What you're actually replacing inside a toilet tank

The inside of a toilet tank holds three main parts that wear out: the fill valve (which lets water in when you flush), the flush valve (which lets water out into the bowl), and the flapper (the rubber seal that sits on top of the flush valve). Most people replace all three at once because they're sold together as a kit, cost between $15 and $40, and the labor is the same whether you replace one or all three. If your toilet runs constantly, won't stop filling, or doesn't flush completely, one of these parts has failed.

You do not need a plumber for this job. The work takes 30 to 45 minutes, requires only a wrench and a screwdriver, and the parts are designed to come out and go back in by hand. The only reason to call a professional is if the tank itself is cracked, if the bolts holding the tank to the bowl won't budge, or if you're not comfortable working with water.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet before you start, then flush to empty the tank.
  • Buy a universal fill valve and flush valve kit from any hardware store—they fit most standard toilets made in the last 20 years.
  • The flapper is the easiest part to replace first; if that alone fixes the problem, you're done in five minutes.
  • Disconnect the water line, unbolt the tank from the bowl, and lift it off to access the fill and flush valves from underneath.
  • Hand-tighten all bolts and connections when reassembling—over-tightening cracks the porcelain or strips the plastic threads.

Shut off the water and empty the tank

Locate the shutoff valve on the water line that runs to the toilet. It's usually a knob or lever on the pipe under the tank, close to the wall. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. Flush the toilet once; the tank will empty and the fill valve will stop refilling it.

If the shutoff valve won't turn or doesn't stop the water, you can still work on the toilet—the tank is now empty and won't refill until you turn the valve back on. If you cannot find a shutoff valve, one may not exist, and you'll need to call a plumber to install one before you can safely work on the toilet.

Try replacing just the flapper first

The flapper is a rubber disc attached to a chain that hangs inside the tank. It sits on top of the flush valve opening at the bottom of the tank. When you push the handle, the chain lifts the flapper, water rushes down into the bowl, and the flapper falls back down to seal the opening. If the flapper is cracked, warped, or no longer seals, water leaks into the bowl constantly and the fill valve runs to top up the tank.

To replace it, unhook the chain from the handle lever, then lift the flapper off the flush valve post. Take it to the hardware store to match the size and shape—flappers come in two or three standard sizes. Screw or clip the new flapper onto the post, reattach the chain with a little slack (the chain should not be tight when the flapper is closed), and turn the water back on. If the toilet stops running and flushes normally, you're done. If it still runs, the fill valve or flush valve has also failed.

Remove the tank from the bowl

With the water still off and the tank empty, locate the two bolts that hold the tank to the bowl. They run vertically through the bottom of the tank and thread into the bowl. You'll see them from underneath if you look under the back of the toilet, or you can feel them by reaching behind the tank from above.

Place a bucket under the tank to catch any remaining water. Use an adjustable wrench to hold the nut on the underside of each bolt while you turn the bolt head from inside the tank with a screwdriver. Turn counterclockwise. If the bolts are stuck, spray them with penetrating oil and wait 10 minutes. Do not force them—if a bolt breaks, removing it requires drilling, which is a job for a plumber.

Once both bolts are loose, carefully lift the tank straight up and set it on its back on a towel or newspaper. The rubber gasket between the tank and bowl will come free; set it aside. You now have access to the fill valve and flush valve from underneath the tank.

Replace the fill valve and flush valve

The fill valve is the assembly on the left side of the tank (when looking from above). It has a large nut holding it to the bottom of the tank. Use an adjustable wrench to turn this nut counterclockwise while holding the valve body steady from inside the tank with your other hand. Once the nut is loose, the valve will slide out from the bottom.

The flush valve is the larger assembly in the center or right side of the tank. It also has a large nut underneath. Turn it counterclockwise the same way. Once loose, the flush valve will slide out from the bottom of the tank.

Take both old valves to the hardware store and buy a universal replacement kit that matches them. Most kits include a new fill valve, flush valve, and flapper. Follow the kit's instructions to install them—they slide in from the bottom, and you hand-tighten the nuts until snug. Do not use a wrench to over-tighten; you'll crack the tank or strip the plastic threads. The nuts should be tight enough that they don't turn by hand, but you should still be able to turn them with moderate wrench pressure if needed.

Reassemble the tank and test

Place the rubber gasket back on the bowl, then carefully lower the tank onto it. Align the bolt holes and insert both bolts from inside the tank. Hand-tighten the nuts from underneath, then use a wrench to snug them down evenly—tighten one bolt a quarter turn, then the other, then back to the first. This keeps the tank level and prevents cracking. Stop when the tank sits firmly on the bowl; you should not be able to rock it.

Reconnect the water line to the fill valve by hand-tightening the compression nut. Turn the shutoff valve back on slowly. Watch the tank fill and listen for the fill valve to shut off when the tank reaches the fill line. Flush the toilet and watch the tank empty and refill. The water should stop running within a few seconds of the tank filling. If it runs continuously, the new fill valve may not be seated correctly—turn off the water, remove the tank again, and reseat the valve.

Common problems and when to stop

If the tank leaks from underneath after you reassemble it, the rubber gasket may be damaged or not seated evenly. Turn off the water, remove the tank again, and inspect the gasket for cracks or hardening. If it's damaged, buy a new one. If it looks fine, make sure the tank bolts are tight and the gasket is centered on the bowl opening.

If water leaks from the fill valve connection at the water line, the compression nut may not be tight enough, or the ferrule (a small metal ring inside the nut) may be damaged. Turn off the water, disconnect the line, and inspect the ferrule. If it's cracked or flattened, buy a new compression fitting kit. If the ferrule looks fine, reconnect the line and tighten the nut a quarter turn at a time until the leak stops.

If the tank cracks during removal or reassembly, or if the bolts break and won't come out, stop and call a plumber. Attempting to drill out a broken bolt or patch a crack will cost more than replacing the tank.

Frequently Asked Questions

Do I need to replace all three parts at once?

No. Start with the flapper if the toilet runs constantly—that's the most common failure and takes five minutes to fix. If the toilet still runs after replacing the flapper, the fill valve or flush valve has failed, and you should replace both at the same time because the labor is identical and they often fail together.

What if my toilet tank is an older model?

Universal kits fit most toilets made after 2000. If your toilet is older, bring a photo of the inside of the tank to the hardware store and ask the staff to confirm the kit will fit. Some older tanks use different bolt patterns or valve sizes, and a kit that doesn't fit will waste your money and time.

Can I replace just the fill valve or just the flush valve?

Yes, but most people replace both because the kits are inexpensive and you've already done the hard work of removing the tank. If you're certain only one has failed, you can buy them separately, but the savings are small.

How tight should I make the bolts when I put the tank back on?

Hand-tighten first, then use a wrench to snug them down until the tank sits firmly and doesn't rock. You should still be able to turn the wrench with moderate pressure—if you're straining hard, you're over-tightening and risk cracking the porcelain. Tighten both bolts evenly, a quarter turn at a time, alternating between them.

What if water leaks from the bottom of the tank after I reassemble it?

Turn off the water and remove the tank again. Check that the rubber gasket is centered on the bowl opening and not torn or hardened. If it looks fine, the tank bolts may not be tight enough—reinstall the tank and tighten the bolts a quarter turn more. If the gasket is damaged, buy a new one.