Can you flush a toilet with one gallon of water?

Flushing a toilet with only one gallon of water may seem impossible, but with the right toilet design and some simple water-saving techniques, it is achievable. In this 5000 word article, we will explore the feasibility of flushing a toilet with a single gallon of water, looking at toilet mechanics, water usage statistics, water-efficient toilets, and water-saving techniques that can help reduce the amount needed to flush.

How Much Water Does a Standard Toilet Use Per Flush?

The amount of water used by a toilet per flush depends on the age and design of the toilet. Older, inefficient toilet models can use 3.5 gallons or more per flush. However, regulations and design improvements have drastically reduced how much water is required for flushing.

Here are some approximate toilet water usage statistics:

  • Pre-1980s toilets: 5-7 gallons per flush
  • 1980s-1990s toilets: 3.5 gallons per flush
  • Current federal standard: 1.6 gallons per flush
  • High-efficiency toilets (HETs): 1.28 gallons or less per flush
  • Ultra low-flow (ULF) toilets: 1.0 gallon or less per flush

As you can see, modern high-efficiency toilet models, including pressure-assist and gravity-flush HETs, can flush efficiently with 1.28 gallons of water or less. The latest ULF toilet designs can flush with only 1 gallon of water.

Toilet Mechanics and the Siphon Flush System

To understand how little water can be used to flush a toilet, it helps to understand how a toilet works. Most modern toilets use a siphon flush system that relies on gravity and the siphoning effect to create a powerful flush.

Here are the basic steps in a siphon toilet flush:

  1. The flush handle is pressed, which lifts a flapper allowing the tank water to empty into the bowl.
  2. The incoming tank water causes the bowl to fill higher than the top of the drain opening, allowing gravity to start pulling the water and waste down.
  3. This siphon effect starts the bowl contents draining, creating a vacuum-like pull as the water flows down the drain.
  4. Fresh water continues flowing from the tank to rinse the bowl and refill the toilet trap.
  5. The tank refills with new water in preparation for the next flush.

The key takeaway is that the flush relies more on gravity and siphoning than water volume. So less water is needed in a well-designed high efficiency toilet.

Water-Efficient Toilet Designs

There are two main types of low-flow, high-efficiency toilets currently on the market:

Pressure-Assist Toilets

These toilets use pressurized air or water to create an air boost that significantly improves flushing power. This allows them to flush effectively with as little as 1 gallon of water.

Gravity Flush HETs

These improved gravity flush toilets optimize the siphon effect and have a redesigned trapway to allow more waste to pass through quickly with less water.

Other water-saving toilet features include:

  • Low-flow flush valves for reduced water usage.
  • Dual-flush options with separate buttons for liquid and solid waste removal.
  • Elongated bowl shapes for faster siphon effect.
  • Smoother, less porous glazes on bowl and trapway.

Water-Saving Techniques

In addition to water-efficient toilet designs, there are also some simple water-saving techniques that can help reduce the amount of water needed per flush:

Adjust Toilet Tank Water Level

Most toilet tanks are factory set to hold more water than needed for an effective flush. Adjusting the tank water level lower through a simple float adjustment can save up to 1/2 gallon with each flush.

Install a Tank Bank or Dam

A tank bank or dam is a simple plastic container that fits inside the toilet tank to displace water. This effectively reduces the amount of water available per flush while still providing enough to flush waste through.

Use a Tank Liner

A tank liner is a plastic bag that can be filled with pebbles and placed in the tank to take up volume. Like tank banks, this displaces water so less is needed for an adequate flush.

Install a Dual Flush Retrofit Kit

For older toilets without dual flush options, a simple button kit can be added to provide separate flush controls for liquid or solid waste removal. This allows matching water usage to need.

Use a Water Displacement Device

Submerging a water bottle filled with pebbles or sand in the tank is an easy DIY method to displace water and reduce the amount used per flush.

Can a Toilet Flush with Just 1 Gallon of Water?

