Toilet Keeps Running: Flapper, Fill Valve and Overflow Fixes

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A toilet that hisses, trickles or refills on its own in the middle of the night is easy to ignore. It still works, after all. But a running toilet can waste a surprising amount of water and quietly raise the water bill month after month. The good news is that most causes are inside the tank, the parts are inexpensive, and the diagnosis takes minutes. When a toilet keeps running, the problem is almost always one of a short list: a worn flapper, a chain that is the wrong length, a fill valve set too high or failing, a float out of adjustment, a cracked overflow tube, or mineral buildup. Occasionally the root cause is outside the toilet, such as household water pressure that is too high. This guide walks through how a tank works, how to find the leak, how to fix each part, and when replacement makes more sense. Cost figures are general 2026 ranges and vary by region.

Part of our Plumbing & Water guide — start there for the full picture →

Why a Toilet Keeps Running

A running toilet means water is either leaking out of the tank into the bowl or flowing into the tank and out through the overflow tube. Either way, the fill valve keeps opening to replace the lost water. The most common causes, roughly in order of how often they show up:

1. A worn or warped flapper that no longer seals against the flush valve seat.
2. A chain that is too short, too long or tangled, holding the flapper slightly open or getting trapped underneath it.
3. A water level set too high, so water spills into the overflow tube.
4. A failing fill valve that will not shut off completely.
5. A misplaced refill tube that siphons water from the tank.
6. A cracked overflow tube or worn flush valve seat.
7. Mineral buildup on the flapper seat or inside the fill valve.
8. High household water pressure that wears out fill valves and keeps them from sealing.

EPA WaterSense notes that most toilet leaks are due to old or worn-out flappers. That is the place to start, but checking the water level first takes only a glance.

How a Toilet Tank Works

Understanding the parts makes the troubleshooting straightforward. A standard gravity-flush tank has two main assemblies.

The flush valve

At the bottom center of the tank is the flush valve: an opening into the bowl, sealed by a rubber or silicone flapper (some toilets use a tower-style canister seal instead). Pressing the handle lifts a chain or lift arm, which raises the flapper. Water rushes into the bowl, and the flapper drops back onto its seat as the tank empties. The tall tube rising from the flush valve is the overflow tube. If the water level ever gets too high, it spills into this tube and down into the bowl, which keeps the tank from overflowing onto the floor.

The fill valve

On the left side of most tanks is the fill valve, sometimes called the ballcock in older toilets. It connects to the water supply line underneath the tank. A float, either a ball on an arm or a cup that slides up a shaft, tells the valve when the tank is full. As the water level drops after a flush, the float drops and opens the valve. As the tank refills, the float rises and shuts the valve off. A small flexible refill tube runs from the fill valve to the top of the overflow tube, sending a little water down into the bowl to restore its water level after each flush.

When everything is adjusted correctly, the tank refills, the bowl refills, the fill valve shuts off, and everything goes quiet. A running toilet means one of those steps is not finishing.

Diagnose the Leak: The Dye Test and Other Checks

Check the water level first

Take the tank lid off and set it somewhere safe; tank lids are heavy and crack easily. Look at the water level relative to the top of the overflow tube. Many tanks have a fill line molded or printed inside. If water is running into the overflow tube, the water level is too high or the fill valve is not shutting off. That points to the fill valve or float, not the flapper.

Run the food coloring dye test

If the water is below the overflow tube but the toilet still runs or refills on its own, the flapper or flush valve seal is the likely culprit. EPA WaterSense describes a simple check: place a few drops of food coloring in the toilet tank, and if any color shows up in the bowl after 10 minutes, there is a leak. WaterSense advises flushing right after the test to avoid staining the tank. WaterSense’s technical guidance for new homes uses the same approach, with a dye tablet or a few drops of food coloring and a 5 to 10 minute wait without flushing; color in the bowl indicates the flapper or flush valve seal is leaking.

Some water utilities hand out free dye tablets during conservation events, which work the same way as food coloring.

