What you are replacing and why it matters

The inside of your toilet tank holds three main parts: the fill valve (which lets water in when you flush), the flush valve (which lets water out into the bowl), and the flapper (a rubber seal that sits on top of the flush valve). When your toilet runs constantly, refills slowly, or won't stop leaking into the bowl, one of these parts has usually worn out or cracked. Replacing them costs between $15 and $50 in parts, takes about an hour, and requires only a wrench and a screwdriver.

You do not need to replace the entire tank. The inside parts are sold separately and snap or bolt into place. Most hardware stores stock universal kits that work on standard toilets made in the last 20 years.

Key Takeaways

  • Turn off the water at the shutoff valve behind the toilet, then flush to empty the tank before you start.
  • The flapper is the easiest part to replace first—it costs $5 to $10, takes five minutes, and fixes most running-toilet problems.
  • If the flapper does not stop the leak, the fill valve or flush valve needs replacement, which requires unbolting the tank from the bowl.
  • Universal repair kits come with instructions specific to your tank design, so read them before you buy to make sure the kit fits your model.
  • If water leaks from under the tank after reassembly, the rubber gasket between tank and bowl is damaged and must be replaced.

Shutting off water and emptying the tank

Locate the shutoff valve on the water line behind the toilet, usually a small knob or lever on the pipe coming from the wall. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. Flush the toilet once—the tank will empty and the fill valve will stop refilling.

If no water comes out on the flush, the valve is already off. If water still flows, the valve may be stuck or broken; in that case, you can still work on the tank, but water will drip from the fill valve as you disconnect it. Have a bucket and towels ready.

Replacing the flapper first

Lift the tank lid and set it on a towel where it will not roll off—it is heavy and breaks easily. Look inside the tank. The flapper is a rubber or plastic disc sitting on top of a hollow tube (the flush valve) at the bottom center of the tank. It is usually connected to a chain or rod that runs up to the flush handle.

Unhook the chain or rod from the flapper by hand. The flapper will either slide off the flush valve stem or be held by a clip on either side—check which before you pull. Slide or unclip the old flapper and set it aside. Slide the new flapper onto the stem in the same direction the old one came off, then reattach the chain or rod. The chain should have about half an inch of slack when the handle is at rest.

Fill the tank by turning the shutoff valve back on, then flush and watch. If the tank refills and stops, and the bowl does not leak water back into the tank, the flapper was the problem. If water still runs into the bowl after the tank is full, move to the next section.

Replacing the fill valve

The fill valve is the tall assembly on the left or right side of the tank (when facing the toilet from the front). It has a float arm or float cup attached to it. When the tank is full, the float rises and shuts off the valve. When you flush, the float drops and the valve opens to refill the tank.

Turn off the water again. Disconnect the water supply line from the bottom of the fill valve by turning the nut counterclockwise with a wrench. Water will drip—have a bucket ready. Reach under the tank from outside and locate the large nut holding the fill valve in place. Turn it counterclockwise with an adjustable wrench until it comes free. The valve will lift out from inside the tank.

Insert the new fill valve through the hole from inside the tank, then hand-tighten the nut from underneath. Use the wrench to snug it—do not over-tighten, or you will crack the tank. Reconnect the water supply line. Turn the water back on and let the tank fill. Adjust the float arm or cup so the water stops about an inch below the overflow tube (a hollow pipe in the center of the tank). Flush and refill to confirm the tank stops at the right level.

Replacing the flush valve

If the flapper replacement did not stop water leaking into the bowl, and the fill valve is working correctly, the flush valve itself is cracked or worn. This repair requires unbolting the tank from the bowl, so you will lose all the water in the tank and bowl.

Turn off the water and flush to empty both. Disconnect the water supply line. Reach under the tank and locate the two or three bolts holding the tank to the bowl—they run through the bottom of the tank and screw into the bowl. Use a wrench on the nut underneath the bowl while you turn the bolt head inside the tank with a screwdriver. Remove all bolts and carefully lift the tank straight up and off the bowl. Set it upside down on a towel.

The flush valve is a large assembly bolted to the underside of the tank. Turn the tank over and locate the large nut holding it in place. Turn it counterclockwise with a wrench until it comes free. The valve will lift out. Insert the new flush valve through the hole, hand-tighten the nut, then snug it with a wrench. Do not over-tighten.

Flip the tank right-side up and set it back on the bowl. The rubber gasket between tank and bowl must be in place—if it is cracked, replace it now before you bolt the tank down. Insert the bolts and hand-tighten them, then use a wrench to snug each one evenly. Reconnect the water supply line, turn the water back on, and let the tank fill. Flush and check for leaks under the tank.

Checking your work and fixing leaks

After reassembly, watch the tank fill and empty through two complete cycles. The tank should fill within two minutes and stop when full. When you flush, all the water should drain into the bowl, and the tank should refill without running water into the bowl afterward.

If water drips from under the tank where it meets the bowl, the rubber gasket is damaged. You will need to unbolt the tank again, remove the old gasket, and install a new one before rebolting. Gaskets cost $5 to $15 and come in universal sizes.

If the tank refills very slowly, the fill valve inlet screen may be clogged. Turn off the water, disconnect the supply line, and turn the water back on briefly into a bucket to flush debris. Then reconnect and test again.

When to call a plumber instead

If the tank itself is cracked (you will see water pooling under the toilet or dripping from the outside of the tank), the tank cannot be repaired and must be replaced. This is a job for a plumber because it requires removing the entire toilet, disconnecting the drain, and installing a new unit.

If you cannot turn the shutoff valve or the bolts holding the tank are stuck and will not budge after you explore steady pressure, stop and call a plumber. Forcing stuck bolts can crack the tank or strip the threads, making the repair much more expensive.

Frequently Asked Questions

Do I need to turn off the water before I start?

Yes. Turning off the water prevents the tank from refilling while you work and stops water from spraying out when you disconnect the supply line. The shutoff valve is on the pipe behind the toilet, usually within reach of your hand.

What if my toilet has a dual-flush button instead of a handle?

Dual-flush toilets use the same flapper and fill valve as single-flush models, but the mechanism connecting the button to the flapper is different. The flapper replacement process is identical. For fill valve or flush valve work, check the kit instructions to confirm it fits your dual-flush model before you buy.

Can I replace just the flapper without turning off the water?

Yes. The flapper sits above the water line inside the tank, so you can replace it without disconnecting the supply line. Flush first to lower the water level, then unhook and swap the flapper. You only need to turn off the water if you are working on the fill valve or supply line.

How do I know which part is broken if my toilet is running?

Add food coloring to the tank and watch without flushing. If the color moves into the bowl, the flapper is leaking. If the tank keeps refilling on its own, the fill valve is not shutting off. If the bowl water level is dropping slowly, the flush valve is cracked.

What if the new parts do not fit my toilet?

Universal kits work on most toilets, but some older or specialty models use different connections. Bring a photo of your tank interior to the hardware store, or remove the old part and bring it with you. Staff can confirm whether a kit will fit before you buy.