Commercial Water Usage Calculator - How to Do It Yourself

How to Calculate Indoor Water Use in Places of Business

Desk with coffee, calculator, notepad and water
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Water is 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 monthly spending, you simply can’t reduce it until you measure it!

Water Usage Calculator: Commercial or Residential

Sit down with a pen and some paper (or my preference, use a spreadsheet program like 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?

To calculate the number of daily occupants in your building (or people in your home), write down how many people pee and poop every day. Depending on the type of building this calculation could be very easy or it could be very hard.

For example, let’s say 5 people live in your home. It’s fairly easy to say that 5 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, five administration staff, six custodial staff and an occasional parent-teacher interview or sports game or play in the gymnasium?


Learn how to calculate a complex equivalent occupancy via LEEDuser.

Once you know that we know how many occupants use water in the space, let’s figure out how much water is used.

Wasteful Culprits

Okay, we know it’s humans who are the true culprits when it comes to wasted water, but it’s easier to blame inanimate objects, so here are some common water-wasting culprits you should be aware of:

  • Flow fixtures. These are any type of fixtures that allow water to flow to and 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 such as 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, they are excluded from the calculations for purposes of this article.

Create a Spreadsheet — Manually

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:

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.

Flow Rates

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)

Occupant Types

Guess what? This is where our occupants come back into the picture. It turns out that sometimes men and women use different amounts of water. For calculation purposes, unless you truly know the occupant water behaviors (for example, if you are calculating your family household use), you will have to make some general assumptions.

Let’s assume that both sexes use the toilet three times per day, twice for number one and once for number two. 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. This now tells you how much indoor water your building uses.

Try an Online Calculator 

If you prefer to use an online calculator, you can try the Kohler Commercial Water Usage Calculator or the BEF's business water usage calculator.

It's important to keep in mind that these calculators are intended to measure approximate water usage for homes or relatively simple businesses and organizations.

The next step? Retrofit your indoor fixtures to minimize water use.