Listen and watch

• A constant hiss or trickle with water visibly flowing into the overflow tube usually means the fill valve or water level.
• The tank refills briefly every so often without anyone using it, sometimes called a phantom flush, usually means water is slowly leaking past the flapper.
• The toilet runs only after flushing and stops when you jiggle the handle often means a chain or handle issue.

A water meter check can confirm whether a leak is costing you. WaterSense suggests checking your water meter before and after a two-hour period when no water is being used; if the meter changes at all, there is probably a leak somewhere in the home.

Fixing the Flapper and Chain

Chain length

The chain should have a little slack, often about a half inch, when the flapper is closed. Too short, and it holds the flapper slightly open. Too long, and the excess can get caught under the flapper as it falls. Adjust by moving the clip to a different link on the handle arm and trim long tails. Also check that the handle itself returns fully and is not sticking.

Cleaning or replacing the flapper

Shut off the water at the supply valve behind the toilet, flush to drain the tank, and inspect the flapper. Run a finger around its edge and the flush valve seat. Warped rubber, a slimy or crumbly surface, or a hard, mineral-crusted seat all prevent a good seal. Wiping the seat with a non-scratch pad can help if the buildup is light.

WaterSense says flappers should be checked periodically and replaced at least every five years to keep a good seal, and notes that an old or worn flapper can cause a toilet to flush on its own or silently leak thousands of gallons a year. Replacement is usually a matter of unhooking the old flapper from two pegs on the overflow tube or sliding it off a ring, then attaching the new one and clipping the chain.

Flappers are not one-size-fits-all. WaterSense suggests bringing the old flapper, or a photo of it, to the hardware store, paying attention to the flapper’s width and whether it has a foam float that may also need replacement. Most toilets use a 2-inch or 3-inch flush valve, and the owner’s manual or manufacturer’s website can confirm the right part.

If you notice dark slime, black spots or a musty smell while the tank is open, that growth is common in toilet tanks and is worth cleaning out at the same time. Our guide to black mold in the toilet tank covers safe cleaning approaches that will not damage rubber parts.

Fixing the Fill Valve, Float and Refill Tube

Adjust the float

If water is going into the overflow tube, lower the water level. On a modern float-cup valve, there is usually a clip or adjustment screw on the side of the shaft; squeeze the clip and slide the float down, or turn the screw to lower it. On an older ball-and-arm ballcock, there may be an adjustment screw on top, or the arm can be bent slightly downward. Aim for the water to stop about an inch below the top of the overflow tube, or at the manufacturer’s marked fill line.

Check the refill tube

The refill tube should be clipped to the top of the overflow tube, aimed down into it, but not pushed far down inside. If the tube sits below the water line inside the overflow, it can siphon water out of the tank continuously, which keeps the fill valve cycling. Clip it so the end stays above the water level.

Clean or replace the fill valve

If the water level is correct but the valve keeps hissing, debris or mineral scale may be preventing the seal from closing. Many fill valves have a cap that can be removed to flush out grit, following the manufacturer’s directions. If that does not help, replacing the whole fill valve is a common homeowner repair. Universal replacement fill valves are widely sold and adjustable for most tank heights. Be sure to shut the supply valve, drain the tank, sponge out the remaining water, and keep a towel handy for the supply line connection.

While you are there, look at the supply line itself. Old plastic or rubber supply lines and corroded shut-off valves are a common source of slow floor leaks. Braided stainless lines are a frequent upgrade.

Cracked overflow tube and worn seat

If the dye test shows a leak but a new flapper does not stop it, look closely at the overflow tube and flush valve seat. A hairline crack in the overflow tube below the water line lets water drain into the bowl constantly. A pitted or warped seat can prevent any flapper from sealing. The fix is replacing the flush valve, which on most two-piece toilets means separating the tank from the bowl and replacing the tank-to-bowl gasket and bolts at the same time. That is a bigger job than a flapper swap, and many homeowners choose to hire a plumber.

