What a float valve does and why it fails

The float valve (also called a fill valve or ballcock) is the part that shuts off the water flowing into your tank once it reaches the right level. When it fails, your toilet either runs constantly, leaks water into the bowl, or won't refill after you flush. Replacing it is a straightforward job that takes about 30 minutes and costs $15 to $40 for the valve itself.

Float valves wear out because they sit in water all day. The internal seals crack, the float arm gets stuck, or mineral deposits clog the valve opening. You'll know it's failing when you hear hissing or trickling sounds from the tank, or when the water level in the bowl stays higher than normal.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet before you start, then flush to empty the tank.
  • The float valve bolts to the bottom of the tank from the inside, with a large nut holding it in place underneath — you'll need an adjustable wrench to remove it.
  • Measure the distance from the center of the fill valve opening to the edge of the tank before you buy a replacement, because valves come in different lengths.
  • The new valve goes in the same way the old one came out: bolt it from inside the tank, tighten the nut from below, then reconnect the water supply line.
  • After installation, adjust the float arm so the water shuts off when the tank is three-quarters full, not to the brim.

Gather your tools and turn off the water

You need an adjustable wrench, a flathead screwdriver, a bucket or towel to catch water, and the replacement float valve. Before you buy the valve, measure the distance from the center of the fill valve opening (the hole in the bottom of the tank) to the nearest edge of the tank. Most valves are either 7 inches or 9 inches long — buying the wrong length means the valve won't fit or the float arm will hit the tank wall.

Locate the shutoff valve on the water line under the toilet, usually a small knob on the pipe coming from the wall. Turn it clockwise until it stops — you should feel resistance, not a hard stop. Flush the toilet once to empty the tank. If water still trickles in, the old valve is leaking and you're doing the right repair.

Remove the water supply line and old valve

Look under the tank where the water line connects. Use your adjustable wrench to loosen the nut where the line meets the valve — turn it counterclockwise. The line will drape down; position your bucket underneath to catch any remaining water. Once the nut is loose enough to turn by hand, unscrew it the rest of the way and set the line aside.

Now look at the bottom of the tank from underneath the toilet. You'll see a large nut (usually 1.25 inches across) holding the valve in place. This is the locknut. Use your adjustable wrench to hold the nut steady while you use a flathead screwdriver to turn the valve itself counterclockwise from inside the tank. Once the valve turns freely, unscrew the locknut by hand. The old valve will come out through the top of the tank.

Install the new float valve

Before you insert the new valve, check that the rubber washer on the bottom is seated properly and not cracked. If it looks dry or hard, replace it with the spare washer that usually comes with the new valve. Insert the valve through the hole in the bottom of the tank from the inside, making sure the outlet (the part the water comes out of) points toward the overflow tube in the center of the tank.

From underneath, hand-tighten the locknut onto the valve. Once it's snug, use your adjustable wrench to tighten it another quarter turn — no more. Over-tightening cracks the tank. The valve should not move when you gently push on it from inside the tank.

Reconnect the water supply and test

Reattach the water supply line to the valve outlet. Hand-tighten the nut first, then use your wrench to snug it another quarter turn. Turn the shutoff valve counterclockwise slowly to let water back in. Watch the tank fill and listen for the hissing to stop when the float arm rises and shuts off the valve.

The water should stop flowing when the tank is about three-quarters full, leaving a gap of about 1 inch below the overflow tube. If water keeps running or shuts off too early, adjust the float arm. Most modern valves have a clip you can slide up or down the arm to change the shutoff point — slide it up to let the tank fill higher, down to shut off sooner. Flush a few times to make sure the valve refills smoothly and doesn't leak from the connection points.

Common mistakes that cause leaks or running water

The most frequent error is over-tightening the locknut, which cracks the tank and creates a slow leak you won't notice for days. Tighten it firmly, but stop as soon as you feel real resistance. Another common mistake is installing the valve backwards — the outlet should point toward the center of the tank, not toward the edge.

If water still leaks from the connection where the supply line meets the valve after you've tightened it, the rubber washer inside the nut may be damaged. Turn off the water, unscrew the nut, and check the washer. If it's cracked or flattened, replace it. If the tank continues to leak from the bottom after installation, the rubber washer under the locknut is not seating properly — remove the valve and check that washer too.

When to call a plumber instead

If the hole in the bottom of the tank is cracked or stripped (the threads won't hold the locknut tight), the tank itself needs replacement and this is beyond a DIY repair. If you turn the shutoff valve and water doesn't stop flowing into the tank, the shutoff valve itself is broken and you'll need a plumber to replace it before you can work on the float valve.

If you're uncomfortable working under the toilet or the tank is very old and fragile, a plumber can do this job in under an hour. The labor cost is usually $100 to $200 depending on your area, plus the cost of the valve.

Frequently Asked Questions

Do I need to drain the whole tank before I start?

No. Flush once after turning off the water, and the tank will be empty enough to work. You'll lose a little water when you disconnect the supply line — that's why you position a bucket underneath — but you don't need to siphon or pump the tank dry.

What if the old valve is stuck and won't turn?

Spray the valve with penetrating oil (like WD-40) and wait 15 minutes. Try turning it again with your screwdriver. If it still won't budge, explore more oil and wait another 15 minutes. Forcing a stuck valve can crack the tank, so patience here saves money.

Can I replace just the float arm instead of the whole valve?

Some float arms are replaceable, but most modern valves are sealed units where the arm is part of the assembly. Check the manufacturer's instructions for your specific valve model. If the arm is the only problem, replacement kits run $8 to $15. If the valve itself is leaking or clogged, you need the whole new valve.

How do I know what brand of valve to buy?

Look at the old valve before you remove it — the brand name is usually stamped on the side. Common brands are Fluidmaster, Kohler, and American Standard. You can buy the exact same brand and model, or any universal fill valve that fits your tank size. Universal valves work in almost any toilet and cost the same.

What if water leaks from under the tank after I finish?

Turn off the water when ready and check that the locknut is tight. If tightening it doesn't stop the leak, turn off the water, remove the valve, and inspect the rubber washer on the bottom. A cracked or flattened washer is the most common cause. Replace it and reinstall the valve.