How to Replace a Toilet Fill Valve
Updated February 25, 2026
Swap a faulty fill valve in 15 minutes to stop a running toilet, phantom flushing, or slow tank refill.
Overview
The fill valve controls water entering the tank after a flush. When it fails, you get a toilet that runs constantly, cycles on and off every few minutes, takes forever to refill, or makes a high-pitched whine. Replacing it is one of the easiest plumbing repairs -- no special skills, one wrench, 15 minutes. The Fluidmaster 400A ($8-12) is the universal standard and fits nearly every toilet made in the last 50 years.
What You'll Need
Safety First
- Turn off the supply valve before removing anything. The fill valve port sprays water straight up when the valve is removed with the water on.
- Have towels on the floor. Even after flushing and sponging, some water remains in the tank and spills when you disconnect the supply line.
Step-by-Step Instructions
- Turn Off Water and Drain the Tank
Turn the supply valve behind the toilet clockwise until it stops. Flush the toilet and hold the handle down to drain as much water as possible. Sponge or towel out the remaining inch of water in the bottom. Place a towel or shallow pan under the supply valve connection point -- water will drip when you disconnect the supply line.
Tip: Supply valve will not turn? Do not force it -- old gate valves seize after years of not being used. Turn off the main house water supply instead, then plan to replace that supply valve after the fill valve job. - Remove the Old Fill Valve
Under the tank, disconnect the supply line from the bottom of the fill valve (the threaded tailpiece sticking through the tank bottom on the left side). Use an adjustable wrench or channel-lock pliers. Then unscrew the plastic locknut that holds the fill valve to the tank -- it is directly above where the supply line connected. Turn counterclockwise. Lift the old fill valve straight up out of the tank. Pull the refill tube off the overflow pipe.
Tip: The locknut is plastic on most fill valves. Hand-loosen it first, then use the wrench if needed. Plastic cracks if you go straight to the wrench with too much force. - Adjust and Install the New Fill Valve
The new fill valve is height-adjustable. Twist the shaft to set the height: the top of the fill valve should sit about 1 inch above the overflow tube. The critical water level mark on the valve should be about 1 inch below the top of the overflow tube. Drop the new valve into the tank hole. The rubber gasket on the bottom seals against the tank. Thread the new locknut on from below -- hand-tight, then a half turn with pliers. Do not overtighten.
Tip: The overflow tube is the open vertical pipe in the center of the tank. Water should never flow into it during normal operation. If it does, the fill valve is set too high. The water level mark on the valve body shows you exactly where the water should stop. - Connect the Refill Tube and Supply Line
Clip the small refill tube from the fill valve onto the top of the overflow tube. This tube sends a small stream of water into the bowl during refill to restore the bowl water level. It should clip on -- not go down into the overflow tube (that siphons water and causes running). Reconnect the supply line to the threaded tailpiece under the tank. Hand-tight, then a quarter turn with pliers.
Tip: The refill tube must be above the overflow tube rim, not stuffed down inside it. Pushing the tube below the water line creates a siphon that slowly drains the tank into the bowl. This is one of the most common causes of phantom flushing after a fill valve replacement. - Turn On Water and Adjust
Turn the supply valve on slowly. Let the tank fill. Watch the water level -- it should stop about 1 inch below the top of the overflow tube, usually at the water level line marked on the tank or the fill valve. Too high? Adjust the float by turning the screw on top of the fill valve (Fluidmaster) or pinching the float clip and sliding the float down (other brands). Too low? Adjust up. Flush 3-4 times and verify the level is consistent.
Tip: After adjusting, flush and wait for the tank to fully refill. The water should stop flowing and the valve should be completely silent. Any hissing after the tank is full means the float needs to be lowered slightly -- water is trickling past the valve.
Pro Tips
- Fluidmaster 400A is the default replacement for a reason: $8-12, universal fit, adjustable height (9 to 14 inches), and available at every hardware store. It fits Kohler, TOTO, American Standard, and every other standard toilet.
- Replacing the fill valve? Replace the supply line at the same time. Braided stainless steel supply lines are $5-8 and last 10+ years. Old chrome or plastic lines are the next thing to fail.
- Some Kohler and TOTO toilets use proprietary fill valves with specific mounting sizes. Check your toilet model before buying a universal replacement. Kohler GP1083167 and TOTO TSU99A.X are the OEM replacements for those brands.
- Fill valve whistling or humming? Before replacing, try cleaning the diaphragm. On a Fluidmaster, twist the cap counterclockwise, lift off, inspect the rubber seal. Sediment on the seal causes noise. Clean it and test. Still noisy? Replace the whole valve.
- Complete toilet rebuild kits ($20-25) include the fill valve, flapper, gaskets, and supply line. If the toilet is 5+ years old, replacing everything at once saves a second trip for the flapper that fails next month.
When to Call a Pro
Fill valve replacement is a DIY job. Call a plumber only if the supply valve behind the toilet will not turn off (valve replacement needed first), if the tank has a crack at the fill valve hole, or if you have a concealed-tank toilet (in-wall carrier) where the valve is inside the wall.
Frequently Asked Questions
When should I call a plumber to replace a toilet fill valve?
Fill valve replacement is a DIY job. Call a plumber only if the supply valve behind the toilet will not turn off (valve replacement needed first), if the tank has a crack at the fill valve hole, or if you have a concealed-tank toilet (in-wall carrier) where the valve is inside the wall.
What are some expert tips to replace a toilet fill valve?
Fluidmaster 400A is the default replacement for a reason: $8-12, universal fit, adjustable height (9 to 14 inches), and available at every hardware store. It fits Kohler, TOTO, American Standard, and every other standard toilet. Replacing the fill valve? Replace the supply line at the same time. Braided stainless steel supply lines are $5-8 and last 10+ years. Old chrome or plastic lines are the next thing to fail. Some Kohler and TOTO toilets use proprietary fill valves with specific mounting sizes. Check your toilet model before buying a universal replacement. Kohler GP1083167 and TOTO TSU99A.X are the OEM replacements for those brands. Fill valve whistling or humming? Before replacing, try cleaning the diaphragm. On a Fluidmaster, twist the cap counterclockwise, lift off, inspect the rubber seal. Sediment on the seal causes noise. Clean it and test. Still noisy? Replace the whole valve. Complete toilet rebuild kits ($20-25) include the fill valve, flapper, gaskets, and supply line. If the toilet is 5+ years old, replacing everything at once saves a second trip for the flapper that fails next month.
What tools do I need to replace a toilet fill valve?
You will need: Adjustable wrench or channel-lock pliers, Replacement fill valve (Fluidmaster 400A or equivalent -- $8-12), Towels and sponge (for draining residual water), Small bucket or pan (to catch water under the tank), New braided stainless steel supply line (recommended while you are in there -- $5-8).
When should I call a professional instead of doing this myself?
Fill valve replacement is a DIY job. Call a plumber only if the supply valve behind the toilet will not turn off (valve replacement needed first), if the tank has a crack at the fill valve hole, or if you have a concealed-tank toilet (in-wall carrier) where the valve is inside the wall.
How difficult is this project?
This project is rated easy. Most homeowners with basic tools can complete it.
How long does this take?
Plan for approximately 15 - 30 minutes. First-timers may need extra time.
How much will this cost?
The estimated cost is $8 - $15. Costs vary by location and materials chosen.