What a fill valve does and why it fails

The fill valve is the mechanism that refills your toilet tank after you flush. Water enters through the fill valve, and when the tank reaches the right level, a float shuts it off. If your fill valve leaks, runs constantly, or fills too slowly, the tank won't hold water properly and you'll hear hissing or running sounds even when nobody has flushed.

Fill valves wear out because they sit in water all day. Mineral deposits from hard water, rust, and sediment clog the internal parts. Rubber seals crack over time. Replacement is usually cheaper and faster than trying to repair the old valve, and a new one typically lasts 5 to 10 years.

Most fill valves cost between $15 and $40 at a hardware store. The job takes 20 to 45 minutes if you've never done it before. You need only a few basic tools and no special skills.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet before you start, then flush to empty the tank.
  • The fill valve sits inside the tank on the left side (in most toilets) and is held in place by a large nut underneath the tank.
  • You will need an adjustable wrench, a flathead screwdriver, and the new fill valve — buy the same brand and model as your old one if possible.
  • After installing the new valve, adjust the float so the water stops filling when the tank is about an inch below the overflow tube.

Gather your tools and materials

Before you touch the toilet, collect what you need. You will need an adjustable wrench (or a wrench that fits the nut under your tank), a flathead screwdriver, a bucket or towel to catch water, and the replacement fill valve itself.

To find the right fill valve, look at the old one or take a photo of it to the hardware store. Common brands are Fluidmaster, Korky, and Toto. If you can't identify the exact model, bring the old valve with you — most stores will match it. Buy one fill valve kit, which includes the valve, the float, and any gaskets you need.

Lay out your tools on the bathroom floor so you don't have to hunt for them mid-job. Have the bucket or towel ready because water will spill when you disconnect the supply line.

Turn off the water and empty the tank

Locate the shutoff valve on the water line that runs to the toilet. It is usually on the wall or floor behind or to the side of the toilet. Turn the handle clockwise until it stops. The handle should point straight down when it is fully closed.

Flush the toilet once. The tank will empty and the fill valve will try to refill, but no water will come in because the shutoff is closed. If water still trickles in, the shutoff valve is worn out — turn it as far as it will go and work quickly, or the tank will slowly refill as you work.

If you cannot find the shutoff valve or it does not work, you can still do the job, but you will need to work faster and have more towels ready. Water will leak from the supply line when you disconnect it.

Remove the old fill valve

Look inside the tank. The fill valve is a tall plastic or metal tube on the left side of the tank (in most standard toilets). It has a float attached to it — either a ball on an arm or a cup that slides up and down the valve itself.

Reach under the tank and locate the large nut that holds the fill valve in place. Use your adjustable wrench to turn this nut counterclockwise. It will be tight, so explore steady pressure. Once it is loose enough, you can unscrew it by hand. Water may drip, so have your bucket or towel underneath.

Disconnect the water supply line from the bottom of the fill valve. There is usually a small nut or a quick-connect fitting. If it is a nut, use your wrench. If it is a quick-connect (a plastic collar), push it in slightly and pull the line straight out. Again, water will drip.

Lift the old fill valve straight up and out of the tank. Some valves are tight and require wiggling, but they will come free. Empty any remaining water from the tank into your bucket.

Install the new fill valve

Before you put the new valve in, check that the rubber gasket on the bottom is in place. Most new valves come with a gasket already attached, but confirm it is there and not cracked.

Insert the new fill valve into the hole in the tank, pushing it straight down until the gasket sits flush against the bottom of the tank. The valve should not wobble. Reach underneath and hand-tighten the large nut, then use your wrench to tighten it firmly — snug, but not so hard that you strip the threads. A quarter-turn past hand-tight is usually right.

Reconnect the water supply line. If it is a nut fitting, use your wrench and tighten until snug. If it is a quick-connect, push the line straight in until you hear or feel a click. Do not force it.

Turn the shutoff valve counterclockwise to turn the water back on. The tank will begin to fill. Watch for leaks around the nut under the tank and where the supply line connects. A small drip is normal for the first few seconds as air escapes, but steady leaking means you need to tighten the connection.

Adjust the float for the correct water level

As the tank fills, watch the float. It should rise with the water and shut off the fill valve when the tank is about an inch below the top of the overflow tube (the hollow pipe in the center of the tank). If the water keeps running past that point, you need to adjust the float.

Most modern fill valves have a clip or adjustment screw on the float. Check your valve's instructions — they usually come in the box. For a clip-style float, pinch the clip and slide the float down the valve shaft to lower the shutoff point. For a screw-style, turn the screw counterclockwise to lower the shutoff. Make small adjustments — a quarter-turn at a time — and test by flushing.

The water level should stop about an inch below the overflow tube. If it stops too low, the toilet will not flush properly. If it stops too high, water will leak into the overflow tube and you will hear constant running. Adjust until the level is right, then flush a few times to confirm the valve shuts off cleanly each time.

Test for leaks and normal operation

Flush the toilet three times and watch the tank fill and empty. The fill valve should shut off smoothly without hissing or sputtering. The tank should refill in 30 seconds to 2 minutes. If it takes much longer, the valve may be partially blocked — remove it and rinse the inlet screen under running water, then reinstall it.

Check under the tank for drips. A few drops in the first hour are normal as the gasket settles, but steady dripping means the nut is loose or the gasket is damaged. Tighten the nut a quarter-turn and retest. If dripping continues, turn off the water, remove the valve, and check that the gasket is seated correctly.

Listen for running water 10 minutes after the last flush. If you hear water trickling, the float is set too high and water is leaking into the overflow tube. Adjust the float down slightly and test again.

Frequently Asked Questions

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

Sometimes, but usually not worth it. Floats are cheap (around $5), but if the float is the only problem, the valve itself is probably still working. If the valve is running constantly or leaking, replacing just the float will not fix it. If you are unsure, replace the whole valve — it is only slightly more work and gives you a fresh start.

What if water is still leaking from under the tank after I tighten the nut?

Turn off the water, remove the valve, and inspect the rubber gasket. If it is cracked, torn, or missing, replace it — most hardware stores sell gasket kits for a few dollars. If the gasket looks fine, the tank hole may be damaged. In that case, you may need a plumber, but try the new gasket first.

How do I know if my fill valve is the problem and not something else?

If the tank runs constantly or refills slowly, the fill valve is almost certainly the culprit. If water is leaking from the bottom of the tank, it could be the fill valve gasket or the flush valve — the fill valve is the easier fix to try first. If the toilet won't flush or the handle is stuck, the problem is elsewhere.

Do I need to turn off the water if I work quickly?

No, but it makes the job much easier and safer. Without the shutoff closed, water will spray or pour when you disconnect the supply line, and the tank will refill as you work. If you cannot find the shutoff valve, you can still do the job — just work fast and have plenty of towels ready.

What if the new valve does not fit in the hole?

Most fill valves are universal and fit standard tanks, but some older or specialty toilets use different sizes. If the valve is too loose or too tight, take it back to the store with a photo of your tank. The staff can help you find the right size or style for your toilet.