Rainwater can be used for all non-potable uses. These include: landscape irrigation, other outdoor uses, toilets and the clothes washing machine. The EPA estimates that on average this totals to about 64% of daily water usage. They also estimate that the average home uses 400 gallons per day (gpd). Given that, the non-potable portion equates to 256 gpd! These averages are helpful in thinking about how much rainwater you would need to capture to meet your demand but it is best to calculate your actual demand and the total possible supply to ensure balance in your rainwater capture system.
Calculating the supply potential for your rainwater system is easy. To illustrate, I have selected a very simple
rectangular roof that measures 13’6” x 16’6” for a total square footage of 222 sf (rounded down). The roof slopes gently to the north and is surfaced with asphalt shingles. There is a great location on the ground to place rain barrels.
Step One: Calculate the roof surface area in square feet. In our example, 13.5 x 16.5 = 222.8sf simplified for this example to 222sf.
Step Two: Gather calculation assumptions:
- inches of rain x 0.62 = US gallons of water captured per inch
- an efficiency factor of 88%. There will be rain that never gets to your catchment vessel so best not to assume 100%.
- Look up the average rainfall for your area. According to the Weather Channel dataset, the average annual precipitation for my area is 48” spread evenly throughout the year.
Step Three: Calculate the potential supply by multiplying the roof surface area sf by your area’s average inches of rain then by 0.62 (the inches to gallons conversion factor) and then by 88% (the efficiency factor). For our example we would calculate:
222sf x 48” precipitation x 0.62 x .88 = 5,814 gallons of rainwater annually
The rainfall in my geographic area is very evenly distributed throughout the year so by dividing by 12 months, I can 
expect about 484 gallons per month from this one roof surface. That equates to 9-50gallon rainbarrels or more sensibly a 500 gallon cistern. Imagine if the rainwater was captured from all of the roof surfaces.
Currently I use about 100 gallons of rainwater a month in the garden in the months of May through September for a total demand of about 500 gallons. That leaves a potential 5,314 gallons for other non-potable uses.
On average, my family uses 40,000 gallons of potable water a year or 109 gpd. Using the EPA averages, 19% or 7,600 gallons of this are used to flush toilets and 16% or 6,400 gallons are used to wash clothes. That means that the rainwater captured from this one roof could almost meet the current demand of one of these non-potable uses. Installing a more water aware washing machine might reduce consumption significantly, maybe as much as 60% which would more than match the water supply from the cistern. That is very compelling.
Next we will explore the steps to follow to start washing clothes with rainwater.
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