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Tutorial: How to Check Toilet for Leaks with Food Coloring

Tutorial: How to Check Toilet for Leaks with Food Coloring

A leak in your toilet tank can cause your water bill to skyrocket and needs to be fixed promptly. To know if your toilet has a leak, you don’t need any fancy tools. All you need is a little food coloring to confirm that there’s a leak.

If you’re wondering how to check your toilet for leaks, simply use food coloring to dye the water in the tank and leave it there for 20-30 minutes without flushing. If the water in the toilet bowl picks up the color from the tank, then you know you have a leak that needs to be fixed.

To understand how food coloring transfers from the tank to the toilet bowl, you need to learn how toilets work. Then, you’ll be better prepared to learn what kinds of issues cause toilet leaks, how expensive the repairs can be, as well as which repairs most people can do themselves.

How Toilets Work

The toilets in your home are called “flush toilets,” and they rely on gravity to send water through its system, into the toilet bowl, and down the drain to carry the contents of the bowl down through the drainpipe to your home’s sewage or septic system.

There are two main parts of the toilet: the tank and the bowl. Water flows from the toilet’s water supply into the tank and then, when the toilet has been flushed, the water flows into the toilet bowl, down the drainpipe, and into plumbing that leads to either a sewage or septic system.

How the Tank Controls Its Water Level

The tank is connected to your home’s water supply, and whether or not water is filling the tank depends on a control called the fill valve. The tank has a specific water level that means the tank is considered “full.” That level is below the actual top of the tank so that the water doesn’t overflow.

A feedback loop controls whether or not water flows into the tank. Changing position as the water drains or rises, a float ball sits on the surface of the water in the tank. If the float ball is below the “full” height of the tank, then an attached float rod removes pressure from the fill valve. That allows water to enter and fill the tank until the float ball is once again at the “full” position, pressing the fill valve and stopping the water.

How Water Flows from Tank to Bowl

When you flush, the lever on the toilet tank pulls up a flapper, which seals the bottom of the tank, and water in the tank is allowed to flow through an attached tube. This U-shaped pipe is this siphon, and it leads to the toilet bowl. Depending on the type of fill valve the toilet uses, different mechanisms help draw water up and over the top U-curve of the siphon and down into the bowl.

The water empties from the tank and floods the toilet bowl. The bowl’s connection to the drain is another upside U-pipe called an “S” bend. When tank water fills the bowl, it causes the water level to exceed the height of the “S” bend and flow down the drainpipe.

This bend is designed to always leave clean water in the bottom of the bowl, which keeps sewage fumes from entering the home from the drainpipe. Because of gravity, unless there’s sufficient water to reach the height of the pipe’s bend, a set amount of water always remains in the bowl, sealing off the home from sewage gases.

Apart from the initial refilling of the tank, when the flushing lever hasn’t been engaged, the water in the tank and the bottom of the bowl shouldn’t change. However, if there’s a leak in the toilet tank, water can continually flow from the tank to the bowl, which can be a costly problem.

How Some Toilets May Differ

Although the majority of toilets have fill valves that use a float to determine whether or not the tank is considered full, some toilets use float-less fill valves, which are a relatively recent invention and operate somewhat differently.

These fill valves are meant to sit below the tank water’s surface. Connected at the base of the tank, float-less fill valves rely on a pressure-sensitive diaphragm to control the water level via the fill valve. These types of fill valves have distinct common issues and can be somewhat unreliable, so if your toilet uses a floatless fill valve it may have other malfunctions you’ll need to deal with.

Why Internal Toilet Leaks Are a Problem

If your toilet has an ongoing internal leak, it might not seem like a pressing issue at first. After all, you won’t have the messy clean-up of a flooded bathroom floor. But these leaks can dramatically increase your water bill, especially if the problem has time to get worse.

