Published on April 8th, 2021 | by Scott Cooney


Dual flush toilets: how they work, whether they’re worth it, adapters, and how much you can save

Dual flush toilets have been around for a while, but still aren’t standard in most building codes. That fact alone made me wonder: are they worth it? And how much money and water do they save? In this article, I’ll answer these questions based on some recent research and experience.

How they work

Toilets operate by siphoning waste down the tube as a result of pressure exerted by water added to the bowl. Flushing liquid waste requires less water to move than solid waste, so a dual flush toilet just adapts how much water is used. Modern dual flush toilets use a little less than 1 gallon for a liquid flush, and 1.25 or so gallons for a solid flush. The fact that 1.25 gallons can flush a #2 is impressive, showing how far the technology has come from the days of “water closets” using up to 5 gallons per flush.

Some have a pair of buttons on top (like in the photo above). What happens if you push both buttons? It just defaults to the #2 flush. Some have a lever like this one below, and signage to show people what to do.

The two button setup is superior, as this lever concept, which tries to emulate the type of toilet handle we grew up with, goes against our typical habit of just pushing these levers down to flush, and therefore has to overcome years of muscle memory. A friend installed a bunch of these on the campus of the University of Hawaii at Manoa, and in his followup research to prove its effectiveness, realized that people don’t read signs, and just ended up pushing the lever down no matter whether it was liquid or solid. So in any conversion for sustainability that involves human behavior, it’s always important to remember people don’t read, and don’t tend to change unless they’re moved to do so.

How much can you save, and is upgrading worth it?

To get to the heart of the matter, let’s look at the cost benefit analysis of upgrading to a dual flush toilet. Let’s say your existing toilet is not that new, and uses 2.5 gallons per flush. Assuming there are three people using a toilet, and each person flushes 6 times a day, 5 liquid and 1 solid. In that scenario, the toilet uses 45 gallons a day. Upgrading this to a dual flush toilet, and assuming everyone uses it the way it is intended, the resulting gallons per day is roughly 18.75, so your savings is 26.25 gallons per day.

The water utility where you are likely charges you in kilogallons (1000 gallons). Where I live, the residential rate for water is about $8.75 per kilogallon (this includes both water and sewer charges). So 26.25 gallons per day is 0.02625 kilogallons, so the savings is about 25 cents a day.

At a purchase price of about $250, a dual flush toilet will then pay for itself in about 1000 days, or 2 and a half or so years. That assumes you do the install yourself vs. paying a plumber (possible for many DIY types, but not likely most people will want to install their own toilet).

There are obviously a number of other assumptions. The number of people, the number of times they flush, and the flow rate of the existing toilet will all make a difference. Also, how social you are: if you host parties and 25 people will flush their beer four times that night, it’ll expedite the payback period.

As eco-upgrades go, this one is mediocre. It is best for people with old toilets that likely need an upgrade anyway, and for high use toilets.


You can also get an adapter for your existing toilet. There are plenty on the market, some with ten plus years of market proof of concept. The primary benefit is the reduced cost, but the other main eco-benefit is that it reduces the need to produce new toilets, and landfill old ones.

If you decide to go this route, make sure you read the reviews of verified purchasers, and scan to see the reviews that are posted after a little time has passed. The best way to ensure you get a quality product is to hear real customer experience 6, 12, 18 months after.


Photos, in order top to bottom, from FlickR creative commons, by SuSanA Secretariat, and Kari Sullivan

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About the Author

Scott Cooney is a serial eco-entrepreneur including being the solo founder of Pono Home,, and CleanTechnica; author of two books; former sustainability consultant with clients including Johnson & Johnson, Eastman Chemical, Wal-Mart, and Duke Energy; former Adjunct teaching the first course in sustainable business in the MBA program at UH Manoa; lover of local, healthy food and especially vegan nachos. Find Scott on Twitter

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