How Dual Flush Toilets Work: A Simple Guide

Two flush options in one toilet

A dual flush toilet works like any other tank-type toilet, but its flush valve can release two different volumes of water depending on which control you use. The reduced flush, usually marked for liquid waste, uses a smaller volume of water. The full flush, for solid waste, releases a larger volume. The US EPA highlights this feature on WaterSense labeled dual flush toilets, noting that you save even more water by choosing the reduced flush mode for liquid waste.

The controls are typically two push buttons on top of the tank or a single lever that tilts two ways. One position produces the reduced flush, the other the full flush. If you are comparing control styles, toilet flush lever vs button explains the tradeoffs. And if you are deciding whether dual flush makes sense at all, single vs dual flush toilets lays out the difference.

How much water each flush uses

Toilets are the largest single use of water indoors. The EPA reports that they account for nearly 30 percent of an average home's indoor water consumption. Older, inefficient toilets can use as much as 6 gallons per flush. The current federal standard is 1.6 gallons per flush, and WaterSense labeled toilets use 1.28 gallons per flush or less, which is 20 percent less than that standard.

On a dual flush toilet, the reduced flush drops below the full flush volume, which is what makes the toilet more efficient overall. Because a dual flush toilet has two modes, the EPA evaluates it on an effective flush volume that accounts for both. A revised specification, currently paused, would instead cap the full flush at the same 1.28 gallons per flush maximum that applies to single-flush models. The EPA estimates that replacing old, inefficient toilets with WaterSense labeled models lets the average family reduce water used for toilets by 20 to 60 percent, saving nearly 13,000 gallons per year.

For context, the average American family uses more than 300 gallons of water per day at home, and roughly 70 percent of that use is indoors. Because toilets drive so much of that indoor use, flush volume is one of the easiest ways to lower a household's water usage. Our toilet flush volume guide explains how to read these numbers, and the best 1.28 GPF toilets guide lists models that meet the WaterSense threshold.

The flush mechanism inside

Inside the tank, a dual flush toilet uses a flush valve that can open at two different rates. Pressing the reduced flush control lifts the valve only partway, or opens a smaller passage, so less water flows from the tank into the bowl. Pressing the full flush control opens the valve fully, releasing the larger volume needed to clear solid waste.

Two valve designs are common. A flapper-style valve uses a rubber or plastic seal that lifts when you flush. A tower-style valve lifts straight up and is often found in dual flush designs because it can offer two distinct openings. See flapper vs tower flush valve for the details on each.

Because the valve moves on every flush, it is the part most likely to wear out, and a worn seal lets water leak from the tank into the bowl. The EPA notes that most toilet leaks come from old or worn-out flappers, and that replacing the flapper is quick and easy. If you are installing a new dual flush toilet, our how to install a toilet guide walks through the process.

Efficiency without losing performance

The low-flow toilets of the 1990s earned a reputation for weak flushes that required a second attempt. The EPA says manufacturers have since redesigned their products, and WaterSense labeled toilets today are independently certified to meet performance criteria, not just water limits.

Those criteria include clearing waste from the bowl, moving waste through the drainline, cleaning the bowl surface, and flushing test media of different sizes and densities. In other words, a certified dual flush toilet has to prove it can clear solid waste on the full flush and still handle liquid waste on the reduced flush. The EPA states that today's WaterSense labeled toilets far outperform typical toilets from the 1980s and 1990s. For the details on how this is verified, see MAP testing toilets explained and WaterSense toilet certification.

What to consider before you buy

Dual flush pays off when the reduced flush actually gets used. If most flushes in your home are for liquid waste, the smaller volume adds up quickly. The EPA's savings figures assume you are replacing an old, inefficient toilet, so the gain is largest when your current model is older and uses more water.

Think about the controls you prefer. Two buttons are easy to identify, and a dual-action lever feels like a standard handle. Also consider bowl shape, seat height, and whether you want a one-piece or wall-hung design. The best dual flush toilets roundup compares models on specs, and one piece dual flush toilets covers that specific design.

If you would rather have one flush volume for everything, a single flush toilet is the simpler choice. Whatever you pick, check for leaks after installation, since a leaking toilet wastes water continuously.

What to pick for your use

Questions

Which button is for liquid waste?

The reduced flush control, which the EPA describes as a mode that uses a smaller volume of water to remove liquid waste. The other button, or the full lever position, gives the larger flush for solid waste. Most tanks are marked so you can tell them apart at a glance.

Do dual flush toilets use less water than single flush toilets?

Under the WaterSense program, a dual flush toilet must use 1.28 gallons per flush or less overall, and the reduced flush uses an even smaller volume. The EPA says replacing old, inefficient toilets with WaterSense models can reduce water used for toilets by 20 to 60 percent for the average family.

Are dual flush toilets more likely to clog?

No. WaterSense labeled toilets must pass the same performance criteria as standard toilets, including clearing waste from the bowl and moving it through the drainline. The EPA says today's labeled toilets far outperform typical models from the 1980s and 1990s.

How much water does the full flush use?

The full flush uses the maximum flush volume of the toilet, which on a WaterSense labeled dual flush toilet is 1.28 gallons per flush or less. The reduced flush uses a smaller volume, but the exact number varies by model. Check the spec sheet for the effective flush volume.

Can I convert a single flush toilet to dual flush?

Some replacement flush valves are marketed as dual flush, but the EPA does not certify them separately. If you want a guaranteed water efficiency and performance rating, choose a complete WaterSense labeled toilet.