Toilet Sink Combo: How to Save Water and Bathroom Space

Last Updated on September 4, 2023 by toilethaven

What is a Toilet Sink Combo?

A toilet sink combo refers to the combination of a toilet and a sink to form one fixture. The sink is mounted on top of the toilet tank instead of the lid. As you wash your hands, the grey water flows from the sink to the tank and is then used to flush the toilet.

A toilet sink combo saves both space and water and is, therefore, a great option for people with small bathrooms and also those who need to bring down their water bills or both.

Toilet sink combos are not as popular in the United States as they are in Japan. They have been in use in Japan since the 1950s. This combo was initially marketed as space-saving toilets owing to the small nature of apartments and bathrooms in Japan.

With the deterioration of the environment and with the need to try and avert the situation, more Japanese bathrooms were installed with toilet sink combos.

Most Americans traveling to Japan are very surprised when they see a toilet sink for the first time. They are almost everywhere.

There are two ways of having a toilet sink combo in your bathroom. You can buy a complete toilet that comes with a sink, or you can buy a toilet sink and retrofit fit in your existing toilet.

The second option seems a particularly good one for people who have otherwise good toilets but only need to add a sink. If you are doing a bathroom remodel, you can consider a brand-new toilet-sink combo. Unfortunately, it is hard to find a complete toilet sink combo in the United States.

How Do Toilet Sink Combos Work?


In ordinary toilets (toilets without a sink on top), a water supply line connects the toilet water supply to the toilet tank. The water supply line runs from the toilet shut-off valve to the tank, where it is connected to the toilet fill valve from underneath the toilet tank.

In a toilet sink combo, the water flows from the supply line to the fill valve, but instead of filling the tank directly, a small pipe is connected from the fill valve to the sink faucet. When you flush the toilet, its refill mechanism will kick in, and water will automatically start flowing from the faucet.

As you wash your hands, the grey water will flow down to the tank through a drain at the bottom of the sink. I was not sure about the practicality of the toilet sink combo until I learned that the water flow rate is slow enough to give you sufficient time to wash your hands.

If you think that your toilet has a higher flow rate than is comfortable/sufficient to wash your hands, look for the water shut-off valve on the wall behind the toilet.

Gently turn it counterclockwise until you get the flow rate you are most comfortable with. This also helps to prevent water from splashing around.

The water you use to wash your hands is not the same water you use to flush the toilet immediately. The tank already had grey water from the previous handwashing; hence, the water you use to wash your hands will be used to flush the toilet by the next user.

It is just like when you use an ordinary toilet. By the time you enter the bathroom, the tank is normally full of water. When you flush the toilet after use, the tank refills so that it has enough flushing water for one more use.

As soon as you flush the toilet, water will automatically start flowing out of the sink faucet and into the sink. When the water level in the tank reaches the set level, the water flowing out of the faucet will be shut off automatically. Some toilet sink combos, however, will need you to turn the water faucet on and off physically.

I should also mention that the toilet sink is not permanently molded on the tank. It is fitted on the top the way a tank lid fits and can thus be removed and returned anytime you need to carry out toilet tank repairs.

Best Toilet Sink Combos Options

Unfortunately, getting a fully integrated toilet sink combo in the USA is not very easy. This idea is yet to resonate well with the big toilet manufacturers like Toto, Kohler, and American Standard. Or maybe it is the people who are not yet ready.

Even if you can get a fully integrated toilet sink combo, you are not always guaranteed other toilet functions like flushing power, which I consider vital in a toilet. So what options are there?

Luckily, you can buy a toilet sink that you can easily attach to the top of your existing toilets. This option provides you with an inexpensive way of converting your regular toilet into a toilet sink combo.

You will, therefore, not need to buy an expensive new toilet or pay a plumber to install it. Attaching a toilet sink to an existing toilet is a very simple task that you can comfortably do on your own.

I know of 2 great products that can help you convert your toilet into a toilet-sink combo. These are:

1. Sink Twice Toilet Sink

On Sale
Sink Twice for Toilet Tanks Measuring 15.25" -...
  • Fits most standard toilets measuring 15.25 in to 16.75 in
  • Touchless faucet activates with hand motion to save water
  • Diverts used water to flush the toilet, saving up to 25,000 gallons per year

Sink Twice is a beautifully curved toilet sink that offers a touchless automatic handwashing solution and also saves water. It is made in the United States, so you are guaranteed good quality.

The Sink Twice sink is compatible with toilet tanks measuring 14 to 16.8 inches at the top of the tank with the lid off. If you have a bigger toilet than that, you can use the Sink Twice expansion kit, which slides on top of the tank to help you have a perfect fit.

You can also buy the Loctite mounting putty tabs to help you fix the sink in some toilet tanks and thus make it sturdy. This will prevent it from wobbling or being pulled off, especially if you have kids.

2. Sink Positive Toilet Sink

  • Daily water savings of up to two gallons per person under normal use.
  • Identifies costly leaks through visible drips.
  • Installs in less than 10 minutes without the need for a plumber.

The Sink Positive toilet sink is another great way of adding a sink on top of your regular toilet. Just like the Sink Twice toilet sink, it is also automatic, so you don’t need to turn the faucet on or off, which further enhances hygiene.

The sink has a soap dish on one side, which will also perfectly fit a handwashing soap dispenser. Installing this sink is so easy and will, on average, take you about 10 minutes. You don’t need to call in a plumber to do it.

Sink Positive toilet sink comes with an expansion kit to help it fit in longer toilet tanks. It measures 16.5 inches in width but can be expanded to fit toilets of up to 21.5 inches in width.

