valve design to prevent backflow and ensure reliable shutoff, and WaterSense certifies complete toilet systems rather than just flush volume, requiring that the fill valve and flush system work together to deliver consistent performance without wasted water between flushes.