How to Fix a Running Toilet Fill Valve: Step-by-Step Guide

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9 min read|Updated: Jun 27, 2026|By Home Repair Expert

Quick Answer

Replacing a toilet fill valve takes 15-25 minutes and costs under $15 in parts. Turn off the water supply, flush to empty the tank, disconnect the water line, unscrew the locknut under the tank, lift out the old valve, install the new one (typically a Fluidmaster 400A), and reconnect. Adjust the float height so the water stops about 1 inch below the overflow tube.

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A running toilet caused by a faulty fill valve is one of the most common plumbing problems in any home, and one of the most wasteful. A constantly running fill valve can quietly send hundreds of gallons of water down the drain every day. The good news is that replacing a fill valve is one of the easiest, cheapest plumbing repairs there is, taking most homeowners under 30 minutes with basic tools.

This guide explains what the fill valve does, how to tell when it has failed, and exactly how to replace it for a quiet, reliable toilet that stops wasting water.

What the Fill Valve Does (and Why It Fails)

The fill valve refills the tank with water after each flush, then shuts off when the tank is full. It is one of two main parts inside your toilet tank, the other being the flapper. Fill valves fail for a handful of reasons:

• Worn internal seals: The seals inside the fill valve perish and no longer shut off cleanly, so water keeps trickling into the tank.
• Debris and limescale in the valve: Sediment, mineral deposits, or rust particles can lodge inside the valve and stop it sealing.
• A misadjusted float: If the float that triggers shut-off is set too high, water rises above the overflow tube and drains away constantly.
• A worn-out diaphragm: In ballcock-style fill valves, the rubber diaphragm wears with age and stops sealing.
• Age: Even good fill valves typically last 5 to 7 years; older ones often fail outright and need replacing rather than repairing.

Tools and Materials You Will Need

• New fill valve (a universal Fluidmaster 400A or Korky model fits most toilets)
• Adjustable wrench
• Sponge and bucket
• A towel
• Rubber gloves
• A flathead screwdriver

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

Step 1: Turn Off the Water and Drain the Tank

Close the shutoff valve at the wall behind the toilet, then hold down the handle to drain as much water as possible from the tank. Sponge out the remainder so the tank is empty and dry.

Step 2: Disconnect the Supply Line

Unscrew the water supply line from the bottom of the old fill valve with an adjustable wrench. Have a towel ready, a small amount of water always comes out.

Step 3: Remove the Old Fill Valve

Unscrew the large lock nut on the underside of the tank that holds the old fill valve in place. Lift the old valve straight up and out of the tank.

Step 4: Install the New Fill Valve

Adjust the height of the new valve to match the tank (it should sit about an inch above the overflow tube), then drop it into the hole. Tighten the lock nut by hand from underneath until it is snug, do not over-tighten or you can crack the tank.

Step 5: Reconnect the Supply, Set the Float, and Test

Reconnect the supply line, open the shutoff valve, and let the tank fill. Adjust the float so the water shuts off about an inch below the top of the overflow tube. Flush several times and check there are no leaks.

Pro Tips to Prevent This Problem

• A universal fill valve fits the vast majority of toilets and costs very little, no need to find a brand-match for most homes.
• Set the float so water shuts off an inch below the overflow tube, too high wastes water, too low gives a weak flush.
• Hand-tighten the lock nut firmly but never use heavy force, porcelain cracks easily under the tank.

When to Call a Professional

A fill valve replacement is one of the most DIY-friendly plumbing fixes there is, so a plumber is rarely needed. Consider calling one only if your toilet still runs after replacing the fill valve and the flapper, which can occasionally point to a cracked overflow tube or an unusual internal fault.

Frequently Asked Questions

How do I know if my fill valve is faulty?

A constantly running toilet, water trickling into the overflow tube, or a tank that takes a very long time to refill are the usual signs. If the toilet keeps running long after a flush, the fill valve has likely failed.

Are universal toilet fill valves any good?

Yes, universal fill valves like the Fluidmaster 400A or Korky models fit the great majority of toilets and work just as well as branded replacements. They are inexpensive and widely available.

How long does a toilet fill valve last?

A quality fill valve typically lasts 5 to 7 years. After that, replacing it outright is usually easier and more reliable than trying to repair worn internal seals.

Final Thoughts

A running toilet caused by a faulty fill valve is a fast, cheap fix that almost any homeowner can do in 30 minutes. Shut off the water, drain the tank, swap the old valve for a universal replacement, and set the float correctly. With this simple repair you can stop pouring water (and money) down the drain, keep your toilet quiet, and avoid an unnecessary call to a plumber.