One of the most common bathroom problems is the toilet running at random times. Hearing your toilet running for a few seconds when you haven’t flushed might startle you, but it’s more than an inconvenience—it’s a problem that’s costing you money. Any water you use unnecessarily is still charged.
Thankfully, this is an easy fix you can handle even if you aren’t a plumbing expert. I’m not terribly handy around the house, and I fixed this problem myself back in 2024.
Understanding toilet functionality
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To understand what’s going on in this situation, it’s important to know the basics of how toilets work. The toilet handle you push down to flush has a lever inside the tank, which has a chain connected to it. That chain’s other side is connected to a flapper, which is the root cause of the randomly flushing toilet problem.
A flapper’s job is to create a seal between the tank and the bowl. When you flush the toilet, the chain raises the flapper, allowing water from the tank to rapidly drop into the bowl through the flush valve. This creates a siphon that pushes wastewater up through the trapway and down into the floor drain, carried off as sewage. (It’s possible to unclog a toilet drain with a cheap gadget, speaking of which.)
From there, the toilet has to refill for next time. The flapper closes to seal the tank again, then the fill valve draws water from the water supply to fill the tank.
The fill valve knows how much water to fill the tank with because of the fill valve float. This is a small part that drops when the water leaves the tank and rises when the water reaches its peak in the tank.
Your issue of a constantly running toilet doesn’t involve every step of the process. But it’s good to know what’s going on when you flush the toilet to understand how it can go wrong—a key lesson of home ownership.
The cause of “ghost flushing”
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The problem of your toilet running when you haven’t flushed it comes down to the flapper; it’s the sole part that provides a barrier between the tank and the bowl. When the flapper doesn’t have a good seal, water slowly leaves the tank and goes into the bowl.
Once the water level has dropped enough, the fill valve float drops. This tells the fill valve it’s time to draw in more water, which is when you hear the sound of a running toilet. It doesn’t run for as long as a normal flush, since the water level in the tank doesn’t fully deplete.
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Most flappers are made of rubber and are prone to disintegrating over time. This can happen due to chemicals in public water, mineral buildup from hard water, an improper fit, or over time due to age. It’s thus the first part you should look to replace when this issue arises.
Replacing the flapper is simple
Thankfully, replacing a toilet flapper is not expensive or difficult. If you want to confirm whether it’s at fault, remove the toilet tank cover and put a few drops of food coloring into the tank’s water. Wait a while without flushing, then check the water in the bowl.
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If the bowl’s water is colored, then you have a leak; replacing the flapper will likely fix it. When selecting a replacement, not all flappers are the same. If you know the model of your toilet, perform a search to see if the manufacturer has a recommended replacement. If not, a universal flapper will likely work fine.
The only aspect you must check before ordering a universal flapper is whether it’s two or three inches. Most toilets, especially those made before 2000, use a 2-inch flapper. Some newer toilets have a three-inch valve instead.
Check which flapper you need
You can confirm this by measuring the flush valve opening. First, shut off the water to your toilet by turning the water supply knob next to the toilet to the right (clockwise) until it stops. Then, hold the toilet handle until all the water drains out of the tank.
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The flapper is the rubber part near the middle of the toilet connected to the chain. Raise it by holding the handle, then measure the size of the valve underneath. When I replaced mine in July 2024, I bought a Korky 2-inch universal toilet flapper for under $10, and it’s worked great.
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If you’re not going to replace the flapper now, turn the water supply under the toilet back on by turning the valve all the way to the left (counter-clockwise).
If you don’t want to order a flapper online or aren’t sure what to get, remove your toilet flapper (following the instructions below), take it to a hardware store, and ask for help choosing a replacement.
Replacing your toilet flapper
After getting your replacement flapper, it’s time to make the switch. Repeat the step above to shut off the water supply to your toilet, then flush it to drain the tank.
My Korky universal flapper has an adjustable band and dial to allow the right amount of water to flow through for your specific toilet. The included instructions tell you how to set these for many common toilet models.
If you have another toilet or don’t know your model, you can check manually by following the longer instructions. This involves marking the minimum water level in the tank with the old flapper, then adjusting the new flapper to match it.
After marking the water level if needed, take off the old flapper by pulling the two side tabs that hold it to the overflow tube, then complete the removal by unhooking the chain from the handle lever. If your toilet was leaking, chances are the old flapper will feel mushy and have pieces flaking off.
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Before you continue, clean the area around the flush valve. Removing any pieces or buildup will make sure the new flapper gets a good seal. Attach the new flapper in the same way the old one was: clip the two side tabs onto the overflow tube, then attach the end of the chain to the handle lever.
You want minimal slack in the chain; if it’s too loose, the handle won’t raise the flapper. If it’s too tight, the flapper won’t sit in place. Turn the water back on (turn the supply line knob to the left), wait for the tank to fill, then turn the water off again and flush.
If you made a mark for the water level earlier, check to see if the current water level is within a half-inch of it. If so, you’re done with the flapper options. Otherwise, follow the instructions in the box to open the band and lower the dial number to adjust it (this might require a few tries).
When you flush, pay attention to see if the chain slack feels right. You can adjust this by following the same water on > fill tank > water off > flush routine as before. Adjust the chain’s tightness by moving the clip to a closer or further link as needed.
Make sure to turn the water back on, then replace the tank cover, and the project is done! You can run the food coloring test again if you want to be sure it’s fixed. You shouldn’t hear the toilet running randomly anymore.
Don’t put up with a running toilet
A running toilet is disruptive, but also wastes water and thus costs you money. If I can perform this fix, you can too. Once you start hearing the toilet running all the time, take a few minutes to fix the issue.
The new flapper won’t last forever, but you should get several years from it at least. And now you know a little more about an important part of your home. Next, why not clear clutter in other rooms?
