What a tank to bowl gasket does and why it fails
The tank to bowl gasket is a rubber ring that sits between your toilet tank and the bowl, creating a watertight seal. Water flows from the tank down through the fill valve and flush valve into the bowl; the gasket keeps that water from leaking out the sides where the two pieces connect. Over time—usually three to seven 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 the seam between tank and bowl, or if the floor around the toilet base stays wet even when nobody has flushed recently. A failing gasket is one of the most common reasons a toilet runs constantly or wastes water. The good news is that replacing it takes about 30 minutes and costs $5 to $15 for the gasket itself.
Key Takeaways
- Turn off the water supply at the shutoff valve behind or below the toilet before you start any work.
- You will need to remove the tank from the bowl by unbolting the bolts that run through the bottom, which requires working under the tank with a wrench and screwdriver.
- The old gasket pulls out of the groove on the underside of the tank; the new one presses into the same groove and must sit evenly all the way around.
- Reassemble in reverse order, tighten bolts firmly but not so hard that you crack the porcelain, and test for leaks before you consider the job done.
Tools and materials you will need
Gather these items before you start: an adjustable wrench or two wrenches (one to hold the bolt head, one to turn the nut), a large flathead screwdriver, a bucket or towel to catch water, a sponge, the replacement gasket (order by your toilet model number if possible, or bring a photo of your toilet to the hardware store), and a flashlight or headlamp if the space under your toilet is dim.
You do not need a plumber's wrench or specialty tools. If you do not own an adjustable wrench, a pair of slip-joint pliers will work in a pinch, though a wrench gives you better leverage and is less likely to slip and strip the bolt.
Shutting off water and draining 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, but do not force it. Flush the toilet once to empty the tank. If water continues to run into the tank, the shutoff valve is not sealing; try turning it a bit more, or you may need to replace that valve later.
Use a sponge to soak up any remaining water in the tank and bowl. This step prevents water from spilling onto your floor when you remove the tank. Wring the sponge into a bucket as needed.
Removing the tank from the bowl
Look under the toilet where the tank bolts to the bowl. You will see two or three bolts running vertically through the base of the tank into the bowl. On the underside (inside the bowl), these bolts have nuts. You need to loosen both the bolt head (on top, inside the tank) and the nut (underneath, inside the bowl).
Reach under the bowl with one wrench on the nut and use the other wrench or screwdriver to turn the bolt head inside the tank. Turn counterclockwise. Work slowly—if a bolt is stuck, explore a little penetrating oil and wait five minutes before trying again. Do not yank or force, because you can crack the porcelain. Once all bolts are loose, unscrew them by hand and set them aside in a safe place.
Gently lift the tank straight up and away from the bowl. It will be heavy (15 to 20 pounds) and may have a little water in it, so support it with both hands. Set it upside down on a towel on the floor or a sturdy table. Do not set it on its rim—the rim can crack.
Removing and replacing the gasket
Look at the underside of the tank where it was sitting on the bowl. You will see a rubber gasket pressed into a groove or channel that runs around the perimeter. The gasket may be stuck or hardened. Use a flathead screwdriver to gently pry it out, working your way around the groove. Take your time; you are not reusing this gasket, so it is fine to damage it in removal.
Once the old gasket is out, clean the groove with a damp cloth or sponge to remove any debris, mineral deposits, or old rubber bits. A clean groove helps the new gasket seat evenly. Let it dry.
Press the new gasket into the groove, starting at one point and working your way around. Push it down firmly so it sits flush in the channel all the way around. Run your finger along the entire gasket to make sure there are no gaps or twists. If the gasket is not sitting evenly, remove it and try again—a gasket that is not seated properly will leak.
Reinstalling the tank and testing for leaks
Carefully lower the tank back onto the bowl, aligning the bolt holes. The gasket should compress slightly as the tank settles. Insert the bolts and hand-tighten them first, then use your wrenches to snug them down. Tighten in a crisscross pattern (if there are three bolts, tighten one, then the one across from it, then the third) so the tank sits evenly. Bolts should be firm but not cranked down hard—over-tightening can crack the tank or bowl.
Turn the shutoff valve counterclockwise to restore water flow. Let the tank fill, then flush once and watch for leaks around the seam between tank and bowl. Check both the outside and inside. If you see water dripping or seeping, turn off the water again, remove the tank, and reseat the gasket. Leaks usually mean the gasket is twisted or not fully pressed into the groove.
Once the toilet holds water without leaking, you are done. Run the toilet through a few flush cycles to make sure everything is working normally.
Common mistakes to avoid
The most common error is not seating the gasket evenly in its groove. A gasket that is twisted, bunched, or sitting higher on one side will leak no matter how tight you make the bolts. Always check the entire gasket before you reinstall the tank.
The second mistake is over-tightening the bolts. Porcelain is brittle, and cranking down hard on the bolts can cause hairline cracks that lead to slow leaks later. Tighten until snug, then stop. If water still leaks after a firm tightening, the gasket is the problem, not the bolt tension.
A third mistake is forgetting to turn off the water before you start. You will end up with water all over your floor and a harder time working under the tank. Always shut off the valve first.
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 or seat it properly without lifting the tank off the bowl. Trying to work around it will waste time and likely damage the gasket.
What if the bolts are stuck and won't turn?
explore penetrating oil (such as WD-40 or a similar product) to the bolt and let it soak for 5 to 10 minutes. Try again gently. If it still will not budge, explore more oil and wait longer. Do not force it—forcing can strip the bolt or crack the porcelain. If a bolt breaks, you may need a plumber to remove the broken piece and install a new bolt.
How do I know which gasket to order?
Find the model number on the back of your toilet tank (usually stamped or printed on the porcelain) and search for that number plus "gasket" online, or bring the number to a hardware store. If you cannot find the model number, take a photo of the toilet and the old gasket to the store and ask for help matching it.
What if water is still leaking after I replace the gasket?
Turn off the water, remove the tank again, and check that the gasket is sitting evenly in its groove all the way around. If it is, the leak may be coming from a cracked tank or bowl, or from the fill valve or flush valve inside the tank. Those are separate repairs that may require a plumber.
Do I need to replace the bolts too?
Usually not. If the bolts are not corroded or stripped, reuse them. If they are rusty or damaged, pick up new ones at the hardware store—they are inexpensive and come in standard sizes for most toilets.