How to Fix a Tank-to-Bowl Leak
Updated February 25, 2026
Stop water leaking between the tank and bowl by replacing the tank-to-bowl gasket, bolts, and washers -- a 30-minute fix that prevents floor damage.
Overview
Water pooling behind the toilet or dripping from where the tank meets the bowl is almost always a failed tank-to-bowl gasket or corroded tank bolts. The rubber gasket between the tank and bowl compresses over time and loses its seal. The brass or steel bolts that hold the tank down corrode, and the rubber washers around them crack. Replacing these parts stops the leak and costs under $15. If you ignore it, the water damages your subfloor -- and subfloor replacement costs $500-$2,000.
What You'll Need
Safety First
- Support the tank when removing the bolts. A two-piece toilet tank weighs 25-40 pounds and sits on a narrow ledge. It can slide off and crack on the floor.
- Do not overtighten tank bolts. Alternate sides and tighten incrementally. Too much force cracks the porcelain tank or bowl -- and that is not repairable.
Step-by-Step Instructions
- Confirm the Leak Source
Dry everything with towels. Put food coloring in the tank water. Wait 15-20 minutes without flushing. Check: colored water between the tank and bowl or on the bolts? That confirms the tank-to-bowl connection is the source. Colored water at the base on the floor? That is the wax ring -- different fix. Clear water on the outside of the tank? That is condensation, not a leak.
Tip: Tank condensation is extremely common in humid climates. The cold supply water chills the porcelain and humid air condenses on it. If the water is uniform across the tank surface (not just at the bolts), it is condensation. An anti-sweat valve ($30-60) fixes condensation by mixing warm water into the supply. - Shut Off Water and Drain the Tank
Turn off the supply valve. Flush and hold the handle to drain. Sponge out all remaining water -- every drop. Water left in the tank will pour out when you lift it off. Disconnect the supply line from the fill valve tailpiece under the tank.
Tip: A turkey baster works surprisingly well for getting the last inch of water out of the tank corners where a sponge cannot reach. - Remove the Tank
Under the tank, find the two (sometimes three) tank bolts. Hold the bolt head inside the tank with a large flathead screwdriver. Underneath, remove the nut with an adjustable wrench. Corroded and will not turn? Apply penetrating oil, wait 15 minutes. Still stuck? Cut the bolt with a mini hacksaw or oscillating tool. Once both bolts are out, lift the tank straight up off the bowl. Set it upside down on a towel.
Tip: Brass bolts resist corrosion far better than steel. When you reassemble, use brass tank bolts ($5 for a set). They will still be removable in 10 years. - Replace the Gasket and Bolt Washers
Peel the old spud gasket (the large rubber ring around the flush valve opening on the bottom of the tank) off. Clean any residue. Press the new gasket on -- it fits over the flush valve nut. Thick side faces the bowl. Insert new tank bolts from inside the tank, with new rubber washers on the inside (tank side) of each bolt. The rubber washer seals the bolt hole in the porcelain.
Tip: Tank-to-bowl gasket kits ($5-10) include the gasket, bolts, washers, and nuts. Fluidmaster 6102 and Korky 481BP are the two most common kits. Buy the kit, not individual parts. - Reinstall the Tank
Lower the tank onto the bowl, aligning the bolt holes and centering the gasket over the bowl inlet. Thread the nuts onto the bolts from underneath -- hand-tight only at first. Then alternate: half turn on the left bolt, half turn on the right. Check the tank is level. Keep alternating until the tank sits firmly on the bowl with no wobble. Do not overtighten. The gasket should compress but the tank should not be squeezed so hard that the porcelain is stressed.
Tip: The tank should sit level and not rock. If it rocks, one side is tighter than the other. Loosen the tight side slightly and tighten the loose side. The goal is even pressure on both bolts. - Reconnect and Test
Reconnect the supply line. Turn on the water slowly. Let the tank fill completely. Flush 4-5 times. After each flush, check: the bolt nuts underneath for drips, the gasket area between the tank and bowl, and the floor behind the toilet. Dry everything with a towel after each flush to catch any new moisture. No drips after 5 flushes? The repair is solid.
Tip: Check again 24 hours later. Some very slow leaks only show up after extended use. A dry towel placed under the connection point overnight is the best test -- any moisture is visible on a dry towel.
Pro Tips
- Tank-to-bowl gaskets come in different thicknesses. Standard is about 3/4 inch. If your old gasket was extra thick (1 inch+), buy the matching thickness or the tank will sit too low and the bolts will not reach.
- While the tank is off, inspect the flush valve seat for corrosion. A corroded seat causes a running toilet. Replacing it now saves a future disassembly.
- Toilets made before 2000 often have brass bolts and rubber washers that crumble when you try to remove them. Budget extra time for cutting corroded bolts.
- If the bolt holes in the porcelain are chipped or cracked, the toilet needs replacing. No gasket or bolt can seal a cracked mounting surface.
- Three-bolt tanks (some Kohler and American Standard models) use a triangular gasket. Make sure you buy the correct gasket for your model -- the standard round gasket will not work.
When to Call a Pro
Call a plumber if the porcelain is cracked at the bolt holes or gasket surface (toilet replacement needed), if the flush valve nut on the bottom of the tank is corroded and will not come off (risk of cracking the tank), or if you have a concealed-tank or wall-hung toilet (different sealing mechanism).
Frequently Asked Questions
When should I call a plumber to fix a tank-to-bowl leak?
Call a plumber if the porcelain is cracked at the bolt holes or gasket surface (toilet replacement needed), if the flush valve nut on the bottom of the tank is corroded and will not come off (risk of cracking the tank), or if you have a concealed-tank or wall-hung toilet (different sealing mechanism).
What are some expert tips to fix a tank-to-bowl leak?
Tank-to-bowl gaskets come in different thicknesses. Standard is about 3/4 inch. If your old gasket was extra thick (1 inch+), buy the matching thickness or the tank will sit too low and the bolts will not reach. While the tank is off, inspect the flush valve seat for corrosion. A corroded seat causes a running toilet. Replacing it now saves a future disassembly. Toilets made before 2000 often have brass bolts and rubber washers that crumble when you try to remove them. Budget extra time for cutting corroded bolts. If the bolt holes in the porcelain are chipped or cracked, the toilet needs replacing. No gasket or bolt can seal a cracked mounting surface. Three-bolt tanks (some Kohler and American Standard models) use a triangular gasket. Make sure you buy the correct gasket for your model -- the standard round gasket will not work.
What tools do I need to fix a tank-to-bowl leak?
You will need: Adjustable wrench (for tank bolt nuts), Large flathead screwdriver (to hold bolt heads inside the tank), Tank-to-bowl gasket kit with bolts and washers ($5-10), Sponge and towels (for draining residual water), Penetrating oil (for corroded bolts), Mini hacksaw (if bolts are too corroded to unscrew).
When should I call a professional instead of doing this myself?
Call a plumber if the porcelain is cracked at the bolt holes or gasket surface (toilet replacement needed), if the flush valve nut on the bottom of the tank is corroded and will not come off (risk of cracking the tank), or if you have a concealed-tank or wall-hung toilet (different sealing mechanism).
How difficult is this project?
This project is rated easy. Most homeowners with basic tools can complete it.
How long does this take?
Plan for approximately 30 - 60 minutes. First-timers may need extra time.
How much will this cost?
The estimated cost is $5 - $15. Costs vary by location and materials chosen.