Calculate how much rainwater you can collect based on the square footage of the collection surface (the roof) and your annual rainfall.
Find the square footage of your roof by measuring the footprint of the roof, the slope of the roof doesn’t affect the catchment potential. For a square or rectangular house, measure the length and width and multiply the numbers together. If you have multiple roof sections do this for each one and add the results together.
After you’ve found the square footage multiply it by 0.56 to determine how many gallons you can collect per inch of rain. This calculation assumes a 90% efficiency. To find how much rain you can collect in an average rain year multiply this number by the average inches of rain.
For example, on a 2,000 square foot roof, you can collect 2,000 x 0.56 = 1,120 gallons/inch of rain. If your average rainfall was 25 inches/year, your annual collection potential is 1,120 x 25= 28,000 gallons/year.