With the right toilet and some water-saving modifications, it is possible to flush a toilet with only 1 gallon of water. Here are some key points:

  • Choose an effective low-flow toilet model such as a ULF toilet or pressure-assist HET designed to work with 1 gallon per flush.
  • Adjust the toilet tank water level down to its lowest functional setting.
  • Use tank displacement devices to eliminate unnecessary water from the tank.
  • Install a retrofit dual flush kit for liquid waste vs solids options.
  • Maintain the toilet seals, valves, and working parts for optimal performance.

With the combination of an ultra-low-flow toilet and simple water reduction modifications, flushing with only 1 gallon is certainly achievable. The key is choosing the right toilet model and optimizing the tank water capacity for minimum functional flow.

Real-World Results: Testing 1-Gallon Toilet Flushing

Extensive testing shows that new ultra-low flow toilets on the market today can flush effectively and clear waste with only 1 gallon of water:

  • Maximum Performance (MaP) Testing: ULF toilets must pass rigorous third-party flushing tests with a rating of 1000 grams or more to meet WaterSense criteria. The highest rated models can clear 1000g wastes with a single, 1-gallon flush.
  • Field Studies: Utilities like the Metropolitan Water District of Southern California have found at least a 20% water savings in households with ULF toilets compared to standard 1.6 gpf models.
  • Water Utility Rebate Programs: Major utilities offer rebates up to $200 for ULF toilets due to their proven water savings ability.

While no toilet model is 100% clog-proof, these real-world findings confirm that the best designed 1-gallon toilets can flush and drain waste just as effectively as older, water-wasting 3.5 gpf models.

MaP Testing Protocol

The Maximum Performance (MaP) testing project has been verifying toilet flush performance since 2003. MaP testing uses real human waste and toilet paper to evaluate flushing efficiency and waste removal score (WRE). The key steps in MaP testing are:

  1. Use specifically formulated soybean paste and toilet paper to simulate human waste.
  2. Fill toilet trapway with 1000g paste and paper waste load.
  3. Flush toilet and assess waste removal score based on percentage of waste cleared from trapway.
  4. A score of 1000 WRE with 1 flush meets WaterSense criteria for premium high efficiency.

The MaP Premium certification for 1000g waste removal with a single flush shows that 1-gallon toilets can flush solid waste just as effectively as inefficient 3.5+ gpf models.

Benefits of 1-Gallon Per Flush Toilets

Switching to a 1-gallon-per-flush toilet offers these savings and benefits:

  • Save thousands of gallons of water per year for the average family.
  • Reduce water and sewer utility bills by 20% or more.
  • Conserve limited freshwater supplies and resources.
  • Reduce wastewater volumes placing less burden on treatment systems.
  • Improve environmental sustainability and carbon footprint.
  • Qualify for utility rebate programs that reward ULF toilet installation.
  • Comply with regulations, codes, and mandates focused on water conservation.

With these significant benefits, upgrading to water-efficient 1-gallon flush toilets provides tremendous value for households, businesses, and the environment.

Key Takeaways on 1-Gallon Toilet Flushing

Here are the key conclusions on flushing toilets with only 1 gallon of water:

  • Modern ULF toilets are designed to flush and clear waste with 1 gallon or less per flush.
  • Adjustments to tank water volume can further reduce water needed for flushing.
  • MaP testing proves 1-gallon toilet models can remove 1000g solid waste loads.
  • Real-world installations show 20%+ water savings vs standard 1.6 gpf toilets.
  • With the right toilet model and simple modifications, 1-gallon flushing is absolutely achievable.

While once deemed impossible, flushing an efficient toilet with only 1 gallon of water per flush is now viable using a combination of innovative toilet designs, performance testing, and water-saving techniques.


Flushing a toilet with only 1 gallon of water is possible through a combination of high-efficiency toilet designs, optimized siphonic performance, and simple water displacement modifications. Extensive testing shows the best new ultra-low flow models can clear waste and meet WaterSense efficiency standards with just 1 gallon per flush. With the significant water savings and environmental benefits, upgrading to these innovative 1-gallon toilets provides maximum value and conservation.

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