What a toilet tank gasket does and why it fails

A toilet tank gasket is a rubber or foam ring that sits between the tank and the bowl, creating a watertight seal. When it wears out or cracks, water leaks from the tank into the bowl, and you'll notice the tank refilling on its own even when nobody has flushed. The gasket typically lasts 5 to 10 years before the rubber hardens and shrinks.

Replacing it is a straightforward job that takes about 30 minutes and costs $10 to $30 for the gasket itself. You don't need a plumber, and the only tools are ones you likely have at home. The hardest part is usually just getting the tank off the bowl without cracking the porcelain.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet before you start, then flush to empty the tank.
  • The gasket sits under the tank where it bolts to the bowl, so you must remove the tank completely to access it.
  • Loosen the bolts from underneath the bowl using an adjustable wrench or socket wrench, not from the top.
  • Slide the old gasket off the flush valve (the pipe sticking up from the bowl) and slide the new one on before reinstalling the tank.
  • Tighten the bolts evenly in a crisscross pattern to avoid cracking the porcelain or creating new leaks.

Gather your tools and materials before you start

You'll need an adjustable wrench or a socket wrench set (usually 7/8-inch or 1-inch bolts), a flathead screwdriver, a bucket or towel, and the replacement gasket. Buy the gasket by the toilet model number, which is usually printed on the back of the tank near the water line. If you can't find it, take a photo of the tank and bring it to a hardware store—staff can match it visually.

Have the bucket or towel ready because water will spill when you disconnect the tank. Even after you flush, a small amount of water stays in the tank and the supply line. Lay newspaper or a towel under the toilet to catch drips and protect your floor.

Turn off the water and empty the tank

Locate the shutoff valve on the water supply line under the toilet, on the left side. Turn it clockwise until it stops—you should feel resistance but don't force it. Then flush the toilet to empty the tank completely. If water still trickles into the bowl after the tank is empty, the shutoff valve may be worn, but you can still proceed; just work quickly and have your bucket ready.

If you can't find the shutoff valve or it won't turn, you can still do this job—just work carefully and have towels ready to catch water when you disconnect the supply line.

Disconnect the water supply line and remove the tank

Use your adjustable wrench to loosen the nut where the supply line connects to the bottom of the tank. Turn it counterclockwise until water drips out, then stop and let it drain into your bucket. Finish unscrewing by hand once the pressure is released. Set the supply line aside.

Now look under the bowl where the tank bolts attach. You'll see two or three bolts (usually two) with nuts on the underside. This is the critical step: loosen these bolts from underneath using your wrench, not from the top. Turn counterclockwise slowly. Once they're loose enough to turn by hand, unscrew them the rest of the way and set them aside in a safe place—you'll reuse them.

Carefully lift the tank straight up and away from the bowl. It's heavy (10 to 15 pounds) and awkward, so use both hands and lift slowly. Set it upside down on a towel on the floor or on a sturdy surface where it won't tip.

Remove the old gasket and install the new one

Look at the underside of the tank. You'll see a large pipe sticking down—that's the flush valve. The old gasket is a rubber or foam ring around the base of that pipe. Slide it off by hand. It may be stuck from years of mineral buildup, so wiggle it gently or use a flathead screwdriver to pry it loose if needed.

Slide the new gasket onto the flush valve the same way. It should sit flat and centered. If it's a rubber gasket, it may feel stiff at first—that's normal. Make sure it's seated all the way down and not twisted.

Reinstall the tank and tighten the bolts evenly

Carefully lower the tank back onto the bowl, aligning the bolt holes. Insert the bolts from underneath and hand-tighten the nuts on top first so the tank sits level. Then use your wrench to tighten the bolts in a crisscross pattern—tighten one bolt a quarter turn, then the opposite bolt a quarter turn, then repeat. This distributes pressure evenly and prevents cracking the porcelain.

Tighten until the tank is snug and doesn't rock, but don't over-tighten. You're looking for firm resistance, not maximum force. Over-tightening is the most common cause of cracks that show up days or weeks later.

Reconnect the water supply and test for leaks

Reattach the supply line to the bottom of the tank by hand first, then use your wrench to tighten the nut. Turn the shutoff valve counterclockwise to restore water flow. The tank will fill. Let it fill completely, then flush once and watch for leaks where the tank meets the bowl.

If water drips from under the tank, turn off the water again and tighten the bolts another quarter turn in a crisscross pattern. Refill and test again. If the leak continues after a second tightening, the gasket may not be seated correctly—turn off the water, remove the tank again, and check that the gasket is centered and flat on the flush valve.

Once the tank holds water without leaking, you're done. The repair is complete and should last another 5 to 10 years.

Common mistakes that cause problems

The most frequent error is tightening the bolts unevenly or too much, which cracks the porcelain bowl. Tighten in a crisscross pattern and stop as soon as the tank is snug. Another common mistake is not seating the gasket fully on the flush valve—if it's twisted or sitting off-center, water will leak no matter how tight the bolts are.

Some people also forget to turn off the water before starting, which creates unnecessary mess and makes the job harder. And a few try to remove the bolts from the top of the tank instead of from underneath, which is nearly impossible and can damage the bolt threads. Always work from underneath the bowl.

Frequently Asked Questions

How do I know if the gasket is the problem and not something else?

If the tank refills on its own without anyone flushing, and you don't hear water running into the bowl, the gasket is almost certainly the culprit. If you hear water trickling into the bowl, the fill valve or flapper inside the tank may be worn instead. You can peek inside the tank by removing the lid—if the flapper (the rubber disc at the bottom) looks cracked or doesn't seal, that's a different repair.

Can I replace the gasket without removing the tank?

No. The gasket sits underneath the tank where it bolts to the bowl, so you must remove the tank to access it. There's no way to reach it from the top or sides.

What if the bolts won't loosen?

Spray them with penetrating oil (like WD-40) and wait 15 minutes, then try again. If they're still stuck, explore more oil and wait longer. Avoid hammering or forcing, which can crack the bowl. If they remain stuck after an hour, you may need a plumber, but this is rare.

Do I need to replace the bolts too?

No. Reuse the old bolts unless they're visibly corroded or damaged. New bolts are inexpensive, but the old ones work fine if they're in decent shape.

The new gasket doesn't fit snugly on the flush valve—is that normal?

Gaskets vary slightly in fit depending on the tank model. If it slides on easily but sits flat and centered, it's fine. If it's so loose it falls off or sits crooked, you may have the wrong gasket for your toilet. Check the model number again and exchange it for the correct one.