What You're Actually Replacing

The "guts" of a toilet are the mechanical parts inside the tank — the fill valve (which refills the tank after you flush), the flush valve (which lets water rush into the bowl), and the flapper (the rubber seal that holds water in the tank until you push the handle). These parts wear out over time, causing running water, weak flushes, or a tank that won't refill. Replacing them is a straightforward job that takes 20 to 45 minutes and costs $15 to $40 in parts.

You do not need special tools. A flathead screwdriver, an adjustable wrench, and a bucket are enough. The work is dry — you'll turn off the water before you start — and the parts are designed to come apart by hand or with gentle leverage.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the tank, flush to empty the tank, and sponge out any remaining water before you begin.
  • The fill valve screws into the bottom of the tank from underneath; the flush valve bolts through the center; the flapper sits on top of the flush valve and can usually be lifted straight out.
  • Buy a repair kit that matches your toilet's brand and model if possible, or a universal kit if the model is unknown — kits include the flapper, fill valve, and flush valve together.
  • Hand-tighten all connections first, then use a wrench only if water leaks; over-tightening cracks the tank.
  • Test by turning the water back on, letting the tank fill, and flushing twice to confirm the tank refills and the bowl drains completely.

Turn Off Water and Empty the Tank

Locate the shutoff valve on the water supply line under the tank — it looks like a small knob or lever attached to the copper or plastic pipe coming from the wall. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. Do not force it; it should turn smoothly.

Flush the toilet once to empty the tank. If water is still sitting in the bowl or tank after the flush, the shutoff valve may not be fully closed. Turn it a bit more and try again. Once the tank is empty, use a sponge or old towel to soak up any remaining water pooled at the bottom — this keeps you dry while you work and prevents water from spilling onto the floor.

Identify Which Parts Need Replacing

Lift the tank lid carefully and set it on a towel where it won't roll. Look inside. The fill valve is on the left or right side of the tank, usually a plastic or brass cylinder about 3 inches tall with a float arm attached. The flush valve is in the center bottom of the tank, a larger cylinder with the flapper (a rubber disc) sitting on top of it. If water is running into the bowl even when you haven't flushed, the flapper is worn. If the tank fills slowly or incompletely, the fill valve is failing. If the toilet runs constantly or the bowl won't drain fully, the flush valve is the problem.

You can replace one part at a time or all three together. Kits that include all three are usually cheaper than buying them separately and are designed to work as a set. Check the brand name stamped on the tank (often on the inside back wall) and look for a kit labeled for that brand — American Standard, Kohler, Toto, and Eljer kits are common. If you cannot find the brand or it is unmarked, a universal repair kit works with most toilets.

Remove the Old Fill Valve

The fill valve screws into the bottom left or right of the tank from underneath. You will need to reach under the tank with an adjustable wrench. Locate the large nut holding the valve in place on the underside. Turn it counterclockwise with the wrench while steadying the valve body from inside the tank with your other hand — this prevents the valve from spinning and damaging the tank. Once the nut is loose enough, unscrew it by hand.

Pull the old valve straight up and out of the tank. Water may drip from the connection; have your bucket ready. Wipe the opening clean with a dry cloth. The new valve will come with a rubber or foam washer; check that it is seated properly in the opening before you install the new valve.

Remove the Old Flush Valve and Flapper

The flapper is the easiest part to remove first. Lift it straight up off the flush valve stem — it usually comes free by hand, though you may need to wiggle it gently. Set it aside. Next, look under the tank for the large bolt holding the flush valve in place. Use your wrench to turn it counterclockwise while steadying the valve from inside the tank. Once loose, unscrew it by hand and pull the old valve out from the top of the tank.

Wipe the opening clean and inspect the rubber gasket that seals the valve to the tank. If it is cracked or hardened, replace it with the gasket included in your repair kit. If it looks intact, you can reuse it, but most kits include a new one — using it ensures a watertight seal.

Install the New Parts

Start with the flush valve. Insert it into the center opening from the top of the tank, aligning the bolt hole. Reach under the tank and hand-tighten the bolt first — do not use the wrench yet. Once it is snug, place the new flapper on the valve stem. It should sit flat and move freely up and down.

Next, install the fill valve. Insert it into its opening from underneath the tank, aligning the threads. Hand-tighten the nut from below. Attach the float arm and clip according to the kit instructions — the float should move freely without hitting the tank walls. Once all three parts are hand-tight, check for leaks by turning the water back on slowly. If water drips from any connection, turn the water off, dry the area, and use the wrench to tighten that nut a quarter turn at a time until the leak stops. Overtightening cracks the tank, so tighten only as much as needed.

Test the Refill and Flush Cycle

Turn the shutoff valve counterclockwise (or flip the lever parallel to the pipe) to restore water flow. Watch the tank fill. It should take 30 to 60 seconds and stop automatically when the float reaches the top. The water level should be about an inch below the overflow tube (a hollow pipe in the center of the tank). If the tank overfills or does not fill, adjust the float arm — bend it slightly downward to lower the water level or upward to raise it.

Flush the toilet and listen for the tank to refill. The bowl should drain completely and the tank should refill without running water into the bowl after the flush is done. Flush a second time to confirm the cycle repeats smoothly. If water continues to run into the bowl after the second flush, the flapper may not be seating properly — lift the tank lid and check that the flapper is centered on the valve stem and not tilted.

Frequently Asked Questions

Can I replace just the flapper without replacing the whole valve?

Yes. The flapper is the easiest part to replace alone and often fixes a running toilet. Lift it off the flush valve stem, take it to a hardware store to match the size and shape, and install the new one the same way. If the toilet still runs after a new flapper, the flush valve itself is worn and needs replacing.

What if water leaks from under the tank after I install the new valve?

Turn off the water, dry the area completely, and tighten the connection nut under the tank a quarter turn with the wrench. Turn the water back on and check again. Repeat until the leak stops. If tightening does not work, turn off the water, remove the valve, and check that the rubber gasket is seated properly in the opening before reinstalling.

How do I know which size fill valve to buy?

Most residential toilets use a standard 7/8-inch fill valve. Check your repair kit packaging or ask the hardware store staff — they can match it to your toilet brand. Universal kits include adapters for different tank sizes, so they work with most toilets.

Do I need to drain the tank completely before I start?

Yes. Flush once to empty most of the water, then sponge out what remains. Working in a dry tank prevents water from splashing on you and makes it easier to see what you are doing.

What if the shutoff valve under the tank won't turn?

Do not force it — you may break the valve. Spray it with penetrating oil (like WD-40), wait 15 minutes, and try again gently. If it still will not budge, you may need to shut off the main water supply to the house instead, which is usually a valve near the water meter or where the main line enters the house.