Although ongoing leaks can be found in many other fixtures in your home that are connected to the water supply, toilets are often the costliest. Compared to leaks in faucets and showers, toilet leaks can run as much as 4 times and 25 times as much, respectively. This is because of both the feedback loop that controls how the tank fills and the toilet bowls connection to the drainpipe. Whatever the root cause may be, as water moves from the tank to the toilet bowl, any excess water always ends up down the drainpipe.

To a certain point, water above the correct “full” position in the tank will end up being sent into the toilet bowl where it will send excess water over the top level of the “S” drain to maintain a consistent amount of water in the bottom of the bowl.

Additionally, if the water is draining from the tank to the bowl too frequently, that “lost” water will be continually made up for by the water supply refilling the tank.

How Much a Running Toilet Can Cost

Over time, these leaks lead to excessive water waste and unnecessarily high bills. Normally, a toilet up to 1.6 gallons of water each time you flush. A toilet that is constantly “running” will often use around 6,000 gallons of water monthly.

To put that in perspective, on average, each person will use their toilet five times in one day. For a family of four, that should equal about 960 gallons of water used by their toilet each month. That is over a six-fold difference in water consumption when you compare a properly functioning toilet to one with an internal leak.

At this point, it should be no surprise that all those gallons add up on the final bill. Depending on several factors, including where you live, such a leak can add $70 to your monthly water bill, which means you’ll be out over $800. That’s certainly a strong incentive to figure out if you have a leak, what’s causing it, and how you can fix it.

How to Use Food Coloring to Identify a Leak

If you add food coloring to your toilet tank, a properly functioning toilet should show no change in color in either the tank water or the toilet bowl water. There are a couple of color changes that you should look out for after you allow the color to sit for 20-30 minutes.

Color Bleed into Toilet Bowl

The most obvious result that you can get from using food coloring to test for an internal leak is if the color in the tank ends up bleeding into the bowl. That’s a clear sign that your toilet has a leak that’s causing the water in the tank not to remain sealed off from entering the bowl via the siphon pipe.

When you flush your toilet, the level uses an attached chain to pull on a part called the flapper. The flapper is what seals off the tank from the siphon and allows the tank water and bowl water to stay separate.

It’s important that the flapper properly seals that connection so that the tank isn’t perpetually leaking water into the bowl, causing the float to drop and open up the fill valve. These leaks, when especially severe, can cause the toilet to continually exhibit a sound of dripping or running water and may result in the costliest water bills.

Often, you may continually hear sudden refilling of the tank if the leak is slow and the depletion of the tank is very gradual. Normally, your tank would only need to refill after it’s drained due to the toilet being flushed. With a malfunctioning flapper, however, your tank will need to either be continually supplied with water or intermittently refilling itself. This phenomenon is known as “phantom flushing” among plumbers, and if you have this kind of leak in your toilet, you need to inspect and possibly replace the flapper to fix any malfunctions that are affecting its ability to seal the bottom of the toilet tank.

Color Diluted in Tank, No Color in Toilet Bowl

Another, more subtle indication that there is an issue with continuously running water in your toilet is if the color of the tank’s water becomes diluted after you’ve let the food coloring sit. If this is the case, over time, the water supply is leaking into the toilet tank via the fill valve.

This can often be a much more complicated repair than other issues as the fill valve needs to be connected to several different parts of the toilet and water supply. Additionally, there are several different types of fill valves, and yours may need to be replaced with a newer version.

If you don’t feel comfortable changing or updating your toilet’s fill valve, you can of course always consult a professional who will be able to make sure that the installation is performed correctly.

No Color Change, But There Are Other Signs of a Leak

In many cases, two things will indicate that there is a potential toilet leak that needs to be addressed: sounds of running water or hissing coming from the toilet or a sudden increase in the water bill.

As previously explained, the running water or hissing might be caused by an issue with the toilet’s flapper. However, if your food coloring test doesn’t reveal any leaking and your water bill is still unusually high, don’t be too quick to dismiss your toilet as the cause. There are still some other underlying issues with your toilet that could be causing excessive water usage. Fortunately, both of these issues are fairly simple fixes.

