DIY / For Students / Water

Calculate Rainwater Harvesting Potential

You'd be surprised how much water falls on your roof! Calculate your space's rainwater harvesting potential.

Calculate Rainwater Harvesting Potential
  • Tape measure or string
  • Pen
  • Paper (or calculator) for calculations
  • Find the average rainfall in your area Link
  • Rainwater Harvesting: For Drylands and Beyond - Brad Lancaster Link

Harvesting the rain with a barrel is a simple way to recycle water. Before you get started collecting it, calculate your catchment to determine just how much rain falls above your head!

By harvesting the rain, we can collect every drop and grow healthy gardens instead of letting it run off to pollute our watersheds and ocean. Here in Southern California, we get about 15 inches of rain/year. It might not be much, but it adds up quickly. In fact, the average Southern California rooftop could collect almost 1,000 gallons of water with just 1″ of rainfall! 

Follow the instructions below to calculate your catchment, build and install a rain barrel of appropriate size, and do a rain dance!

Ready for a challenge?

Calculate how much rain the houses on your entire street could collect (assuming their catchment area is the same). Install a rain barrel at your home, and then inspire your neighbors to do the same!

  • Using your tape measure or string, measure the width and length of your home, classroom, office, chicken coop etc. Any structure with a roof can harvest the rain!
  • Calculate the square footage (width x length [floor in feet] = catchment area [square feet])
  • Awesome! Let's assume the square footage is equal to it's roof. Now you can estimate how much rainwater you could collect.
  • Harvested Water (Gal) = Catchment Area (sqft) X Rainfall Depth (inch) X Conversion Factor (0.623)
  • For example, let's pretend your square footage is 3,000 sq. ft. (the size of the average Southern California home), and it rains 1 inch.
  • 3000 x 1 x 0.623 = 1869
  • That's 1,869 gallons of water that we could potentially divert from the drain and into our garden!