leaking toilet tank - man removing toilet tank bolts and gasket

How to Fix a Leaking Toilet Tank (3 Problems and Solutions!)

A leaking toilet tank can cause lots of water damage, so it needs to be fixed quickly.

These types of leaks often go unnoticed until the floor beneath the toilet soaks up the leaking water and begins to rot.

Before you go to the length of removing or replacing your toilet, determine whether the tank is truly leaking or simply “sweating.”

Determine if You Have a Leaking Toilet Tank

First, find out whether the supply locknut, a tank bolt, or the flush valve outlet is leaking.

Lie down under the toilet and dry each of those areas with a paper towel. Using a flashlight, watch to see where the first drop of water develops.

Another trick is to put food coloring into the toilet tank. Now take a white rag or tissue paper and place it under the tank bolts. If the color changes, you know the toilet tank is leaking.

If you confirm that the tank is leaking, then go ahead and turn off the water supply valve under the toilet, flush the toilet, and sponge-dry the inside of the tank. You’ll need to do this for any of the below solutions.

Leaky Toilet Flapper - add food coloring to tank
One way to identify where a toilet tank is leaking is by adding food coloring into the tank. Then place a white rag or paper towel over potential problem areas. If it turns color, you know where the leak is.

What You Need to Get Started

Prepare to fix the toilet tank leak with the following tools and materials. The job itself — regardless of which solution below you need — shouldn’t take more than 30 to 60 minutes.

  • Paper towel
  • Food coloring (optional)
  • Tailpiece gasket
  • 2 toilet tank bolts and gaskets
  • Flashlight
  • Sponge
  • Adjustable wrench
  • Screwdriver

Causes of Leaking Toilet Tanks (and the Solutions)

1. If the Supply Locknut is Leaking

The supply locknut is the main nut the screws to the bottom of the toilet tank. It connects the tank to the water supply line. If this is the area that’s leaking, then you need to replace the toilet fill valve. See our article titled “How to replace a toilet fill valve.”

leaking toilet tank - removing water supply locknut attached to toilet tank

2. If One or Both Tank Bolts are Leaking

This problem is easier and less time-consuming to solve. It’s either a corroded tank bolt or a worn-out gasket (or both) causing the leak.

  1. Using an adjustable wrench and screwdriver, remove one of the tank bolts and its gasket.
  2. Take the old gasket to a hardware store or plumber’s supply house and buy two replacement gaskets.
  3. Reinstall the tank bolt with one of the new gaskets, and then replace the other tank bolt gasket.
  4. Turn on the water supply valve and check for leaks. Use the same techniques as described above for determining if it is still leaking. If it is, try tightening the bolts a bit more.
leaking toilet tank - two tank bolts located under a toilet tank
These are the two tank bolts that need to be removed and replaced. Notice the black rubber material under the tank bolt nut. That’s the gasket that creates a watertight seal. You’ll need this piece replaced too.

3. If the Flush Valve Outlet is Leaking

Sometimes, the leak comes from the flush valve outlet. If this is the problem, you’ll need to remove the tank to repair the leaking toilet tank.

The tailpiece of this outlet can be found on the bottom of the tank. It’s where the water from the tank flows into the bowl. So, it will be the largest nut under your tank. Here’s how to fix it.

  1. Remove the top supply slip nut and the two tank-bolt nuts under the tank. Now lift the tank straight up.
  2. Set the tank upside down on the floor. Remove the spud washer by hand. Take that piece to a hardware store and purchase a replacement.
  3. Place the new spud washer over the flush valve tailpiece.
  4. With the new spud washer in place, lower the tank onto the base. You want the tank bolts to go through their holes in the base of the tank. Now attach the nuts and tighten until snug with the adjustable wrench.
  5. Reinstall the supply slip nut and turn on the water supply valve. Check for leaks. Tighten the tank bolds another quarter turn, if necessary.
leaking toilet tank - removing top supply slip nut under toilet tank
You’ll need to begin by removing the tank from the toilet. The first steps are to remove the water supply slip nut and the two tank-bolt nuts from under the tank. You’ll want to replace the flush valve tailpiece that you remove with a brand-new replacement.


Here are some great resources for fixing common toilet problems.

Final Thoughts

Identifying where a leak is coming from is always the first step in a leaking toilet tank. Thankfully, using a paper towel and perhaps a bit of food coloring in the tank will help you quickly determine where the leak is coming from.

If the leaking toilet tank occurs in any of the three problem areas noted above, you should have no trouble repairing the issue. Just follow the above instructions and bring the necessary tools, materials, and your DIY attitude!

Call 1-Tom-Plumber

Don’t hesitate to contact us here or call us at 1-Tom-Plumber (1-866-758-6237) if you need help with a leaking toilet tank.

1-Tom-Plumber’s certified team of plumbers and drain technicians respond immediately to any emergency plumbing, drain cleaning, or water damage problem. We also handle the excavation of underground water lines and sewer main lines. Our immediate-response team is available every day and night of the year, even on holidays.

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