How to Fix Different Types of Leaks

Once you’ve identified leaks in your toilet tank or water supply, you need to determine if the fix is a DIY job or something for which you should call in a professional. Many of these causes can easily be fixed without hiring out for the job, but if you’re not comfortable doing these yourself, you can always call for a plumber to make sure the job is done properly.

Fixing a Malfunctioning Toilet Flapper

If the issue with your toilet is causing water to leak from the tank into the bowl, the flapper, which seals the connection between the tank and siphon pipe, is malfunctioning. The underlying cause will usually come from one of three issues: a degraded flapper, built-up sediment around the flapper, or a flapper caught on its chain.

Fixing a Degraded Flapper

Often, you may have a degraded flapper if your toilet is relatively old. Over time, this part can become worn down, which reduces its ability to properly seal the entrance to the toilet’s siphon. As a result, water can seep through these worn-down areas and run through the siphon and into the bowl, which is how the food coloring can reach the bowl when you test for a leak.

If this is the case, you’ll need to replace the flapper. However, make sure that you’re not doing so prematurely, as the same problem can occur if there is debris from the water supply that has accumulated around the flapper’s edges. When this happens, the flapper isn’t able to securely close off the water’s access to the toilet bowl.

Removing Built Up Sediment

To fix this and clean the debris, you’ll need to drain the tank by turning off the water supply and flushing the toilet first. Then you can clean away any built-up sediment that is there. This also allows you to check the condition of the flapper. If the flapper has become rigid and is no longer flexible, you’ll still need to replace it even if there was debris contributing to the problem.

Removing a Caught Chain from the Flapper

Another cause for a malfunctioning flapper is if the chain attached to it from the flushing lever isn’t the right length. An overly long chain will often get caught on the underside of the flapper and prevent it from effectively sealing off the siphon. If this is the cause of your tank’s leak, you’ll need to drain the tank and cut the chain to the right length.

How to Replace a Toilet Flapper

If you’ve determined that you need to fully replace the flapper and you want to do it yourself, you’ll need to first drain the tank by cutting off the water supply and flush the toilet.

Next, you need to remove the tank from the toilet itself. Placing it upside down on the ground will allow you to access the old flapper to remove it as well as to install the new one. You’ll have to disconnect the chain that attaches the old flapper to the flushing lever by releasing its clip. Usually, a new flapper will come with its own attached chain, so you won’t need to keep the old one.

Then, you need to remove the flapper’s side pieces from the flush valve’s pegs, which stick out from the walls of the flush valve pipe. You’ll need to hook the new flapper into this position and attach the new flapper’s chain to the flushing lever.

After that, you can replace the tank and turn the water supply back on to test that you have the chain on the right length to allow the toilet to flush without an improperly sealed flapper. Make sure to run at least two or three tests to verify that everything is working properly.

Replacing a Fill Valve

Another issue is if the toilet’s water supply is leaking into the tank, causing the tank to overflow and drain excess water into the bowl slowly over time. This can also add up to a substantial difference in your water bill if it isn’t repaired.

If the food coloring test reveals that the leak isn’t between the tank and bowl, you may have an issue with the fill valve. First, try tightening the screws that attach to the water supply line and tank. If the signs of leaking persist after this, you’ll know that your toilet’s fill valve needs to be replaced.

The fill valve is responsible for controlling whether or not water enters the tank, so if there’s an issue with the function of yours, it can cause your water bill to skyrocket. A malfunctioning fill valve is a fairly common repair to make for a toilet, but you need to know what type of fill valve your toilet has and what kind you should get to replace yours if it isn’t working.

Depending on the age of your home, your toilet may have an outdated fill valve. If it needs to be replaced, you most likely need to get a newer type called an anti-siphon fill valve. The anti-siphoning function protects your water supply from becoming contaminated, so these replacements can also be a good idea before you ever have an issue with your toilet.

