Which Part Is Causing the Problem?
A running toilet wastes up to 200 gallons of water per day, according to the EPA WaterSense program. That adds roughly $70 a month to your water bill for a toilet that will not shut up. The fix is almost always one of three parts, and you can identify which one in under 60 seconds.
Quick diagnosis: Add 3 to 4 drops of food coloring to the toilet tank. Wait 15 minutes without flushing. If colored water appears in the bowl, the flapper is leaking. If the tank water rises to the overflow tube and the fill valve keeps running without shutting off, the fill valve or float is the problem.
The flapper is the rubber seal at the bottom of the tank that lifts when you flush. Chlorine in tap water hardens the rubber over time, and it stops making a tight seal. The fill valve is the tower on the left side that refills the tank after every flush. When its internal diaphragm fails, it runs continuously.
Inside a Running Toilet: Which Part Failed?
Tools and Materials
| Item | What to Buy | Cost |
|---|---|---|
| Universal flapper | Korky 2020BP or Fluidmaster 502 | $5-$8 |
| Fill valve | Fluidmaster 400A (fits 99% of toilets) | $8-$12 |
| Fill valve seal cap | Fluidmaster 242 seal (for 400-series) | $3 |
| Small sponge or towel | For residual tank water | Already own |
Total cost: $5-$12 depending on which part failed. Buy the flapper first. It accounts for roughly 80% of running toilet problems. If the flapper swap does not fix the issue, return to the store for a fill valve.
I keep a spare Fluidmaster 502 in the bathroom vanity. Universal flappers fit every toilet I have owned across three apartments and two houses.
Step 1: Turn Off the Water Supply
Find the shut-off valve on the wall behind the toilet, near the floor. It is a small oval or lever handle connected to a chrome supply line. Turn it clockwise until it stops. Do not force it past resistance. Old shut-off valves can snap if you crank too hard.
If the valve is seized, turn off the main water supply to the house instead. The main shut-off is usually in the basement, crawl space, or garage near the water meter.
Flush the toilet and hold the handle down to drain as much water from the tank as possible. Sponge out the remaining water at the bottom of the tank. A few tablespoons left is fine and will not interfere with the repair.
Step 2: Remove the Old Flapper
Unhook the flapper’s side ears from the pegs on the overflow tube. The overflow tube is the vertical plastic pipe in the center of the tank. The flapper chain connects to the flush handle arm above it. Unclip the chain or slide it off the hook.
Inspect the flush valve seat — the circular opening where the flapper rests. Run your finger around the rim. If it feels rough, pitted, or crusted with mineral buildup, wipe it clean with a scouring pad or fine steel wool. A new flapper cannot seal against a damaged seat.
The Korky 2020BP fits 2-inch flush valves found on most toilets made after 1990. Older toilets may use a 3-inch opening. Measure the diameter before you buy if your toilet was installed before 1995.
Step 3: Install the New Flapper
Slide the new flapper’s ears onto the overflow tube pegs. Connect the chain to the flush handle arm. Leave half an inch of slack in the chain. A chain pulled too tight holds the flapper open. A chain with too much slack lets the flapper drift sideways and miss the seal.
Adjust the chain length so the flapper lifts fully when you press the handle but drops straight down when you let go. Test this by hand before putting the tank lid back.
Turn the water back on and let the tank fill. Watch the flapper through the water. If tiny air bubbles stream up from underneath it, the flapper is not sealing. Push down gently on the flapper with your hand. If the bubbles stop, the chain has too much slack and is preventing a full rest. Remove one link and try again.
Step 4: Diagnose the Fill Valve
If the flapper replacement did not stop the running, the fill valve is the culprit. Look at the water level in the tank after it fills. If water pours into the overflow tube and never stops, the fill valve fails to shut off.
A healthy fill valve shuts off when the water reaches about half an inch below the top of the overflow tube. If yours keeps trickling, the internal diaphragm seal is worn out. On Fluidmaster valves, you can replace just the seal cap for $3 instead of the whole unit.
The Fluidmaster 400A is the most common replacement fill valve in North America. It fits tanks 9 to 14 inches deep and adjusts from 7 to 13 inches in height. At $8-$12 at Home Depot or Lowe’s, it is the standard repair part every hardware store stocks.
Step 5: Replace the Fill Valve
Turn off the water and flush to drain. Place a shallow bucket or towel under the tank where the supply line connects. Unscrew the supply line nut from the bottom of the fill valve by hand or with pliers.
Loosen the plastic lock nut under the tank that holds the fill valve in place. It typically spins by hand. Pull the old fill valve straight up and out of the tank.
Slide the new Fluidmaster 400A into the hole. Adjust its height so the critical level mark on the valve body sits at least one inch above the top of the overflow tube. Tighten the lock nut underneath by hand, then give it a quarter-turn with pliers. Do not overtighten. The plastic threads strip with almost no warning.
Reconnect the supply line and hand-tighten the nut. Give it an eighth-turn with pliers. Turn the water on and check both connections for leaks before continuing.
Step 6: Set the Water Level
Attach the refill tube to the top of the overflow tube using the clip provided in the box. The tube must point down into the overflow. If it sprays outside the overflow, the bowl will not refill properly after flushing.
Adjust the water level by squeezing the adjustment clip on the side of the fill valve and sliding it down. The correct water level sits about half an inch below the top of the overflow tube. Flush and watch where the level settles. Adjust again if needed.
Step 7: Test Everything
Flush the toilet three times. Listen between flushes. The fill valve should shut off completely within 30 to 60 seconds after each flush. No hissing. No trickling. No sound at all.
Wait five minutes and check the bowl for ripples or color if you used the food coloring test earlier. A properly sealed flapper keeps the bowl water perfectly still. Put the tank lid back on. You are done.
Common Mistakes That Waste Your Time
Replacing the flapper without cleaning the seat. Mineral crust on the flush valve seat creates a bumpy surface the new flapper cannot seal against. Wipe the seat with a scouring pad every time you swap a flapper. I skipped this step once and burned an hour re-replacing a flapper that had nothing wrong with it.
Buying the wrong flapper size. 3-inch flappers look similar to 2-inch versions on the shelf. Measure your flush valve opening before leaving the house. The diameter is stamped on the face of the old flapper on Korky and Fluidmaster models.
Tightening the chain instead of removing links. Yanking the chain shorter creates slack in a different direction and pulls the flapper sideways. Remove links from the hook end instead. The flapper must drop straight down and land centered on the opening.
Replacing the entire fill valve when only the seal cap failed. Fluidmaster 400-series valves have a replaceable seal cap on top that costs $3. Pop the cap off by twisting it 90 degrees, swap the black rubber seal inside, and snap it back on. I keep a spare cap in the bathroom drawer. It fixes most fill valve failures in under two minutes.
When to Call a Plumber
Most running toilets are a flapper or fill valve failure — both are DIY fixes in under 15 minutes. Call a plumber for these specific situations:
The tank itself is cracked anywhere. Water seeping through a hairline crack means the porcelain has failed and the tank needs replacement.
The flush valve seat is deeply pitted or gouged and cannot be cleaned smooth with a scouring pad. A new flapper will not seal against a damaged seat no matter what you do.
You replaced both the flapper and fill valve and the toilet still runs. This points to a problem inside the flush valve assembly that sits below the flapper, which requires pulling the tank off the bowl to access.
A plumber charges $75-$150 for toilet repair in most markets. If water pools around the base of the toilet, the wax ring seal between the toilet and the floor drain has failed. That is a separate repair that takes about 45 minutes with experience and involves lifting the toilet off the floor.