How to Calculate Rainwater Collection


Rainwater collection is also known as rainwater harvesting. Rainwater collection involves the collection and storage of rainwater run off. Typically a collection barrel or similar object is positioned to catch water as it runs from rooftops and other structures. Many people use untreated rainwater to irrigate flower beds, landscaping and gardens. Other uses include car washing and water for flushing toilets. Collecting rainwater is also beneficial to the environment as it reduces the amount of water that may become contaminated from oil and pesticides and then flow into a stream, river or pond. Calculating the potential for harvesting rainwater can help with planning and use.

Things You'll Need

  • Pencil and paper
  • Tape measure
  • Calculator
  • Determine the amount of water that may be collected during a year. This will involve finding the cubic feet of water per year. Figure the square footage (A) of the collection area. This will be the area from which rainfall will be collected, such as a roof

  • Multiply the number (A) by the estimated or average inches of rainfall per year (B) for the location. The total will be referred to as (C). Refer to historical weather data and forecasts to figure the estimated rainfall per year.

  • Divide the multiplied number (C) by 12, which represents inches per foot, to determine the cubic feet of water per year which may be collected (D).

  • Convert the cubic feet of water collected per year (D) to gallons. Multiply the the total (D) by 7.43 to convert to gallons per year.

Tips & Warnings

  • Keep in mind that the calculations do not take evaporation or other leakage into consideration.

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