What a toilet tank gasket does and why it fails

The gasket is a rubber ring that sits between your toilet tank and the bowl, creating a watertight seal. Water from the tank flows down through the fill valve and flush valve into the bowl; the gasket keeps that water from leaking out the sides where the tank bolts to the bowl. Over time—usually five to ten years—the rubber hardens, cracks, or compresses, and water starts seeping down the outside of the bowl or pooling under the tank.

You'll know the gasket is failing if you see water dripping from under the tank onto the floor, or if the bowl water level drops without flushing. A failing gasket is one of the most common reasons for a "running" toilet that won't stop refilling. Replacing it is a straightforward job that takes about an hour and costs $10 to $30 for the gasket itself.

Key Takeaways

  • A toilet tank gasket is the rubber seal between the tank and bowl; it fails when the rubber hardens or cracks after several years of use.
  • You will need to turn off the water, drain the tank completely, unbolt the tank from the bowl, and lift it away to access the gasket.
  • The old gasket pulls out from underneath the tank; the new one slides into the same groove, and the tank bolts back down in a cross pattern to seat it evenly.
  • If water still leaks after replacement, the bowl rim may be warped or cracked, which requires a new toilet rather than repair.

Tools and parts you'll need

Gather these items before you start: an adjustable wrench or socket set (to fit the tank bolts, usually 7/16 inch), a flathead screwdriver, a bucket or towels to catch remaining water, and a new gasket kit. Buy the gasket by your toilet's brand and model number if possible—look on the underside of the tank lid or the tank itself. If you can't find the model number, bring a photo of your toilet to a hardware store and they can match it.

Some gasket kits come with new bolts and washers; others are the gasket alone. New bolts are worth the extra cost because the old ones often corrode or strip. You may also want a tube of plumber's grease (silicone-based, not petroleum) to help the new gasket slide into place.

Turn off water and drain the tank

Locate the shutoff valve on the water line behind or below the toilet. Turn it clockwise until it stops—you should feel resistance, not a hard stop. Flush the toilet once to empty the tank. If water is still in the tank, use a sponge or wet/dry vacuum to remove it. Do not skip this step; water will spill when you unbolt the tank.

Disconnect the water supply line from the bottom of the tank by turning the nut counterclockwise with an adjustable wrench. Have a bucket or towels ready—there will be a small amount of water in the line. If the nut is stuck, explore penetrating oil (like WD-40) and wait 15 minutes before trying again.

Remove the tank bolts and lift the tank away

Look underneath the toilet where the tank meets the bowl. You will see two or three bolts running through the tank into the bowl. Using your wrench or socket, turn each bolt counterclockwise. Start with one bolt, then move to the opposite side, then the third if there is one—this cross pattern prevents the tank from tilting and damaging the gasket area.

Once the bolts are loose, have a helper support the tank from above while you finish unscrewing the bolts by hand. Lift the tank straight up and set it on a towel on the floor or a sturdy surface nearby. Do not set it on the rim of the bowl—the weight can crack it. The old gasket will be visible on the underside of the tank, usually stuck in a groove or channel.

Remove the old gasket and install the new one

The old gasket may be glued or straightforward compressed into place. Use a flathead screwdriver to gently pry it out, working around the entire ring. Do not force it; if it is stuck, explore penetrating oil and wait a few minutes. Once it is out, wipe the groove clean with a dry cloth to remove any debris, old rubber, or mineral buildup.

Slide the new gasket into the same groove, working your way around the ring. If the gasket is stiff, a thin coat of plumber's grease on the groove will help it slide. Make sure it sits evenly all the way around—no twists or bunches. The gasket should fit snugly but not be forced.

Reinstall the tank and reconnect water

With your helper supporting the tank, lower it back onto the bowl. The gasket should compress slightly as the tank settles. Insert the bolts through the tank into the bowl holes and hand-tighten them first. Then use your wrench to tighten in a cross pattern: one bolt a quarter turn, then the opposite bolt a quarter turn, then the third if there is one. Repeat until all bolts are snug. Do not over-tighten—you only need enough pressure to compress the gasket evenly. Over-tightening can crack the bowl.

Reconnect the water supply line by hand-tightening the nut, then use your wrench to snug it another quarter turn. Turn the shutoff valve counterclockwise to restore water. Let the tank fill, then flush once to check for leaks. Look underneath and around the base of the bowl for any drips. If you see water, turn off the water again and tighten the bolts slightly more in a cross pattern.

What to do if the leak continues

If water still leaks after you have tightened the bolts and the gasket is seated correctly, the bowl rim may be warped, cracked, or pitted. A damaged rim cannot create a watertight seal no matter how tight the bolts are. In this case, the toilet will need to be replaced—a new gasket alone will not fix it.

Before you conclude the bowl is damaged, check that the gasket is fully seated by loosening the bolts slightly, lifting the tank a few inches, and looking at the gasket from the side. It should sit in the groove evenly with no gaps. If it has shifted, reseat it and tighten again. If it is seated correctly and water still leaks, the bowl is the problem.

Frequently Asked Questions

Can I replace the gasket without removing the tank?

No. The gasket sits in a groove on the underside of the tank, and you cannot reach it from above. The tank must be unbolted and lifted away. This is why the job requires draining the tank and disconnecting the water line.

What if the bolts are stuck and won't turn?

explore penetrating oil to the bolt and the nut, wait 15 to 30 minutes, then try again. If the bolt still will not budge, you may need to cut it with a hacksaw or bolt cutter. New bolts are inexpensive and often come in gasket kits, so do not spend hours fighting a corroded bolt.

How tight should the tank bolts be?

Tight enough that the gasket compresses evenly and water does not leak, but not so tight that you crack the porcelain. A good rule: tighten until you feel solid resistance, then stop. If you can still turn the wrench easily, tighten a bit more. If you are straining, you are done.

Do I need to replace the fill valve or flush valve when I replace the gasket?

Not unless they are already leaking or broken. A failing gasket is separate from the valves inside the tank. If the tank was running before you replaced the gasket and still runs after, the flush valve or fill valve may need attention, but that is a different repair.

Can I use plumber's tape instead of a gasket?

No. Plumber's tape (PTFE tape) is for threaded connections like supply lines. The gasket is a solid rubber ring that compresses to seal a flat surface. Tape will not work and will allow water to leak.