What Tank Bolts Do and Why They Fail

Tank bolts are the large bolts that hold your toilet tank to the bowl. They run vertically through the base of the tank and screw into the bowl underneath, with rubber washers and metal washers on both sides to create a watertight seal. Over time—usually three to five years—these bolts corrode, crack, or the rubber washers deteriorate, causing leaks around the base of the toilet or making the tank feel loose when you sit down.

Replacing tank bolts is a straightforward repair you can do yourself with basic tools. The job takes 30 to 45 minutes and costs $10 to $25 for a replacement bolt kit. You do not need to remove the tank from the bowl, though you will need to drain it first.

Key Takeaways

  • Drain the tank completely by turning off the water supply and flushing, then use a sponge to remove remaining water from the tank bottom.
  • Tank bolt kits come with new bolts, rubber washers, and metal washers—buy a kit sized for your toilet rather than individual bolts.
  • Remove old bolts from underneath the bowl using a wrench or socket, then slide the new bolt up through the hole and hand-tighten before using a wrench.
  • Tighten bolts evenly in a crisscross pattern to avoid cracking the porcelain, and stop when resistance increases noticeably—do not force them.
  • Turn the water back on and watch for leaks around the base for 10 minutes before using the toilet normally.

Gather Your Tools and Materials

You will need an adjustable wrench or a socket set (usually 7/8-inch or 1/2-inch, depending on your toilet), a sponge or towel, a bucket, and a tank bolt replacement kit. These kits are sold at hardware stores and online retailers under names like "toilet tank bolt kit" or "toilet repair kit." Do not buy individual bolts—kits include the correct rubber and metal washers, which are essential for a watertight seal.

Check the bolt size before you buy. Most standard toilets use 5/16-inch or 3/8-inch bolts, but your kit packaging will tell you which size fits your model. If you are unsure, take a photo of your toilet's underside or bring the old bolt to the store.

Turn Off Water and Drain the Tank

Locate the water supply valve on the wall or floor behind the toilet. Turn it clockwise until it stops—you should feel resistance, but do not force it. Flush the toilet once to empty most of the water from the tank.

Water will remain in the tank bottom. Use a sponge to soak it up and wring it into a bucket. This step prevents water from spilling onto the floor when you work underneath. Once the tank is dry, you are ready to access the bolts from below.

Remove the Old Bolts from Underneath

Get under the toilet with your wrench or socket. You will see the bolt heads protruding from the underside of the bowl, usually two or three of them. Hold the wrench on the bolt head while you use another wrench to turn the nut on top of the bolt (inside the tank). Turn counterclockwise to loosen.

Once the nut is loose enough to turn by hand, remove it completely and set it aside. Slide the old bolt down and out from underneath. You may need to wiggle it slightly. Remove all old bolts this way, then pull off the old rubber and metal washers from the bolt holes in the bowl.

Install New Bolts with Washers

Take a new bolt from your kit. Slide it up through the hole in the bowl from underneath. The bolt should pass through easily. Once it is through, slide the rubber washer onto the bolt (the rubber side faces down toward the bowl), then the metal washer on top of that.

From inside the tank, thread the nut onto the bolt by hand until it is snug but not tight. Repeat this for all bolts. Once all bolts are in place with hand-tight nuts, you are ready to tighten them properly.

Tighten Bolts in a Crisscross Pattern

Use your wrench to tighten the bolts, but do not tighten one bolt all the way and then move to the next. Instead, tighten them in a crisscross pattern—if you have three bolts, tighten the first, then the one across from it, then the third. If you have two bolts, alternate between them. This distributes pressure evenly and prevents the porcelain from cracking.

Tighten each bolt a quarter-turn at a time. You will feel the resistance increase as the rubber washer compresses. Stop when you feel solid resistance—the bolts should be snug, not cranked down hard. Over-tightening is the most common mistake and can crack the bowl. If water still leaks after you turn the water back on, tighten a quarter-turn more, but only if necessary.

Restore Water and Check for Leaks

Turn the water supply valve counterclockwise to restore water to the tank. The tank will refill. Watch the base of the toilet where the tank meets the bowl for any water dripping or pooling. Wait at least 10 minutes, then flush once and watch again.

If you see water leaking, turn off the supply again and tighten the bolts a quarter-turn more in the same crisscross pattern. Restore water and check again. Most leaks stop after one additional tightening. If water continues to leak after a second tightening, the bowl itself may be cracked, and you will need to replace the toilet.

Frequently Asked Questions

Can I replace just one tank bolt, or do I need to replace all of them?

Replace all bolts at the same time, even if only one is visibly corroded. Bolts that look fine are usually close behind, and replacing them all ensures an even seal and prevents future leaks. A full kit is inexpensive and takes only a few extra minutes.

What if the bolt spins but the nut will not tighten?

The bolt threads may be stripped or damaged. Remove that bolt and replace it with a new one from your kit. If the new bolt also spins without tightening, the hole in the bowl may be stripped, and you will need a plumber to repair or replace the toilet.

Do I need to remove the tank to replace the bolts?

No. You can replace bolts with the tank in place by working from underneath the bowl. Removing the tank is more work and unnecessary for this repair.

How tight should the bolts be?

Tight enough that the tank does not rock when you press on it, but not so tight that you strain to turn the wrench. If you can turn the wrench with one hand and feel solid resistance, the bolt is tight enough. Porcelain cracks easily under excessive force.

What if water leaks from inside the tank, not around the base?

That is a different problem—likely a faulty fill valve or flapper, not the tank bolts. Tank bolt leaks appear at the base where the tank meets the bowl. If water is leaking inside the tank, you may need to replace internal components instead.