What You're Actually Replacing Inside a Toilet Tank

The insides of a toilet tank are not one part — they are a system of three main pieces that work together to fill the tank after you flush and stop the water when the tank is full. The fill valve (also called a ballcock) lets water in. The flush valve at the bottom of the tank lets water out into the bowl when you push the handle. The float or float cup rises with the water level and tells the fill valve when to shut off. When your toilet runs constantly, refills slowly, or won't stop leaking, one of these three has usually worn out and needs replacement.

You do not need to replace all three at once. Most of the time, one part fails and the others keep working. This guide covers how to identify which part is broken, remove it, and install a new one. The whole job takes 20 to 45 minutes and costs between $15 and $60 per part, depending on what you buy.

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, flush valve, and float are the three parts most likely to fail; you can often replace just the broken one instead of all three.
  • A constantly running toilet usually means the flush valve is leaking or the float is stuck; a slow-filling tank usually means the fill valve is clogged.
  • Flush valve replacement requires draining the tank completely and working from underneath; fill valve and float replacement are quicker and happen from the top.
  • New parts come with instructions specific to the brand; follow those instructions over general steps if they differ, because tank designs vary.

Turn Off Water and Empty the Tank

Before you touch anything inside the tank, locate the shutoff valve on the water supply line under the toilet. It is a small knob or lever on the pipe that runs from the wall to the bottom of the tank. Turn it clockwise (or flip the lever to the off position) until it stops. Then flush the toilet once. The tank will empty and stay empty because the water supply is now closed.

If the shutoff valve is stuck or missing, you can still do this job — you will just need to work faster and have towels ready. If you cannot turn the valve at all, stop and call a plumber; forcing it can break the valve and create a bigger problem.

Identify Which Part Is Broken

A constantly running toilet usually points to the flush valve. Listen to the tank after you flush. If water keeps trickling into the bowl even when nobody is using it, the flush valve is leaking. You can confirm this by adding food coloring to the tank water (not the bowl). If the color appears in the bowl within 15 minutes without anyone flushing, the flush valve seal has worn out.

A slow-filling tank or one that does not fill at all usually means the fill valve is clogged or broken. After you flush, the tank should refill in 30 seconds to two minutes. If it takes longer or does not fill, the fill valve needs attention. A toilet that runs on and off throughout the day, or one that keeps refilling even when nobody has flushed, usually means the float is stuck or the fill valve is not shutting off properly.

If you are not sure which part is broken, start by checking the flush valve, because that is the most common failure. If the flush valve is fine, move on to the fill valve and float.

Replace the Flush Valve

The flush valve sits at the bottom center of the tank. To replace it, you must drain the tank completely. After you have turned off the water and flushed, use a sponge or small cup to remove any remaining water from the tank. You need the tank as dry as possible to work underneath.

Locate the large nut that holds the flush valve to the bottom of the tank. You will see it from underneath the toilet. Use an adjustable wrench to hold the nut while you use a second wrench to unscrew the bolt from above. Once the bolt is loose, you can lift the old flush valve out of the tank from the top. Remove the rubber seal (called a gasket) that sits under the valve.

Insert the new flush valve into the hole from the top, making sure the rubber gasket sits flat underneath. From below, thread the bolt back through the nut and tighten it by hand first, then use the wrench to snug it down. Do not over-tighten; you only need it tight enough that it does not leak. Reconnect the chain or rod that attaches the flush valve to the handle, then turn the water back on and test.

Replace the Fill Valve

The fill valve is usually on the left side of the tank (when you are facing it from the front). It is the part the water line connects to. To replace it, first turn off the water and flush to empty the tank. Disconnect the water supply line by turning the nut where it meets the fill valve. You may need an adjustable wrench for this.

From underneath the tank, locate the large nut that holds the fill valve in place. Use a wrench to unscrew it while you hold the valve steady from above so it does not spin. Once the nut is loose, you can pull the old fill valve out from the top of the tank. Remove the rubber washer underneath.

Insert the new fill valve into the hole with the rubber washer in place underneath. Thread the nut back on from below and tighten it by hand first, then with a wrench until snug. Reconnect the water supply line. Some fill valves come with an adjustment screw for water level; follow the instructions that came with your new valve. Turn the water back on and let the tank fill, then check that it stops filling when full and does not leak.

Replace the Float or Float Cup

The float is the part that rises and falls with the water level. Older toilets have a ball-shaped float on the end of a rod; newer ones have a cup-shaped float that slides up and down on the fill valve itself. If your float is stuck or cracked, you can replace just the float without replacing the entire fill valve.

For a ball float, locate where the rod connects to the fill valve (usually with a clip or pin). Remove the clip or pin, slide the rod out, and unscrew the old ball. Screw the new ball onto the rod, reinsert the rod, and find it with the clip or pin. For a cup float, locate the clip that holds it to the fill valve, slide the clip off, and slide the old cup down and away. Slide the new cup onto the fill valve and find it with the clip.

After you install a new float, you may need to adjust it so the water level stops at the correct height. Most tanks should fill until the water is about one inch below the overflow tube (the hollow pipe in the center of the tank). Check your new float's instructions for how to adjust it — some have a screw, others require you to bend the rod slightly. Turn the water back on and watch the tank fill. If the water level is too high or too low, adjust the float until it stops at the right height.

Test Your Work and Watch for Leaks

After you install any new part, turn the water back on slowly and let the tank fill completely. Flush the toilet and watch the tank refill. Listen for any hissing or trickling sounds that would indicate a leak. Check underneath the toilet for water dripping from the connections you tightened.

Let the tank sit for 10 minutes without flushing, then check the bowl. If water has appeared in the bowl without anyone flushing, the flush valve is still leaking and needs adjustment or replacement. If the tank does not fill or fills very slowly, the fill valve may still be clogged; you may need to clean it or replace it again. If everything looks dry and the tank fills and stops normally, the job is done.

Frequently Asked Questions

Do I need to replace all the parts at once or just the broken one?

Replace only the broken part. Most of the time, one piece fails while the others keep working. Replacing all three at once wastes money. However, if your toilet is very old (15+ years), the other parts may fail soon after, so you might consider replacing them all together to avoid multiple repair trips.

What if I turn the shutoff valve and nothing happens?

The valve may be stuck from years of not being used. Try turning it slowly and gently; do not force it. If it will not budge, you can still do the repair — just work quickly and have towels ready to catch water. If you must force the valve, stop and call a plumber, because forcing it can break the valve itself.

How do I know if the fill valve is clogged versus broken?

A clogged fill valve fills the tank very slowly or not at all, but the valve itself may still be good. Before you replace it, try cleaning it. Turn off the water, remove the fill valve, and rinse the internal parts under running water to remove mineral buildup. If cleaning does not help, replace the valve.

Can I use any brand of replacement part, or does it have to match my toilet?

Most fill valves and flush valves are universal and fit any standard toilet tank. However, check the measurements and connection type before you buy. Bring a photo of your tank or the old part to the store if you are not sure. Float designs vary more by brand, so match the float type to your fill valve if possible.

What if the toilet still runs after I replace the flush valve?

The fill valve may not be shutting off properly, or the float may be stuck or set too high. Check that the float moves freely and is not hitting the tank wall. Adjust the float down slightly so the tank stops filling sooner. If the tank still fills and refills on its own, the fill valve needs replacement or cleaning.