What's Inside a Toilet Tank and Why It Fails

The inside of your toilet tank holds three main parts: the fill valve (which lets water in when you flush), the flush valve (which lets water out into the bowl), and the flapper (a rubber seal that sits on top of the flush valve). When any of these wear out, your toilet either runs constantly, won't fill, or leaks water into the bowl.

Most people don't need to replace all three at once. A running toilet is usually a worn flapper. A toilet that won't stop filling is usually a stuck fill valve. A toilet that leaks silently into the bowl overnight is often a flapper that no longer seals. You can replace just the part that's broken, or replace the whole assembly if you want a fresh start.

The good news: this is a job you can do yourself with basic tools and about 30 minutes. You don't need a plumber unless the tank itself is cracked or the bolts holding the tank to the bowl are seized.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet before you start, then flush to empty the tank.
  • You can replace just the flapper, just the fill valve, or buy a complete repair kit that includes all three parts.
  • Most hardware stores stock universal flappers and fill valves that fit standard tanks, but bring a photo of your current parts to match the size.
  • The flapper is the easiest part to replace and usually costs under $10; the fill valve takes longer but is still a straightforward swap.
  • If bolts are stuck or the tank is cracked, stop and call a plumber rather than force it.

Turn Off Water and Empty the Tank

Locate the shutoff valve on the water line that runs to the toilet. It's usually a small knob or lever on the pipe under the tank, near the wall. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. Then flush the toilet once to empty the tank completely.

If the shutoff valve won't turn or leaks when you try, you can still work on the tank—just work slowly and have towels ready. If it leaks badly, you may need a plumber to replace the shutoff valve itself before you proceed.

Decide Whether to Replace Just the Flapper or the Whole Assembly

If your toilet runs constantly or water trickles into the bowl when nobody has flushed, the flapper is almost certainly the problem. A flapper is a rubber or plastic disc attached to a chain that lifts when you push the handle. Over time, the rubber hardens and no longer seals against the flush valve seat.

If your toilet won't fill after you flush, or fills very slowly, the fill valve is stuck or worn. If you're not sure which part is broken, start by replacing the flapper—it's cheaper and faster, and if that doesn't fix it, you know the fill valve is next.

You can also buy a universal repair kit that includes a new flapper, fill valve, and overflow tube. These kits cost $15 to $30 and let you replace everything at once. They work in most standard tanks, but check the package to make sure it fits your tank size.

Replace the Flapper

Lift the tank lid and set it somewhere safe where it won't break. Look at the bottom center of the tank. You'll see a large opening (the flush valve) with a rubber or plastic disc sitting on top of it. That's the flapper. A chain runs from the flapper up to the handle.

Unhook the chain from the handle lever by unclipping or unscrewing the attachment point. Then lift the flapper straight up and off the two small posts (called ears) that hold it in place. The old flapper will come free.

Slide the new flapper onto those same two posts, making sure it sits flat and centered on the flush valve opening. Reattach the chain to the handle lever. The chain should have a tiny bit of slack—about a quarter-inch—when the handle is at rest. If it's too tight, the flapper won't seal. If it's too loose, the handle will feel mushy. Adjust by moving the chain clip up or down on the handle lever.

Turn the water back on and test. The tank should fill, and the toilet should flush normally without running afterward.

Replace the Fill Valve

The fill valve is the assembly on the left side of the tank (when facing it from the front). It's a plastic or brass cylinder with a float arm attached. Water comes in through this valve, and the float rises as the tank fills, shutting off the water when full.

Before you remove it, take a photo of how the float arm is positioned and which way the water inlet tube points. This helps you set up the new one the same way.

Locate the large nut underneath the tank where the fill valve connects. Use an adjustable wrench to hold the nut while you unscrew the valve from inside the tank with your other hand. Once the nut is loose, the valve will pull out from the top. Water may spill—have a towel ready.

Slide the new fill valve into the hole from the top, making sure the rubber washer sits flat against the bottom of the tank. Thread the nut back on underneath and tighten it by hand first, then snug it with the wrench. Don't over-tighten or you'll crack the tank.

Reattach the water inlet tube and adjust the float arm to match your photo. Turn the water back on slowly and watch the tank fill. It should stop filling when the water reaches the line marked inside the tank (usually about an inch below the overflow tube). If it fills too high or too low, adjust the float arm by bending it slightly or using the adjustment clip on the valve itself—check your valve's instructions.

Reconnect the Water Supply and Test

Once the new part is in place, turn the shutoff valve back on by turning it counterclockwise (or flipping the lever parallel to the pipe). Open it slowly so water doesn't rush in and cause a leak.

Watch the tank fill. Listen for any hissing or spraying inside the tank. The fill valve should shut off automatically when the tank is full. Flush the toilet and listen for the tank to refill smoothly without running on.

Check under the tank for drips. If you see water pooling, the nut may not be tight enough, or the rubber washer may not be seated properly. Turn off the water, drain the tank, and try tightening the nut a quarter-turn more. If water still leaks, the tank itself may be cracked and you'll need a plumber.

When to Call a Plumber Instead

Stop and call a plumber if the shutoff valve won't turn or leaks when you try to use it. Forcing it can break the valve and flood your bathroom.

Also call a plumber if the bolts holding the tank to the bowl are seized and won't budge, or if you see cracks in the tank itself. Cracked tanks can't be repaired and must be replaced as a unit, which requires disconnecting the tank from the bowl—a job that's easier with two people and professional tools.

If you've replaced the flapper and the toilet still runs, or you've replaced the fill valve and the tank still won't fill, there may be a problem with the flush valve seat itself (the surface the flapper seals against). A plumber can clean or replace that, or recommend whether a new toilet makes sense.

Frequently Asked Questions

Do I need to turn off the water to replace just the flapper?

Yes. Even though the flapper is a small part, turning off the water first prevents accidental overflow and lets you work without water dripping on you. It takes 30 seconds and is worth the safety.

What if the new flapper doesn't fit the posts on my tank?

Bring a photo of your tank's flush valve to a hardware store and ask for a universal flapper that matches. Most tanks use the same two-post design, but some older or specialty toilets use a different system. A store employee can help you find the right one, or you may need a plumber to identify your tank model.

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

If the toilet runs constantly and water is trickling into the bowl, it's the flapper. If the toilet won't fill after you flush, or the handle feels stuck, it's the fill valve. If you're unsure, replace the flapper first since it's cheaper and faster—if that doesn't fix it, the fill valve is next.

Can I use any universal repair kit, or do I need one made for my toilet brand?

Universal kits work in most standard tanks regardless of brand. Check the package to confirm it fits your tank size (most are one-size-fits-most). If your toilet is very old or unusual, bring a photo to the store and ask whether the kit will work.

What should I do if water leaks from under the tank after I install the new fill valve?

Turn off the water, drain the tank, and check that the rubber washer under the fill valve is seated flat and centered. Tighten the nut underneath a quarter-turn more, then test again. If it still leaks, the tank may be cracked and you'll need a plumber to replace it.