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How to Calculate Indoor Building Water Use

Reducing Indoor Water Use for Flow and Flush Fixtures


Empty boardroom chair with notepads and glasses of water on table
Tom Merton/OJO Images/Getty Images

Everyone is talking about the resource: water. It’s integral to life and it’s a precious commodity worldwide. We know that with the increase in population, the toxins in our environment and the dwindling supply of fresh clean water, that we must reduce the amount of water we use daily.

So how do we do that?

The first step to reducing water use is to understand how much we are using in the first place – similar to tracking our monthly spending – you can’t reduce it until you measure it!

Sit down with a pen and some paper (or my preference, excel) and let’s get started making a spreadsheet.

We’ll need to know:

  • How many occupants use water daily?
  • What flow or flush fixtures do they use?
  • What are the amounts of water used for each flow or flush fixture?

Daily Occupants

To calculate the number of occupants in your building (or people in your home), write down how many people pee and poop every day. I love getting to say those words out loud! Depending on the type of building this calculation could be very easy or it could be very hard.

For example, let’s say five people live in your home. It’s fairly easy to say that five people occupy the home and use water for basic functions daily. Easy.

Now, let’s look at a commercial setting like an office or a school. What happens when 20 full time office workers and 10 part-timers get together? Or when there are 100 teachers and 1500 students, and then sometimes there are 10 part-time sub teachers, 5 administration staff, 6 custodial staff and an occasional parent teacher interview or sports game or play in the gymnasium? Hard.

Learn how to calculate a complex equivalent occupancy via LEEDuser or contact me directly.

Okay, so know that we know how many occupants use water in the space, let’s figure out what they are using.

Indoor Water Wasting Culprits

(Okay, okay, it’s we humans who are the true culprits when it comes to wasted water, but it’s easier to blame inanimate objects).

  • Flow fixtures are any type of fixture that allows water to flow from it, such as a faucet (either in a kitchen, bathroom or janitor sink) or a shower.
  • Flush fixtures are those fixtures that can be flushed. So, what comes to your mind is probably a toilet or a urinal.
  • Washing Machines.
  • Dishwashers.
  • Process Water. Process water is any water that is used for an institutional or mechanical purpose. For example, cooling towers, commercial laundries, car washes, etc would qualify, however, are excluded from the calculations for purposes of this article.

Create a Spreadsheet

Remember that spreadsheet that I mentioned earlier? Now would be the time to start a column listing each of the types of flow and flush fixtures in the building. The list might look like this:

  1. Type of Fixture
    Conventional Toilet
    Bathroom Sinks
    Janitor Sink
    Kitchen Sink

    Then, add a column for the number of each fixture.

    Now that we know how many of each type of fixture we have, let’s find the flow rate. There are a couple of ways you can do this. One method is to find out directly from the manufacturer – whether it be listed on the website, or on a specific product cut-sheet for the fixture. The other method is to go to the fixture itself and read it off the fixture. For toilets, look at the text adjacent to the toilet seat, between the hinge of the seat and the tank itself. For urinals, it can be found on the top of the urinal underneath the flush valve. For sinks, check out the metal piece where the water flows from and it should be inscribed there. In the shower, I advise you leave the water off, and look directly at the shower head. If for whatever reason you are unable to see the flow rate, then you will have to go directly to the product manufacturer.

    Add the flow rate to the table you have created:

  2. Flow Rates

  3. Conventional Toilets are measured in gal/flush (gpf)
    Urinals are measured in gal/flush (gpf)
    Bathroom Sinks are measured in gal/minute (gpm)
    Janitor Sinks are measured in gal/minute (gpm)
    Kitchen Sinks are measured in gal/minute (gpm)
    Showers are measured in gal/minute (gpm)

    Guess what? This is where our occupants come back into the picture. It turns out that men and women both use different amounts of water (for anyone that has teenage girls, I bet you can attest to this!). For calculation purposes, unless you truly know the occupant water behaviors (for example, if you are calculating your family household use) we will have to make some generally assumptions.

  4. Occupant Types
  5. Let’s assume that both sexes use the toilet three times per day, twice for pee and once for poop (yes, again, I love saying pee and poop!). That said, if a building has urinals then it’s true that men use less water because they use this fixture, so let’s add another two columns to the spreadsheet:


    Remember those occupancy calculations from earlier? Well let’s use a finer tooth comb and list the number of female and the number of male occupants.

    Now, plug in the number of occupants in each user category, et voila! You now know how much indoor water your building uses.

Check out the following table for a sample.

The next step? Retrofitting your indoor fixtures to minimize water use. You'll want to be sure that all of your fixtures are labeled by WaterSense.

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