According to the EPA, the average number of windows per house is 15% of the floor area (see Table 3: Calculation of the Average Number of Windows per House). To calculate the total number of windows for each house, divide the total window area by the area of a single window (15 sq. ft. or 3 ft. by 5 ft.). Then multiply that number by the average number of windows per house. The average window size is 30 sq. Ft. Multiply the total number of windows by 30 to get an estimate of the total square footage of windows in houses. It should be noted that this calculation assumes that all the windows are equal size. In reality, some windows will be larger than others and therefore consume more energy than others. However, this calculation provides a good starting point for calculating how much energy your house uses. Table 3: Calculation of Window Area and Number of Windows Per House Floor Area in Square Feet Number of Windows* Window Area Square Footage 300 15 sq.ft. 120,000 240 20 sq.ft. 60,000 480 30 sq.ft. 90,000 720 40 sq.ft. 45,000 960 50 sq.ft. 33,000 1120 60 sq.ft. 27,500 1440 70 sq.ft. 22,200 1680 80 sq.ft. 19,440 1920 90 sq.ft. 17,280 2160 100 sq.ft.