What you're actually replacing inside a toilet tank

The "guts" of a toilet are the mechanical parts inside the tank that fill it with water and let it drain when you flush. These parts wear out over time and cause running water, weak flushes, or leaks. You do not need a plumber for this job — the parts are inexpensive, the work is straightforward, and most homeowners can do it in 30 minutes to an hour with basic tools.

The main parts you might replace are the fill valve (also called the ballcock), which refills the tank after a flush; the flush valve, which sits at the bottom and lets water drain into the bowl; and the flapper, a rubber seal that sits on top of the flush valve and lifts when you flush. You may also replace the overflow tube, a hollow pipe that prevents the tank from overflowing, or the supply line, the hose that brings water into the tank.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet before you start any work inside the tank.
  • The flapper is the cheapest and easiest part to replace, and fixing a running toilet often means replacing only the flapper.
  • Fill valves and flush valves require draining the tank completely, which takes a few minutes longer but follows the same basic steps.
  • Bring a photo of your current fill valve or flush valve to the hardware store, because different toilets use different sizes and styles.
  • If water leaks from under the tank or the bowl will not stop running after you replace the flapper, you may need a professional to reseat or replace the flush valve.

Turn off the water and drain the tank

Before you touch anything inside the tank, locate the shutoff valve on the water supply line under the toilet. It looks like a small knob or lever attached to the pipe coming from the wall. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. This prevents water from flowing into the tank while you work.

Next, flush the toilet to empty the tank. If the shutoff valve is stuck or leaking, you may need to hold down the handle longer or place a bucket underneath to catch drips. Once the tank is empty, you can lift the lid and set it somewhere safe — it is porcelain and breaks easily.

Replacing the flapper

The flapper is a rubber disc or cup attached to a chain that lifts when you push the flush handle. Over time it hardens, warps, or develops a poor seal, causing water to leak slowly into the bowl and the toilet to run constantly. Replacing it is the fastest fix and costs $3 to $8.

To remove the old flapper, unhook the chain from the flush lever arm (the metal rod that moves when you push the handle). Then slide the flapper off the pins or hooks that hold it on the overflow tube. Take the old flapper to the hardware store or use it to measure, because flappers come in different sizes — the most common are 2-inch and 3-inch.

Slide the new flapper onto the pins, then reattach the chain. The chain should be taut but not tight — if it is too tight, the flapper will not seal completely. Adjust the chain length so there is about a half-inch of slack when the flapper is closed. Turn the water back on and test the flush. If the toilet still runs, the problem may be the fill valve or a damaged flush valve seat.

Replacing the fill valve

The fill valve controls how much water enters the tank after a flush. If water runs continuously or the tank fills very slowly, the fill valve may be worn. Replacement costs $15 to $30 and takes 20 to 30 minutes.

With the tank already drained, locate the fill valve — it is usually on the left side of the tank and has a water supply line connected to it. Disconnect the supply line by turning the nut that holds it counterclockwise. You may see a small amount of water drip out. Place a bucket or towel underneath to catch it.

Under the tank, you will see a large nut holding the fill valve in place. Use an adjustable wrench to turn it counterclockwise and remove the old valve. Slide the new fill valve into the hole from inside the tank, then hand-tighten the nut underneath. Do not over-tighten — snug is enough. Reconnect the supply line and turn the water back on. The tank should fill within a few seconds. Adjust the water level by turning the adjustment screw or clip on the fill valve until the water stops about an inch below the top of the overflow tube.

Replacing the flush valve

The flush valve sits at the bottom center of the tank and lets water drain into the bowl when you flush. If water leaks from under the tank or into the bowl even when the flapper is new, the flush valve may be cracked or the rubber seal may be damaged. Replacement costs $20 to $40 and requires draining the tank completely.

With the tank empty, look at the bottom center. You will see a large nut underneath the tank holding the flush valve in place. Use an adjustable wrench to turn it counterclockwise. From inside the tank, you can now lift out the old flush valve. Some models have a rubber gasket or spud washer underneath — keep this if it is still in good condition, or replace it if it is cracked or compressed.

Slide the new flush valve into the hole, making sure the rubber gasket sits flat underneath. Hand-tighten the nut from below, then use the wrench to snug it — again, do not over-tighten. Reconnect the flapper and chain, turn the water back on, and test. If water still leaks from under the tank after replacement, the tank itself may be cracked and you will need a professional assessment.

Replacing the supply line

The supply line is the hose that brings water from the shutoff valve into the tank. If you see water pooling under the toilet or dripping from the connection, the line may be kinked, cracked, or the fitting may be loose. A new supply line costs $5 to $15.

Turn off the water at the shutoff valve. Use an adjustable wrench to disconnect the old line at both ends — one end connects to the shutoff valve, the other to the fill valve. Slide the new line into place and hand-tighten both connections, then use the wrench to snug them. Turn the water back on and check for leaks at both connections. If water still drips, turn off the water, remove the line, and reconnect it — sometimes a second try seats the fitting properly.

When to call a professional plumber

Most toilet tank repairs are straightforward, but a few situations call for professional help. If water leaks from under the tank even after you replace the flush valve, the tank itself may be cracked. If the shutoff valve under the toilet will not turn or leaks when closed, a plumber can replace it. If you replace the flapper and fill valve and the toilet still runs constantly, the problem may be a damaged flush valve seat that needs to be reseated or the tank may need replacement.

Also call a professional if you cannot disconnect the supply line or the large nuts under the tank — corrosion or over-tightening can make them stuck, and forcing them risks cracking the porcelain. A plumber has the tools and experience to remove stuck fittings without damage.

Frequently Asked Questions

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

Yes. Always turn off the shutoff valve under the toilet before you open the tank or disconnect any lines. This prevents water from flowing while you work and makes the job cleaner and safer.

What if I do not know which part is broken?

Listen to the toilet. A constant hissing or trickling sound usually means the flapper is leaking — replace it first, as it is the cheapest fix. If the tank fills very slowly or not at all, the fill valve is likely the problem. If water pools under the toilet, the supply line or a connection is leaking.

Can I replace parts without draining the tank?

You can replace the flapper without draining the tank — just turn off the water and flush once. For the fill valve or flush valve, you must drain the tank completely to avoid water spilling everywhere and to see what you are doing.

How do I know what size flapper to buy?

Take the old flapper to the hardware store and compare it to the options, or measure the diameter — most are 2 or 3 inches. If you are not sure, bring a photo of the inside of your tank and a store employee can help you match it.

What if the new part does not fit?

Check that you bought the right size and style for your toilet model. If the part physically does not fit the holes or connections, return it and bring the old part or a photo to the store. Different toilet brands use different mounting systems, and the staff can help you find the correct replacement.