Three Step Rainwater Capture Calculation

Calculating the supply potential for your rainwater system is easy.

Step One: Pick a roof surface and calculate the roof surface area in square feet.

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%.
  • average rainfall for your area.  One good place to find this is the Weather Channel average data.

Step Three: Calculate the potential amount 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:

Example 1: 100sf x 1” precipitation x 0.62 x .88 = 54.6 gallons of rainwater per 1″ rain event

Example 2: 100sf x 48″ annual precipitation x .62 x.88 = 2,619 gallons of rainwater per year


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