Mineral Buildup, Hard Water and High Pressure

Water conditions can make repairs short-lived. Hard water, which is common in parts of Colorado, leaves calcium and magnesium deposits on flapper seats and inside fill valves. A seat crusted with scale may not seal even with a new flapper, and grit can keep a fill valve from closing. A water softener reduces scale throughout the plumbing, though it is a bigger decision with its own maintenance and costs.

High water pressure is the other hidden factor. EPA WaterSense notes that water pressure in supply mains can reach 100 psi or more and that high pressure can damage plumbing fixtures and lead to premature equipment failure and leaks. WaterSense recommends incoming service pressure between 45 and 60 psi. A fill valve fighting excessive pressure may hiss, fail to shut off, or wear out within months. If toilets in the house keep needing new fill valves, test the pressure with an inexpensive hose-bib gauge, as described in our water pressure test guide. If pressure is high, the house may need a new or adjusted pressure reducing valve.

How Much Water and Money a Running Toilet Wastes

EPA WaterSense reports that household leaks can waste 180 gallons per week and that toilets are often the culprit. WaterSense’s statistics page puts the average family’s leak losses at 180 gallons per week, or 9,400 gallons a year, and its Fix a Leak Week page says fixing easily corrected household leaks can save homeowners about 10 percent on their water bills. A toilet that runs constantly can waste far more than one with a slow, silent flapper leak, so the actual cost depends on the size of the leak and local water rates.

Toilets are also the largest indoor water user to begin with. WaterSense says toilets account for nearly 30 percent of an average home’s indoor water consumption, and older, inefficient models can use as much as 6 gallons per flush. Along the Front Range, where many utilities use tiered rates and encourage conservation, a leak that pushes a household into a higher tier can make the bill jump noticeably.

Repair Costs and When to Replace the Toilet

As general 2026 ranges, which vary by region and brand:

• Flapper: often about $5 to $25 for the part.
• Fill valve: often about $10 to $35 for a universal valve.
• Complete tank rebuild kit: often about $25 to $60, including flapper, fill valve and sometimes flush valve parts.
• Plumber service call for a running toilet: commonly around $150 to $350 depending on parts, access and whether the tank must be removed.
• New toilet installed: commonly a few hundred dollars to $1,000 or more, depending on the model and any flange or floor repairs.

Replacement starts to make sense when the porcelain tank or bowl is cracked, when parts for an older or unusual model are hard to find, when repairs keep failing, or when the toilet is an older high-volume model. WaterSense notes that design improvements now allow toilets to use 1.28 gallons per flush or less, which is 20 percent less than the federal standard of 1.6 gallons per flush, and that replacing old, inefficient toilets with WaterSense labeled models can reduce toilet water use by 20 to 60 percent. Some utilities offer rebates for labeled toilets, so check with your water provider before buying.

A cracked bowl, a toilet that rocks, or water around the base is a different problem from a running tank and may mean a failed wax ring or damaged flange. If the issue is slow drainage rather than running water, see our guide to a clogged toilet.

Running Toilets in a Home Inspection

Home inspectors typically operate toilets as part of checking plumbing fixtures and may note a running toilet, a loose bowl, or signs of leaks around the base. They do not usually rebuild tanks or measure water pressure. A running toilet on an inspection report is usually a minor repair item, but several running toilets or repeatedly failed fill valves can hint at high water pressure worth checking further. For more on the plumbing side of an inspection, see our plumbing and water inspections section and the main guide to hiring a home inspector. Homeowners who want added protection against bigger leaks can also consider an automatic water shut-off valve.

References

• Fix a Leak Week — U.S. Environmental Protection Agency WaterSense
• Residential Toilets — U.S. Environmental Protection Agency WaterSense
• Statistics and Facts — U.S. Environmental Protection Agency WaterSense
• Free of Leaks Tech Sheet (WaterSense Labeled Homes) — U.S. Environmental Protection Agency
• Service Water Pressure Tech Sheet (WaterSense Labeled Homes) — U.S. Environmental Protection Agency

Seeing running toilets, dripping faucets or banging pipes in a house you plan to buy on the Front Range? Send us a note and we will help you connect with an inspector who checks plumbing fixtures carefully.