Texas rainfall patterns historically tend to be inconsistent. Quite often wet years are following by dry ones. Rainwater collection systems need to be designed to capture sufficient amounts of water to meet a family's needs during the dry years. In conservative households the average gallons per day individual falls between 25 and 40 gallons. Utilizing this information enables one to calculate the size of roof area and tank capacities needed.The net return of one inch of rain on one square foot of roof equals .6233 gallons. Consequently, a one inch of rain on 1000 square feet yields 623 gallons. During the calculation phase, one must remember that the roof area can only be calculated by the roof footprint (square footage of the area under roof). Square footage calculated due to roof pitch will not increase the amount of return.