What You Need to Do Before You Start

Toilet tank bolts corrode, crack, or strip over time, and replacing them stops leaks at the base of the tank and prevents water damage to your floor. You can do this job yourself with basic tools in about 30 minutes, and the bolts themselves cost between $5 and $15 for a full set.

Before you touch anything, turn off the water supply valve behind the toilet and flush to empty the tank. If you cannot locate the valve or it does not shut off completely, place a bucket under the tank to catch water that will spill when you remove the bolts.

Toilet tank bolts come in two types: the bolts that hold the tank to the bowl (the most common replacement), and the bolts that hold the fill valve or flush valve inside the tank. This guide covers replacing the bolts that attach the tank to the bowl, which is the job most people face.

Key Takeaways

  • Turn off the water supply and flush the tank completely before removing any bolts.
  • Tank-to-bowl bolts are accessed from underneath the toilet, and you will need a wrench or socket set to remove the nuts on the underside.
  • Replacement bolts are sold as kits that include the bolt, rubber washer, and nut, and cost $5 to $15 per set.
  • Tighten new bolts firmly but not aggressively — over-tightening cracks the porcelain tank.

Gather Your Tools and Materials

You will need an adjustable wrench or a socket set with a wrench that fits the nut size (usually 5/16 inch or 3/8 inch). Have a flathead screwdriver on hand to help pry the rubber washers out if they stick. A bucket or towel placed under the tank catches any remaining water.

Buy a replacement bolt kit before you start — do not remove the old bolts and then hunt for new ones. Kits are sold at any hardware store or home center and include the bolt, rubber washer, and nut. Bring the old bolt or a photo of it to the store if you are unsure of the size.

If the bolts are severely corroded or stuck, a penetrating oil like WD-40 sprayed on the nut 10 to 15 minutes before you work makes removal easier. Have a rag nearby to wipe your hands and the bolt heads.

Remove the Old Bolts from Underneath

Lie on your back or crouch behind the toilet to access the bolts from underneath. You will see two or three bolts running vertically through the base of the tank, with nuts on the underside. The rubber washers sit between the tank and the bowl.

Hold the bolt head steady with one wrench while you turn the nut counterclockwise with another wrench. If you have only one wrench, grip the bolt head with a flathead screwdriver from inside the tank (you may need to reach in from the top) while you turn the nut below. Work slowly — if the bolt spins without the nut turning, you are not holding it firmly enough.

Once the nut is loose, unscrew it by hand and set it aside. Pull the bolt out from the top of the tank. The rubber washer will likely stick to the underside of the tank; pry it out gently with a flathead screwdriver and discard it.

Install the New Bolts and Washers

Slide the new rubber washer onto the bolt first, then insert the bolt down through the hole in the tank from the top. The washer should sit flat against the underside of the tank. If the hole is slightly misaligned, wiggle the tank gently until the bolt slides through.

From underneath, slide the rubber washer that came in your kit onto the bolt, then thread the nut onto the bolt by hand. Tighten the nut with your wrench, turning clockwise. Stop when the nut is snug — you should feel resistance but not have to strain. Over-tightening cracks the porcelain tank.

Repeat this process for each bolt. Most toilets have two or three tank bolts. Tighten them in a cross pattern if there are three (tighten one, then the one across from it, then the third) to keep pressure even on the tank.

Check for Leaks and Refill

Once all bolts are installed and tightened, turn the water supply valve back on slowly. Let the tank fill completely, then flush once to test the system. Watch the base of the tank and the floor underneath for any water dripping or pooling.

If you see a leak at one of the bolts, turn off the water, empty the tank, and tighten that bolt a quarter turn more. Refill and check again. If the leak continues, the bolt may be damaged or the hole in the tank may be cracked — in that case, the tank itself will need to be replaced.

Leave the toilet off for 30 minutes after installation, then check again. Sometimes a small amount of water seeps out as the rubber washers settle, but this should stop quickly.

When Bolts Are Stuck or Corroded

If a bolt will not budge, spray the nut with penetrating oil and wait 15 to 20 minutes. Try again with steady, even pressure on the wrench — jerking or sudden force can snap the bolt. If it still will not turn, explore more oil and wait another 15 minutes.

As a last resort, you can cut the bolt with a hacksaw or rotary tool from underneath, but this is messy and takes longer. Most stuck bolts will loosen with patience and oil. If the bolt head breaks off inside the tank, you will need to drain the tank completely and fish out the piece before installing a new bolt.

Severely corroded bolts are a sign that your toilet has been leaking for a while. Once you replace the bolts, inspect the floor underneath and around the base of the toilet for soft spots or discoloration in the wood or subfloor. Water damage may require repair beyond the bolt replacement.

Frequently Asked Questions

How tight should I make the bolts?

Tighten until you feel firm resistance and the nut will not turn further with moderate hand pressure on the wrench. The goal is to compress the rubber washer enough to seal, not to crush it. If you hear a cracking sound, you have gone too far — back off when ready.

What size bolts does my toilet need?

Most toilets use 5/16-inch or 3/8-inch bolts, typically 1.5 to 2 inches long. Bring your old bolt to the hardware store or take a photo, and the staff can match it. Kits are labeled by size, so you cannot install the wrong one by accident.

Can I replace just one bolt instead of all three?

You can, but it is better to replace all of them at once. If one bolt has corroded, the others are likely corroding too, and replacing them all prevents a second leak a few months later. The job takes only slightly longer with three bolts than with one.

What if water is still leaking after I tighten the bolts?

The leak may be coming from a different source — the fill valve, the flush valve, or a crack in the tank itself. Check that water is actually dripping from the bolt area and not running down from the tank top or seeping from the bowl. If the leak is definitely at the bolts, the tank may be cracked and will need replacement.

Do I need to drain the tank completely?

No. Turning off the water and flushing empties most of it. A small amount of water will spill when you remove the bolts, which is why you place a bucket underneath. If you want to avoid any spill, you can sponge out the remaining water from inside the tank before you start.