Fill valves can come in the following variations:

  • Plunger/piston fill valves
  • Brass body, diaphragm fill valves
  • Plastic body, diaphragm fill valves
  • Float cup fill valves
  • Float-less fill valves

Plunger or piston fill valves are the oldest variation, and they should be replaced in most cases as they aren’t able to protect your water supply from potential contamination, which fails to meet current plumbing codes.

For all other kinds, you should match whichever new fill valve you purchase to the old one so that it can exactly match your toilet’s setup.

How to Replace a Toilet Fill Valve

First, remove the toilet tank lid and drain it from water by cutting off the water supply from below the tank and flushing the toilet. You’ll need to make sure to get the toilet as dry as possible, so you may need to repeatedly flush to empty the bowl. Finally, try using a sponge or cloth towel to remove any water that’s still there.

This will allow you to then disconnect the control pipe that sits underneath the tank directly below where the fill valve meets the water supply. You’ll need to use a twister wrench to move the attachment counterclockwise to remove the pipe.

After this, you can unscrew the fill valve’s attachment to the tank using pliers. Be mindful to do this carefully to avoid dropping the screw into the opening of the control pipe.

Once you’ve done that, you can remove the tank’s lid and pull out the old fill valve from its connection to the tank. From there, you can install the replacement fill valve. Make sure to securely tighten its screw to hold the new fill valve in place and prevent any other leaks from the water supply into the tank.

You’ll then need to adjust the float height for the new fill valve based on the instructions provided by the manufacturer. Finally, you can reattach the water supply to the toilet tank. Afterward, be sure to test that the toilet flushes properly. You may need to readjust the float height if you notice that the tank overfills past the top of the siphon.

Replacing the Float

Another potential cause of a leak is that your toilet tank’s “fill” position is set too high. Normally, the level of your toilet tank’s “fill” position should be no more than half an inch to an inch under the top of the siphon that brings the water to the toilet bowl. All you need to do is lower the float’s resting height to below the top of the siphon.

Remove the toilet tank’s lid when the tank is full of water. Observe the float ball to see if it is floating on a full tank, which indicates that the float is still functional and just needs to be adjusted to a lower height. On the other hand, if the tank is full and the float has sunk, you’ll need to replace the float because yours is no longer working.

To change the “full” position of the tank, use a screwdriver to adjust the toilet valve’s screw. This should be connected at the end of the float rod. Turning the screw clockwise will lower the ball’s “full” position and allow your tank to fill to a level below the top end of the siphon, saving you money on your next water bill.

If you’ve ruled out all the other potential causes of your toilet leaking and you’re still seeing a water bill increase and hearing continuous running water in your toilet, its fill valves might have degraded or malfunctioned.


Identifying a leak in your toilet is a critical first step in reducing any potential water waste that could be leading to an increased water bill each month. Using food coloring to diagnose the issue is a simple, cost-effective way to know for sure what’s going on.

If the issue is that water is leaking from the toilet tank into the toilet bowl, you’ll be able to quickly identify the issue by seeing the food coloring seep from the tank to the bowl after you leave it to sit for at least 20 minutes, possibly longer.

While some advice online may suggest a shorter wait time, you should leave it a bit longer so you can be sure to identify more gradual leaks. If there’s a slow leak and you catch it because you let the food coloring sit a bit longer, you’ll have invested very little time to catch an expensive problem before it gets significantly worse.

Some of the internal leaks are a bit more difficult to identify, so be sure to consult a licensed plumber if you’re unsure about any potential results of your test with food coloring. If you realize that the cause of a constantly “running” toilet in your home is an issue with how high the “full” position is set for your toilet, that’s a fairly simple fix that most people would be comfortable doing.

On the other hand, replacing a fill valve can be a bit more involved, especially if you need to replace an older model with one that meets modern plumbing standards. Although many people feel comfortable inspecting their toilets and replacing or adjusting parts as needed, it’s always best to get professional advice if you feel the need.