- Goals - gutters, first flush filter, etc.
Harvested water (gallons) = catchment area (square feet) [roof footprint] x rainfall depth (inches) x 0.623 [conversion factor]
[one gallon equals 231 cubic inches; one square foot equals 144 square inches]
Harvested water (liters) = catchment area (square meters) [roof footprint] x rainfall depth (millimeters)
[one liter equals 1000 cubic centimeters; one square meter equals 10000 square centimeters; one centimeter equals 10 millimeters]
The above formula can be used to calculate the volume size required to store water from a known size of roof, or to calculate the roof size required to supply a known demand of water usage. Required volume size of water storage maybe should be equal to harvested water during peak rainfall minus average water usage, per week or month.
In Missouri, the average family's indoor water usage is about 50 gallons per person per day. Flushing the toilet accounts for about 42 percent of the total, the highest percent of indoor water usage. (There are several solutions to mitigate or eliminate freshwater usage for the flushing of toilets.)
Nationally, outdoor water use accounts for 30 percent of household use, yet that can be much higher in drier parts of the country and in more water-intensive landscapes. (There are several solutions to mitigate or eliminate freshwater usage for residential landscape irrigation.)