How Low-flow Toilets Flush Your Waste Without Wasting Water

By: Barbara Boughton  | 
Low-flow toilet
Low-flow toilets came onto the market in the 1990s, to mixed reviews. Since then they've improved substantially and can flush your waste without wasting water. gomolach/Shutterstock

Having a toilet shouldn't be a privilege, but it is. One in four people don't have a decent toilet, according to WaterAid America, and that contributes to poor hygiene, serious illness and even death. But toilets, other than low flow toilets, require a lot of water to do their dirty work. And water is a precious commodity we can't afford to, well, flush down the toilet.

After outdoor irrigation, toilets use more water than any appliance or fixture in a residential home. Older, inefficient toilets use as much as 7 gallons (26.5 liters) per flush. And old toilets are also often the cause of household leaks.


But what about low-flow or ultra-low-flow toilets (ULFTs)? Since 1992, U.S. law has mandated all new toilets sold in the United States use 1.6 gallons (6 liters) or less per flush. Of course many homes still have inefficient toilets and haven't replaced them with newer, low-flush toilets. But the Environmental Protection Agency (EPA) says the average family could save as much as 13,000 gallons (49,210 liters) of water a year if they did. So how do low-flow toilets get the job done?

Ultra-low Flush Toilets Come to Your Bathrooms

First let's talk about how low-flow toilets got to your bathroom. In the early 1990s, Congress mandated the first real law on toilet efficiency as part of the Energy Policy Act. Toilet manufacturers had to redesign their toilets to use 1.6 gallons (6 liters) or less per flush. Some were effective. Some weren't. Many manufacturers just reduced the size of the toilet tanks, and consumers complained the new toilets just didn't function effectively.

That led to a new testing protocol called Maximum Performance (MaP) testing. In 2002, a group of 22 Canadian and U.S. organizations developed the program to test toilets based on their flushing ability. The scores represent how much waste (soybean paste and toilet paper) that the toilet can totally remove in a single flush. It's now the de facto flushing performance test for toilets in North America.


Advances in Low-flow Toilets

An illustration of a person's hand pushing the flush button on a low-flow toilet.
There are several types of low-flow toilets. One off-shoot is the dual-flush, which has two buttons on the top of the tank, one for liquid waste and one for solids. Thebirdss/Shutterstock

Today's low-flow toilets are much more effective than those that first came onto the market. Low-flow toilets usually use one of two methods to clear waste: gravity or power flush. Both types use no more than 1.6 gallons (6 liters) of water or less per flush.

As the name implies, gravity toilets use gravity to remove the water and wastes from the toilet bowl. You just flush and water goes into the bowl, then into a trap that creates a natural syphoning effect that helps everything head into the sewer pipes. Gravity-assisted toilets are easy to repair; the technology has been the same for decades, it's just been improved.


Power flush toilets, by contrast, use pressurized air in the tank to force the water and waste as you flush. The air is compressed in a vessel inside the tank each time it refills. Pressure-assisted toilets are very efficient, but they're noisy and usually more expensive than gravity fed toilets.

How to Choose a Low-flow Toilet

Water swirls around as a low-flow toilet flushes.
One thing to look for when purchasing a low-flow toilet is the WaterSense label from the Environmental Protection Agency. MITstudio/Shutterstock

As we've said, modern low-flow and high-efficiency toilets perform far better than the water-wasters of the past. But there are differences among toilets, and the following factors influence how well a new toilet performs.

  • Waste removal: Some toilets still remove waste from the bowl better than others.
  • Noise: High-pressure toilets are louder than gravity toilets and might not be ideal for some homes.
  • Design: Round bowls also tend to be better at flush performance than elongated bowls because they're smaller, which means the released water can be more efficient at removing waste.

What should you look for when buying a low-flow toilet to get the most out of your investment?


  • Look for models that are labeled as high-efficiency. These toilets will give you the most water and financial savings. And, in some locations in the United States, sizable rebates are available to water utility customers for these toilets.
  • Make sure you get a toilet with the WaterSense label from the EPA. A WaterSense labeled-toilet is a high-performance, water-efficient option that has been tested by MaP.
  • Expensive toilets are not necessarily better toilets. WaterSense has approved toilets at a variety of price points.


Retrofitting Older Toilets to Save Water

The internal mechanisms of a low-flow toilet.
If you simply can't replace your inefficient toilet, you can retrofit the flushing mechanism so you still use less water with each flush. Zhabska T.S./Shutterstock

If you'd like to save water, but don't want to invest in a low-flow toilet, there are a few ways to retrofit a toilet so that it uses less water.

The most common approach is to use a household displacement device. These devices are placed inside the tank in a place where they won't affect flushing and do just what you'd think — displace water. Because they take up room inside the tank, the tank doesn't have to fill up with as much water each time you flush. Some typical displacement devices include plastic bottles, or bags filled with water or bricks.


But displacement devices and not recommended as long-term solutions. Bricks can deteriorate and cause damage to the flushing mechanism and plastic bottles or bags usually need to be weighted (with pebbles, for example) to make sure they're stable.

Another way to retrofit a toilet is to use a displacement dam. These plastic dams are wedged into the tank on both sides of the flush valve to decrease the amount of water per flush. One advantage they have over homemade displacement devices is that they're less likely to move around in the tank and disrupt the flushing mechanism.

One more retrofit device is the early-close flapper. It shuts off the water flow to the bowl before the toilet tank is empty. Early close flappers are adjustable so homeowners can adjust the water level so they can save water and still achieve a clean flush.

Still, all retrofit devices generally affect how well a toilet functions, and retrofitted toilets simply don't work as well as new low-flow and high-efficiency toilets in clearing waste. And since low-flow toilets with excellent performance are available at a range of price points, it's hard to find reasons to retrofit instead.


Lots More Information

Related Articles

  • "About Ultra Low Flush Toilets." A Massachusetts Water Resources Authority Publication.
  • Interview with Ernesta Jones, Environmental Protection Agency
  • Interview with John Koeller, Alliance for Water Efficiency
  • Interview with Terry Love, Love Plumbing & Remodel.
  • "It's All in The Flush! The High Impact of Low-Flush Toilets."
  • Maas, William. "Five Tips for Choosing a Low-Flow Toilet." Green Home Guide.
  • "Residential Rebate Programs." Pasadena Water & Power Department.
  • Water-Efficient Toilets. Massachusetts Water Resources Authority.
  • "WaterSense." Environmental Protection Agency.