Dual-Flush and Low-Flow Toilets: How They Save Water

A toilet made before 1994 could use as much as 6 gallons every time it flushed. A WaterSense-labeled toilet sold today uses 1.28 gallons or less to do the same job. That is not a small tweak. According to the EPA, toilets account for nearly 30 percent of the water an average home uses indoors, so the gallons-per-flush number on the side of the tank is one of the few plumbing figures that shows up directly on your water bill.

This guide explains the two ways modern toilets cut that number, dual-flush designs and single low-flow designs, and the part most people get wrong: how a toilet can use less than a quarter of the old water volume and still clear the bowl in one flush. The short answer is that engineers did not simply pour in less water. They redesigned the path the water travels.

This post is about the mechanism and the math of water saving. For the physical styles of toilets (one-piece, two-piece, wall-hung), see our guide on toilet types. For choosing a seat height, see our guide on comfort-height versus standard toilets. If your toilet flushes weakly or leaves waste behind, that is a performance problem, covered in our guide on why a toilet won’t flush, not a water-saving feature.

How Dual-Flush Toilets Use Two Flush Volumes

A dual-flush toilet gives you two flush options from one tank: a smaller volume for liquid waste and a larger volume for solid waste. The control is usually two buttons on top of the tank, or a two-stage lever, where one push sends down a partial flush and the other sends the full flush.

The logic is simple. Most flushes in a household are liquid-only, and those do not need the full volume that solid waste does. By matching the water to the job, a dual-flush toilet avoids spending a full flush on a task that a partial flush handles cleanly.

The way the federal WaterSense program rates these toilets is worth understanding, because the single number you see advertised is an average, not the volume of either button. Under the WaterSense specification currently in effect (Version 1.2), a dual-flush toilet’s rated water use is its “effective flush volume,” calculated as the average of two reduced flushes and one full flush. That weighting assumes you use the reduced flush about twice as often as the full flush, which reflects typical bathroom use. So a dual-flush toilet advertised at 1.28 gallons per flush does not use 1.28 gallons on either setting. The reduced flush uses less, the full flush may use more, and the rated figure is the blended result.

The EPA released a revised specification (Version 2.0) in May 2024 that would change this approach, setting a flat 1.28-gallon maximum for both single-flush toilets and the full-flush mode of dual-flush models. The agency paused that version’s effective date, and Version 1.2 remains in force until further notice. The practical takeaway for a homeowner does not change: a WaterSense dual-flush toilet is certified to save water and to perform, regardless of which version sets the math behind the label.

What “Low-Flow” Means and How Standards Changed

“Low-flow” is a loose term for a toilet that meets or beats the federal water-use limit. The limit itself comes from the Energy Policy Act of 1992, which set a national maximum of 1.6 gallons per flush for new residential toilets. That standard took effect on January 1, 1994 for residential buildings and January 1, 1997 for commercial buildings, and it is enforced by the Department of Energy. Before it, common toilets used 3.5 gallons per flush or more, and older models used as much as 6.

So a 1.6-gallon toilet is technically the federal floor, not a special efficiency feature. The term that carries real meaning today is “high-efficiency toilet,” or HET, which the DOE defines as a toilet that uses no more than 1.28 gallons per flush. That 1.28 figure is the same one the EPA’s WaterSense program uses for its label, and it is 20 percent below the 1.6-gallon federal standard.

Here is the cleaner way to keep the numbers straight. A toilet at or under 1.6 gallons per flush is legal and standard. A toilet at or under 1.28 gallons per flush is high-efficiency, and if it carries the WaterSense label, it has also passed performance testing. The label matters because efficiency and flush power are certified together, which is exactly what the early low-flow toilets failed to deliver.

How Modern Bowls Keep Flush Power at Lower Volume

This is the part that explains why the “they clog more” reputation is outdated. The first 1.6-gallon toilets, rushed to market in the early 1990s to meet the new standard, often did flush poorly. The common fix was a second flush, which defeated the water savings entirely. But the problem was never the water volume by itself. It was that manufacturers shrank the volume without redesigning the rest of the toilet.

A flush is a system. The amount of water matters, but so does how fast that water enters the bowl, the shape of the bowl, and the width of the channel the waste has to travel through. Early low-flow toilets kept the narrow trapways (the passage from bowl to drain) of their water-hungry predecessors, and a narrow trapway clogs easily when there is less water pushing waste through it.

Over the following years, manufacturers reworked the whole system. According to reporting from This Old House, the trapways were widened by around 2003, which directly addressed the clogging complaints. Engineers added larger flush valves and valve seats so the tank dumps its water into the bowl faster, turning less water into a stronger surge. They reshaped the internal waterways and glazed the trapway surfaces so waste slides through with less resistance, and they used computer simulation and hundreds of test units to refine bowl geometry. The result is that 1.28 gallons of well-directed water can clear a bowl that 3.5 gallons of poorly-directed water used to need.

Some toilets reach low volumes a different way, using a pressure-assist system that uses building water pressure to compress air in a sealed vessel inside the tank and release the flush with extra force. Those are common in commercial settings and some homes. The sealed pressure vessel is not a homeowner-serviceable part, so repairs to it should go to a licensed plumber.

WaterSense Labeling: What It Certifies

For a buyer, the WaterSense label does the vetting in one step, because it certifies two things at once: the toilet uses 1.28 gallons per flush or less, and it has independently passed a performance test. Efficiency alone is easy to claim. The performance half is what separates a modern WaterSense toilet from the failed early low-flow models.

