Is Toilet Water The Same As Sink Water? (Explained)


Toilets are used many times every day, flushing gallons of water to push waste out of the home. As you wash your hands, you may wonder if the toilet water is the same as the water from the sink.

The toilet water that comes in to fill the toilet tank is the same as the sink water. Clean water comes into your home via a well or municipal water line. Pipes then divert from the main source to bring this water to each water fixture and appliance that needs it, including your toilet.

However, once the water fills the inside of the toilet tank and bowl to collect and flush your waste, it is not considered potable, or drinkable. 

How Water Gets To The Toilet

Municipal and plumbing codes have strict requirements for water supply lines to buildings and homes for safe drinking and disposal. 

Water generally enters the home via a 3/4- or 1-inch pipe from the mainline supply source. The main pipe is located deep in the ground to avoid freezing. 

This supply line connects to the home’s water heater, with two main service pipe paths for hot and cold water that are 1/2-inch in diameter. Pressure moves water through the pipes when valves are opened.

Cold Water Service Lines

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The cold water line sends water to every water fixture and appliance in the home. These service lines are installed in straight lines, 90-degree angles, or downward-facing slopes to encourage water to flow. 

The entire system and each line has a valve to turn off the water as needed. Fixtures will have their own intake water pipe, which extends off of the service line, with a valve for stopping the flow for repairs or leaks.

If you have installed a bidet on your toilet, this is also connected to the cold water service line. When the bidet is turned on, it delivers clean water for cleansing purposes.

Hot Water Service Lines

The hot water line that branches off of the mainline empties into the home’s water heater. A sensor in the heater will warm up the contents. 

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This water will be sent through the plumbing lines of the fixture, such as a shower or sink, when the faucet hot control valve opens up to pull out hot water. 

The service lines for hot water run parallel to the cold water lines. 

Mixing Hot And Cold Water

Cold and hot water only mix directly if they both lead into the same spout, such as a sink or shower. By opening the faucet, you’ve opened a closed valve that allows the water from service lines to come out. 

Each fixture that uses cold and hot water will have two lines that come in. 

A toilet only has one line coming in for cold water use only, whereas a sink will have two lines; one for hot and one for cold.


Wastewater is disposed of through separate drain pipes that are not connected to your clean water supply. This “gray” water exits the system into a sewer or septic system. 

Even though clean cold water comes into the toilet bowl, the toilet bowl is not a sanitary drinking vessel. 

Not only is there waste that sits in the toilet, but there are also toxic toilet bowl cleaner chemicals that many households use.

How A Toilet Uses Water

Essentially a toilet has two parts that hold water: the tank and the bowl. 

The tank is behind the bowl with a cover. It fills up and holds clean reserve water from the cold water service line. It refills when it empties water into the bowl upon flushing. 

The inner mechanisms of a toilet tank use a float, flush valve, and more that work in conjunction with the handle. When the handle is pressed to flush the toilet, wastewater from the bowl moves out and down the drain. 

Reserved and clean water from the tank moves into the bowl and then refills for the next flush.

This video offers a simple explanation of how a toilet works:

Gallons Per Flush (GPF)

According to the Environmental Protection Agency (EPA), toilets consume about 30% of a home’s water supply. Older and insufficient toilets can use up to six gallons of water with every flush.

To find out how much water your toilet uses for flushing, look for a label or marking near the seat’s hinges or on the underside of the tank’s lid. 

If you cannot find the information, you can search online or do the following:

  1. Remove the tank’s cover and mark the level inside where the water sits at with a pencil.
  2. Turn off the water service supply line behind the toilet. This will prevent water from refilling into the tank after flushing.
  3. Flush the toilet to empty the tank.
  4. Find a container that you know the measurement of. For example, you could use an empty gallon milk jug.
  5. Fill the container and pour it into the tank.
    • Keep track of how many times you pour the filled container into the tank. 
  6. Once you reach the pencil line, you can stop pouring and determine the total amount of water you needed to fill it.
  7. Once you know how much water your toilet uses you can roughly determine how much water you use daily. 
    • For example, if your tank holds three gallons of water, and you use the bathroom on average five times a day, then you have flushed away 15 gallons of water per day. 
    • For larger families and increased toilet usage, that can add up rather quickly!
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How To Make A Toilet Use Less Water

New toilets use far less water than older models, typically from 1.28 to 1.6 gpf. When you consume less water, your water bills will also go down. 

If purchasing a new toilet, look for a “WaterSense” label or a low flush option, and an information label indicating the amount of water used. 

Many toilets now offer dual controls for low and high volumes of flushing. The higher volume uses more water to flush solid waste.

Stop Leaks And Replace Parts

If your toilet tank is leaking or malfunctioning, you can waste hundreds of gallons weekly. 

Check the flapper and mechanisms for signs of mineral deposits or decay in the toilet tank to make sure all parts are working as they should. Most of these parts are easy to replace on your own with a quick trip to the home improvement store.

To determine if you need to replace parts in the tank, you can do a simple check to test for leaks.

  1. Pour some food dye into the tank. 
  2. Let the dye sit in the tank for one hour, without flushing.
  3. If the hue seeps into the toilet bowl, a part is leaking. If not, then you are not losing or wasting water out of the tank due to a malfunctioning part.

Flapper Replacement

The flapper valve in the toilet is often an issue with water usage, including in newer toilets. The flapper is typically made of rubber or plastic and is attached to a chain inside the tank. 

When you push the flush handle, the chain pulls the flapper open to allow water to flow into the bowl. Too much water may be getting in if the flapper is misaligned or wearing down.

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Thankfully, you can replace this part without replacing the entire toilet. Measure the length of the chain, know the make and model of your toilet, and get a replacement at the home improvement store.

Replace The Filler Valve

The filler valve is responsible for allowing water to fill up the tank after flushing. If it is worn or damaged it could use water inefficiently.

You can replace this part or use a professional plumber to help you out.

Rubber Gasket Replacement

The service line has a rubber gasket where it connects to the toilet. If you see moisture or water building up and dripping from the line onto the floor, this gasket likely needs replacing.

Turn off the valve on the water supply line, remove the old gasket, and replace it with a new one. Check for leaks to ensure that it is well connected, and make adjustments as needed.

Other Ways To Use Less Water

If you are not ready to install a new toilet, there are some other options. 

Reduce The Fill Space In The Tank

Fill a ½-gallon plastic jug with water. Seal the cap and place it inside the tank, keeping it away from any moving parts.

Alternatively, you can use a glass container filled with marbles that are also sealed. Do not use a material that will degrade over time, which could affect the functioning of the tank’s moving parts.

Use the gpf technique mentioned above to determine how much water is being used. 

Observe how the toilet flushes after solid waste. If it struggles to move the waste out, decrease the size of your container in the tank to allow more water in.

Float Adjustment

The float inside a toilet works with the tank’s mechanism to indicate that the water level has been reached inside, turning off the supply valve.

You can move the float down inside the tank by adjusting the connecting rod to angle it lower. Typically you will see an adjustable screw or dial to lower it. 

This part can also wear out over time and can be easily replaced with a quick trip to the home improvement store.

Install A Dual Flush Kit

Instead of buying a new toilet, purchase a dual flush kit that works with your model and brand of toilet. Follow the kit’s instructions for installation or use a plumber. 

This will allow you to use less or more water depending on the amount of waste in the bowl.