Quick Answer
To calculate rain harvesting potential based on roof size, multiply the roof area by the average annual rainfall, then account for losses due to evaporation and overflow.
Calculating Roof Area and Rainfall
To start, you’ll need to determine the roof’s square footage. Measure the length and width of the roof, then multiply these numbers together. For example, a roof that’s 30 feet long and 40 feet wide would have an area of 1,200 square feet. Next, research the average annual rainfall in your area. According to the National Oceanic and Atmospheric Administration (NOAA), the average annual rainfall for the contiguous United States is around 30 inches. Convert this value to inches per square foot: 30 inches / 144 square feet per square inch equals approximately 0.2083 inches of rainfall per square foot per year.
Accounting for Losses and Calculating Harvestable Water
Once you have the roof area and average annual rainfall, you can calculate the total amount of water that could be harvested. However, you must also account for losses due to evaporation and overflow. A common estimate is to reduce the total by 20-30% to account for these losses. For our example, let’s assume a 25% reduction. Multiply the roof area by the rainfall per square foot: 1,200 square feet x 0.2083 inches = 250.8 inches of rainfall per year. Convert this value to gallons: 250.8 inches x 0.622 gallons per inch equals approximately 156.4 gallons. Then, apply the 25% reduction: 156.4 gallons x 0.75 = approximately 117.3 gallons of harvestable water per year.
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