Its molded plastic construction is of very high quality, and from a distance, it looks like porcelain. This product is slightly more expensive than the Sink Twice toilet sink.

Advantages of a Toilet Sink Combo

The following are the main advantages of installing a toilet sink combo or even adding a toilet sink on top of your existing toilet:


1. It Saves Water

Most of the modern low-flow toilets use 1.28 or 1.6 gallons of water per flush. You never know how much water that is until you see it flow from a faucet and how long it takes to flow.

It is even worse for people who still have the old toilets with a flush rate of 3 gallons per flush. It is, however, reassuring knowing that you are killing two birds with one stone.

So, how much water do you save by using a toilet-sink combo? New bathroom faucets have a flow rate of 1 gallon per minute, which is the average time used to wash one’s hands. Old bathroom faucets have a flow rate of up to 2 gallons of water per minute.

Using a toilet sink combo means every time you use the toilet, you will save at least 1 gallon of water. Assuming you use a toilet twice a day, it means you will save 60 gallons of water per month.

Multiply that by the number of people using the toilet. Even though two daily toilet use is still a conservative figure, you stand to save so much water by using this system yearly. This will, in return, help you cut down on your water bills.

Apart from just saving on water bills, toilet sink combos are also environmentally friendly. Using less water helps us to conserve our natural water resources for a greener environment.

2. Perfect for Small Spaces

Sometimes, you have such a small bathroom such that installing a stand-alone sink seems impossible. A toilet sink combo, therefore, comes in handy in such a situation.

Apart from a toilet and sink, you also need to have other bathroom items like a shower/bathtub and bathroom storage units. By installing a toilet sink combo, you are left with enough space to add any extra items you may want in the bathroom without it looking stuffed.

3. Improves Hygiene

There are a few people who somehow, for whatever reason, don’t wash their hands after using a toilet. Some people refuse to wash their hands because they don’t want to touch faucets that other people have touched with their dirty hands.

With a toilet sink combo, however, as soon as you flush the toilet, the water automatically starts pouring out. Even if you were really not looking forward to washing your hands, you will find yourself unknowingly doing so.

4. Keeps the Tank Clean

Contrary to what some people might think, the grey, soapy water does not make the toilet tank dirty or smelly but rather keeps it clean all the time. What most people also don’t know is that adding a bit of dish soap to your toilet’s overflow tube helps to improve its flushing power, especially in areas with hard water.

How does this happen? Calcium, which is the one element found in plenty of hard water, clogs up the toilet bowl rim holes and the siphon jet. During flushing, water from the tank enters the bowl slowly, resulting in a weak flushing toilet.

The soapy water in the toilet sink combo helps to soften the water, therefore preventing minerals from being deposited and clogging the rim and siphon jets. You will, therefore, always have a strong flushing toilet.

Disadvantages of a Toilet Sink Combos

So, what are the cons of installing a toilet sink combo? Although this system is simple and practical, it still has shortcomings. The following are the disadvantages of installing a toilet sink combo:

1. Not readily available

Although toilet sink combos are so popular and readily available in Japan and other countries, the same cannot be said of the United States. Getting a good sink-toilet combo is presently almost impossible.

The only option you are left with is to install a sink on top of your existing toilet. That is a great idea for someone who already has a toilet, but for someone looking for a new fully integrated toilet sink combo, it’s just inconvenient.

2. Sink not Accessible to Some People

Unfortunately, disabled people, especially those using a wheelchair, cannot access a sink on top of a toilet. They would normally need to have a space below the sink, which, in this case, is occupied by the toilet. The sink-toilet combo will just not work for them.

3. Cold Handwashing Water

One other thing that you may not like about the toilet sink combo is that it is connected to the cold water system. If you are one of those people who like washing your hands with hot water, then you might not enjoy this innovation as much as you would have liked.

There are, however, some valves that are installed to preheat the incoming water. These are the same valves used to prevent toilet sweating. If you want to follow that path, then you can completely resolve that challenge.

How to Add a Sink to a Toilet

If you have an existing toilet that you need to make a toilet sink combo from, adding a sink at the top of the toilet is a very simple procedure that will take you not more than 10 minutes. In this case, I am using the Sink Twice toilet sink.

Once you receive your sink, make sure that you have the following parts:

  • Basin
  • Faucet with a tube
  • Nut
  • Fill cycle diverter

Here is how to install the sink on top of the toilet:

  • Look for a hole at the top of the basin. This is where the faucet tube should go in through.
  • Feed the tube through the hole until the bottom of the faucet rests on top of the basin.
  • To secure the faucet firmly on the basin, feed the nut through the faucet tube and screw it on the faucet threads found on the underside of the basin.
  • Remove the lid from the toilet tank.
  • You will see the toilet fill valve with a small tube connected to the overflow tube. The overflow tube is the large pipe in the middle of the tank. It prevents the toilet from overflowing by channeling excess water in the tank down to the bowl.
  • If you have a high-pressure toilet, connect the refill tube to the faucet tube using the T-shaped fill cycle diverter. The orientation should be such that one end of the filly cycle diverter is connected to the refill tube, the other to the faucet tube, and the last facing down inside the overflow tube. You can cut the tubes down in size if the tubes are too long.
  • If you have a low-pressure toilet, remove the refill tube from the fill valve and connect the faucet tube directly to the fill valve.
  • Carefully place the sink on top of the toilet.
  • Use the expansion kit if required.
  • Flush the toilet to ensure the system works as anticipated/designed.


It is without a doubt that toilet sink combos are a valuable combination, especially for people with small bathrooms and those seeking to save water. Whether this system will be as popular in the USA as it is in Japan is something that remains to be seen.