WaterSense toilets are independently certified by a third party rather than self-declared by the manufacturer. Performance is verified through testing modeled on the Maximum Performance (MaP) protocol, which measures how much waste a toilet can clear in a single flush. To earn the label, a tank-type toilet must clear a defined amount of test media from the bowl, the EPA specification calls for removing 350 grams of soybean-paste test media plus four balls of toilet paper in four of five test flushes, along with passing bowl-cleaning and drain-line transport checks. In practice, the EPA reports that the vast majority of labeled models can clear far more than the minimum.

For a homeowner, the label removes the guesswork. You do not have to research a specific model’s flush ratings, because the certification has already confirmed both the water use and the bowl-clearing ability.

Real Water and Money Saved Over a Year

Here are the figures the EPA publishes for replacing old, inefficient toilets with WaterSense-labeled models. The average family reduces the water it uses for toilets by 20 to 60 percent, which adds up to nearly 13,000 gallons of water saved per home every year. In dollars, that is more than $170 per year on water costs, and more than $3,400 over the lifetime of the toilets.

A few honest caveats keep that math grounded. The savings are largest when you are replacing a genuinely old, high-volume toilet, on the order of 3.5 to 6 gallons per flush. If you are swapping a 1.6-gallon toilet from the late 1990s for a 1.28-gallon model, the per-flush gap is much smaller, so the annual savings shrink accordingly. Your actual dollar savings also depend on your local water and sewer rates, which vary widely across the country, and on how many people use the toilet. A busy household full of flushes saves more real water than a rarely-used guest bath.

The pattern still holds in either case: the toilet you flush thousands of times a year is one of the few fixtures where a lower number on the spec sheet turns into measurable savings, year after year.

Do Low-Flow Toilets Clog More Often? The Honest Answer

A modern, well-designed low-flow toilet does not clog more than an old high-volume one, and a WaterSense-labeled model has been tested specifically for its ability to clear waste. The folk belief that “low-flow toilets always clog” comes from real experience with the first generation of 1.6-gallon toilets in the early-to-mid 1990s, which paired less water with unchanged, narrow trapways. That tradeoff has since been engineered out through wider trapways, larger flush valves, and redesigned bowls, as described above.

That said, two things are true. First, any toilet clogs if it is asked to flush too much at once or items it was never meant to handle, and low-volume designs are slightly less forgiving of abuse than a 3.5-gallon flush. Second, model-to-model quality still varies, which is exactly why the third-party performance certification behind the WaterSense label is useful. If you are choosing a low-flow or dual-flush toilet and clogging worries you, the label is the most direct signal that the model was tested to clear the bowl, not just to sip water.

Frequently Asked Questions

What is the difference between a low-flow toilet and a high-efficiency toilet?
“Low-flow” is an informal term for any toilet that meets the federal limit of 1.6 gallons per flush or less. A high-efficiency toilet (HET) is more specific: it uses no more than 1.28 gallons per flush, which is 20 percent below the federal standard. A WaterSense-labeled toilet is a high-efficiency toilet that has also passed independent performance testing.

Does a dual-flush toilet really use 1.28 gallons every time?
No. The advertised figure for a dual-flush toilet is an average, the effective flush volume, calculated under the current WaterSense specification as the average of two reduced flushes and one full flush. The reduced (liquid) flush uses less water than that, and the full flush may use more. The single number is the blended result, not the volume of either button.

Will a 1.28-gallon toilet save much over the 1.6-gallon one I have now?
Less than you might hope. The big savings come from replacing very old toilets that used 3.5 to 6 gallons per flush. Going from 1.6 to 1.28 gallons is a real but modest improvement. The EPA’s headline savings figures assume you are replacing an old, inefficient toilet, not a recent one.

Do low-flow toilets need two flushes to work?
A properly designed, WaterSense-labeled toilet should clear the bowl in one flush, because it has been performance-tested to do so. The double-flush habit comes from the poorly designed early-1990s low-flow models, whose narrow trapways were later redesigned. If a newer toilet routinely needs a second flush, that points to a performance problem with that specific unit rather than a limitation of low-flow toilets in general.

How much of my water bill is the toilet?
The EPA estimates toilets account for nearly 30 percent of an average home’s indoor water use, making them the single largest indoor water user in many households. That is why the gallons-per-flush rating has an outsized effect on long-term water consumption.

This article is general information about how water-saving toilets work, not professional advice. Installation and any code-required or pressure-assist repairs should be handled by a licensed plumber, and local plumbing codes vary by jurisdiction.

Sources

• U.S. EPA, WaterSense, Residential Toilets: https://www.epa.gov/watersense/residential-toilets
• U.S. EPA, WaterSense, Notice of Intent to Revise the Specification for Tank-Type Toilets (effective flush volume and Version 2.0): https://www.epa.gov/system/files/documents/2023-06/ws-products-toilets-v2-noi.pdf
• U.S. EPA, WaterSense, Tank-Type Toilets Performance Overview: https://www.epa.gov/system/files/documents/2022-05/ws-products-perfomance-toilets.pdf
• U.S. Department of Energy, Best Management Practice #6: Toilets and Urinals (federal 1.6 gpf, HET 1.28 gpf): https://www.energy.gov/femp/best-management-practice-6-toilets-and-urinals
• U.S. Department of Energy, Water Closets (Flush Toilets), appliance standards: https://www.energy.gov/eere/buildings/water-closets-flush-toilets
• Alliance to Save Energy, Energy Policy Act of 1992 toilet standards and 1994 effective date: https://www.ase.org/blog/congress-set-toilet-standards-1992-heres-data-showing-theyre-saving-water-and-energy
• This Old House, Low-Flow Toilets: How They Reduce a Home’s Water Use (trapway and bowl redesign history): https://www.thisoldhouse.com/bathrooms/21015448/low-flow-toilets