Dual flush toilets save water compared with an old toilet, but often not much compared with a 1.28 gallon single-flush model. Field studies found them averaging about 1.24 gallons per flush, because people use the reduced flush less than the ratings assume. Real savings depend on your household's habits and on having no leaks.
How does a dual-flush toilet work, and how does WaterSense count it?
### What are the two flushes?
A dual-flush toilet has two modes, an idea that started in Australia. Harrison's 2010 conference paper says Caroma's 1980 design led Australia to make dual flush toilets required in almost all new buildings. EPA describes a "reduced flush" that uses a smaller volume of water to remove liquid waste, and a "full flush" for solids. The national standard caps the full flush at 1.6 gallons and the reduced flush at 1.1 gallons, according to MaP Testing, the independent toilet-testing program.
### What is the effective flush volume?
Under the WaterSense tank-type toilet specification that is in effect (Version 1.2), a dual-flush toilet must have an "effective flush volume" of no more than 1.28 gallons. EPA calculates it as the average of two reduced flushes and one full flush. MaP's example: [1.6 + (2 x 1.1)] / 3 comes to 1.27 gallons.
That math assumes people use the reduced flush twice as often as the full one. It's an assumption, not a measurement. In May 2024 EPA published Version 2.0, which drops the calculation and instead requires a full flush of 1.28 gallons or less. EPA has paused that version's effective date, and says Version 1.2 remains in effect until further notice. So a dual-flush toilet on the shelf today may still carry the older 1.6-gallon full flush.
What did field studies actually measure?
### What do the residential studies show?
The best-known residential data is a 2010 reanalysis of Aquacraft field studies in East Bay Municipal Utility District (EBMUD) and Seattle homes, written by Aquacraft staff and a Kohler engineer. It compares those homes with two other studies:
| Study | Small-to-large flush ratio | Average gallons per flush |
|---|---|---|
| Seattle and EBMUD homes | 0.8 | 1.29 |
| Oregon SWEEP study | 1.86 | 1.3 |
| Utah study | 1.47 | 1.20 |
Combining the studies, the authors estimated an average of about 1.24 gallons per flush for a 0.8 and 1.6 gallon dual-flush toilet, and a small-to-large ratio of about 1.4. They say that if the toilets had all performed at exactly 0.8 and 1.6 gallons, the average would have been 1.15. So the real number was close to the 1.28 gallon rating of a single-flush high-efficiency toilet, not far below it. Against the 1.6 gallon federal standard, 1.24 is still about 22 percent less, so the toilet does save water. The question is how much more it saves than a good single-flush model, and the answer here is not much.
Two limits. The sample was small (65 dual-flush toilets in the Aquacraft homes), and the authors call their findings "suggestive, but not definitive." And a Kohler engineer co-wrote it, so weigh that.
### What do other sources say?
They point the same direction. MaP's John Koeller says manufacturer claims of a 4-to-1 ratio aren't substantiated by facts developed independent of manufacturers, and that real residential use is somewhere between 1:1 and 2:1. EPA says it received feedback from some stakeholders that dual-flush toilets with a 1.6 gallon full flush may not realize the 20 percent savings in real-world use because of user confusion, user preference or other factors. Its own 2024 review of studies concluded users may choose the full flush more than once every three flushes, so labeled toilets might not deliver the 20 percent savings WaterSense promises.
One outlier is worth knowing. A 2010 conference paper by Masaye Harrison studied a LEED-certified commercial building and calculated a 32 percent water savings against a conventional 1.6 gallon flush. Only 55 percent of occupants said they used the small flush consistently, and the paper suggests they may have been used to it after a year in the building. It's one building, not a home.
Why do the savings fall short?
### People press the safe button
Koeller says savings depend on whether users understand the purpose of a dual-flush toilet or just take the "safe" choice and use the full flush every time. Harrison's survey found 21 percent of occupants said the flip handle was difficult to use, which the paper ties to how the handle is designed. If the two buttons look alike, guests will guess.
### A leak beats any flush volume
EPA says household leaks can waste 180 gallons per week, toilets are often the culprit, and most toilet leaks come from an old or worn flapper. Dual-flush toilets have a flush-valve seal too. If it doesn't seat, water trickles into the bowl all day, and no flush setting fixes that. The food-coloring test in EPA's fact sheet works on any toilet: put a few drops in the tank, wait 10 minutes, and check the bowl. Our pages on how to fix a running toilet and how to pick the right flapper cover the repair.
### Double flushing
A reduced flush that doesn't clear the bowl can lead to a second flush, and the savings vanish. Flush performance matters as much as flush volume, which is why how to choose a toilet tells you to check the MaP score.
What does this mean in Phoenix?
The Phoenix incentive doesn't care whether a toilet is dual flush. The City of Phoenix's program pays toward a WaterSense-labeled toilet of 1.28 gallons per flush or less, and requires the old toilet to use 3 gallons per flush or more. The city says residents can expect each new toilet and irrigation controller to save approximately 10,000 gallons per year. You get most of that from swapping out an old toilet, whichever flush type you pick.
Should you buy a dual-flush toilet?
If you're replacing a toilet from the 1980s or 1990s, any WaterSense model gets you most of the savings. Dual flush adds a bit more only if everyone in the house actually uses the reduced flush. A high-scoring single-flush toilet at 1.28 gallons or less is a safe choice, and it takes the guesswork out. Koeller goes further for places where users are strangers to the toilet: he says many water efficiency specialists do not recommend dual flush for non-residential installations, and that single-flush models in the 1.0 to 1.2 gallon range remove the behavior factor. That's worth thinking about for a rental or a guest bath that gets a lot of visitors.
### How do I get the savings if I do buy one?
- 1Teach the household which button does what, and tell guests.
- 2Use the reduced flush for liquid waste only, the way EPA describes it.
- 3Run the food-coloring leak test once or twice a year.
- 4Replace a worn flapper or flush-valve seal early. EPA calls a flapper an inexpensive part and says replacing one is quick and easy.
What a swap involves is in what drives the cost to replace a toilet. HQ Plumbing & Air is licensed and insured and gives free estimates, with an upfront price before we start.