How to Replace a Toilet Fill Valve (Step-by-Step)

What a Toilet Fill Valve Does

The fill valve is the tall assembly on the left side of your toilet tank. It's connected to the water supply line underneath and controls all the water flowing into the tank after each flush. When you flush, the water level drops, a float mechanism on the fill valve detects the drop, and the valve opens to let fresh water in. As the tank refills, the float rises with the water level. Once it reaches the set point, the valve shuts off.

Modern fill valves use an internal diaphragm — a small rubber disc — to control the water flow. When the diaphragm wears out, gets stiff, or collects debris from the water supply, the valve can't shut off properly. That's when you get a toilet that hisses, runs continuously, fills too slowly, or cycles on and off by itself.

Signs Your Fill Valve Needs Replacing

A failing fill valve shows up in a few distinct ways:

Constant hissing. If you hear a steady hiss from the tank even when nobody has flushed, the fill valve's diaphragm isn't sealing and water is trickling through continuously. The toilet runs and won't stop. Unlike a flapper leak (which causes intermittent phantom flushing), a failed fill valve often produces continuous running because the valve physically can't close. Slow tank refill. If the tank takes noticeably longer to fill after a flush than it used to, the valve may be partially clogged with sediment or the diaphragm is restricting flow. The toilet won't fill at all. In severe cases, the valve seizes completely and no water enters the tank after a flush. Water level is too high or too low. If the tank overfills and water pours into the overflow tube, or if the tank never fills to the proper level despite adjustments, the valve's float mechanism has failed.

Before replacing the entire valve, try cleaning it first. Lift the cap off the top of the valve (it usually lifts and twists), pull out the diaphragm, rinse it, and flush debris from the valve body by briefly turning on the water with a rag over the opening. If cleaning doesn't fix it, the valve needs to be replaced.

Types of Fill Valves: Fluidmaster vs. Korky vs. Others

Two brands dominate the residential fill valve market:

Fluidmaster 400A is the most widely used fill valve in the U.S. It's the one already installed in the majority of toilets. It uses a float cup that rides up and down the valve shaft. Adjustment is simple — you squeeze a clip on the float cup and slide it up or down to set the water level. Replacement units cost $8 to $12.

Korky QuietFILL is the main alternative. It's designed to be quieter than the Fluidmaster and uses a slightly different float mechanism. Some plumbers prefer it for its build quality. It costs $10 to $15.

Both are universal-fit and work in virtually every standard residential toilet. They adjust from about 9 inches to 14 inches in height to fit different tank sizes. Installation is identical for both brands.

Older toilets may have a ballcock valve — the kind with a brass rod and a floating ball on top. If your toilet has one of these, it's been decades since it was current. Replace it with a modern fill valve rather than trying to repair it. The old ballcock valves are noisier, less reliable, and harder to find parts for.

Step-by-Step: How to Replace a Toilet Fill Valve

Total time: 20 to 30 minutes. Tools needed: channel-lock pliers, a small bucket or towel, and the new fill valve.

Shut off the water supply at the angle stop behind the toilet. Flush the toilet to drain most of the tank. Use a sponge or towel to soak up the remaining water at the bottom — you want the tank as dry as possible.

Disconnect the supply line from the bottom of the fill valve. Place a towel or bucket underneath to catch residual water. Use channel-lock pliers if the coupling nut is tight.

Remove the old fill valve. Underneath the tank, you'll see a large plastic mounting nut where the fill valve passes through the tank hole. Unscrew it by hand or with pliers — turn counterclockwise. Once the nut is off, lift the old fill valve straight up and out of the tank.

Adjust the new fill valve height. Before you install it, set the height to match the old one. Most fill valves have a twist-to-adjust shaft — hold the base and twist the top portion to extend or retract it. The critical measurement: the top of the fill valve should sit about one inch below the top of the overflow tube. If it's too high, water can siphon back through the valve.

Install the new fill valve. Make sure the rubber washer is seated on the bottom of the valve (it comes pre-installed on most units). Drop the valve into the tank hole, center it, and thread the mounting nut from underneath. Hand-tighten, then snug with pliers — firm but don't overtighten or you'll crack the tank.

Attach the refill tube. The small rubber tube that comes with the fill valve clips onto the top of the valve and routes into the overflow tube. This tube sends a small stream of water into the bowl during refill to restore the bowl's water seal. Clip it to the top of the overflow tube with the included clip — don't push it down inside the tube.

Reconnect the supply line. Thread it onto the fill valve's inlet and tighten. Hand-tight plus a quarter turn with pliers is enough.

Adjust the Water Level and Test

Turn the water back on slowly and watch the tank fill. The water should rise steadily and the fill valve should shut off once the water reaches the fill line marked inside the tank — or about one inch below the top of the overflow tube.

If the water level is too high, squeeze the adjustment clip on the float cup and slide it down. If it's too low, slide it up. Make small adjustments and flush between each one to check.

Flush three or four times and watch the full cycle each time: the tank should drain, the fill valve should open, water should rise steadily, and the valve should shut off cleanly at the correct level. Listen for any hissing after shutoff — there should be none.

Check all connections for leaks: the supply line coupling at the bottom, the mounting nut underneath the tank, and the refill tube clip at the overflow. Tighten anything that drips — but gently. Plastic threads strip easily.

When to Call a Professional

A fill valve replacement is a manageable DIY job, but a few situations call for professional help:

If the shutoff valve behind the toilet won't close — meaning you can't turn off the water to work on the fill valve — you've got a shutoff valve problem that needs to be fixed first. If the tank has a crack, replacing internal parts won't help. Hairline cracks in porcelain can be hard to spot but will leak under the pressure of a full tank. If you've replaced both the fill valve and the flapper and the toilet still runs, something less obvious is going on — a warped flush valve seat, a problem with the overflow tube, or an issue with the supply line pressure.

At The Toilet Guys, fill valve repairs and replacements start at $100, parts included. We carry Fluidmaster and Korky valves on the truck for same-visit repairs. If your toilet is old enough that multiple parts are failing, we'll give you straight talk about whether repairs or a full replacement makes more sense. Replacements start at $499 installed.