How Low-Flow Toilets Save Water

Plus, How Much Water They Save and Buying Tips

Close-up of handle on toilet

Kinga Krzeminska / Getty Images

The average household sees five toilet flushes per day. With older toilets using about six gallons per flush, a person could easily get through 11,000 gallons of water per year this way.

Water is, of course, an increasingly precious resource, and a growing number of people are going "low flow" to help conserve it. Many advanced low-flow toilets use 1.28 gallons per flush, 77% less water than what's used by conventional toilets.

While the Environmental Protection Agency acknowledges the flaws in early low-flow models—we're talking double flushing and other "performance issues" in the '90s—its website notes how companies have been working to "redesign and reengineer" their models. Low-flow toilets go through rigorous testing before they get stamped with the organization's widely recognized WaterSense certification, now a national plumbing standard.

Learn how low-flow toilets work, whether they're better than other types of alternative toilets, and tips for choosing one.

How Do Low-Flow Toilets Work?

Overhead view of clean toilet bowl mid-flush

Calvin Chan Wai Meng / Getty Images

There are a few different types of low-flow toilets, including single-flush, more than one type of dual-flush, and pressure-assisted models. Each varies in design complexity and performance. Compare the four most well-known categories of low-flow toilets.

Single-Flush

Single-flush low-flow toilets function the same as high-flow toilets. They're made with all the same components and share the same siphonic system—the siphon acting like an internal plunger, sucking water from the toilet bowl down the sewer pipe. The only difference is that they use less water.

Dual-Flush Washdown

Whereas most high-flow toilets use a siphonic system, many dual-flush systems use what's called a washdown system. Washdown toilets have two buttons on the cistern: one for liquid waste (half flush), one for solid waste (full flush). They use gravity and the weight of the water itself to push waste from the bowl down a larger, less curvy trapway than siphonic toilets have.

Dual-flush washdown toilets are some of the least expensive, most basic, and easy-to-maintain low-flow toilets available, but without the suction of the siphonic system, they are sometimes less effective than regular toilets in expelling every bit of waste.

Dual-Flush Siphonic

Though most dual-flush toilets use the washdown system, a small portion are siphonic instead. Dual-flush siphonic toilets typically come with a push lever, not buttons, and work similarly to a straw with a hole in the middle: When the half-flush is pulled, the "hole" in the siphon stops the water flowing from the cistern at the halfway mark, resulting in less water used.

Pressure-Assisted

Another type of low-flow toilet that's rarely used in homes is the higher-tech pressure-assisted system, which features an air-tight secondary tank inside the main tank. As water fills the secondary tank, it pressurizes the air. That pressurized air gets released into the bowl when the handle is pushed, therefore increasing the velocity of the flushing water.

Pressure-assisted toilets are very expensive and notoriously loud. They also have more moving parts, which also makes them prone to breakage.

Can a Toilet Be Waterless?

Person pouring sawdust into composting toilet with scoop

SolStock / Getty Images

Low-flow toilets can help users drastically reduce their water consumption at home, but other alternative toilets eradicate the need for toilet water completely.

Dry-Flush Toilets

Dry-flush toilets, used more commonly in recreational vehicles than inside homes, come with a replaceable cartridge of bags that automatically line the toilet after each flush. When the handle is pulled, the bag is twisted above the waste and discarded—voila! A clean toilet.

Dry flush toilets run on battery power. While they're great for water consumption, they're also terrible for pollution. Most bags designed for dry flush toilets are not even biodegradable.

Composting Toilets

Another option is the composting toilet, in which waste gets mixed with a fibrous substance like sawdust, coconut coir, or peat moss that helps turn it into compost. Here's how a composting toilet works: You do your business, then follow that by throwing a scoop or so of carbon-rich material down the hole. Some require water, but usually no more than a pint per flush. The whole process can be done manually or automatically with a more high-tech toilet that features an automated mixing element (these can also come with heating systems or exhaust fans). Later, the compost created from waste can be safely used on non-edible gardens.

Though the process seems quite archaic, this type of toilet is becoming increasingly popular. Even so, it's uncommon to use a composting toilet inside the home because of the mess and, depending on the model, the smell.

Which Low-Flow Toilet Type Is the Most Environmentally Friendly?

Waterless types aside, the best toilet model for water conservation is a dual-flush because the half-flush option uses only about .8 to 1.1 gallons. Pressure-assisted toilets can use as little water as their dual-flush counterparts, but their fragile parts often break and create waste.

Things to Consider When Choosing a Low-Flow Toilet

Ready to take the next step in home water conservation? Both your wallet and the planet will thank you for it. Here are some tips for first-time buyers.

  • Make sure the toilet you choose is EPA WaterSense-certified, meaning it has been tested and does not use more than 1.28 gallons per flush.
  • Check its MaP score, which demonstrates efficiency. If it's 500 or higher, the toilet has demonstrated excellent performance.
  • Pressure-assisted toilets can be especially noisy with all the whooshing involved with flushing. Make sure to listen to it first so you know it won't be a nuisance in your home.
  • Read reviews! Pressure-assisted low-flow toilets are rather new on the scene, and some tend to break easily. Broken parts are more than annoying; replacing them constantly can create an abundance of plastic waste. The key is to research thoroughly before buying.
Frequently Asked Questions
  • Are low-flow toilets difficult to install?

    Swapping a high-flow toilet for a low-flow toilet is easy, but there are a couple of things to consider. One is the "rough-in distance," which is the distance between your drain and the wall. You can choose a toilet with a smaller rough-in distance than your original toilet, but not vice versa. Another is one-piece versus two-piece toilets, the latter being more common and easier to install because they're lighter.

  • Do low-flow toilets clog more easily?

    Low water volume historically meant that low-flow toilets would clog more easily. Many advancements have been made—and rigorous testing put in place—to improve the mechanisms and prevent this from happening.

  • How much water do low-flow toilets save?

    WaterSense-certified toilets use 1.28 gallons (or less) per flush. That's 77% less water than is needed for conventional toilets, which use about six gallons per flush.

View Article Sources
  1. "Toilets." Princeton University. 2020.

  2. "Residential Toilets." Environmental Protection Agency.

  3. "Residential Toilets." Environmental Protection Agency.