how many square feet are in a door
1 Multiply the perimeter by the ceiling height to get the total wall area: 50 ft. x 8 ft. = 400 sq. ft. 2  Doors are usually 21 square feet (there are two in this exercise): 21 sq. ft. x 2 = 42 sq. ft.  Windows average 15 square feet (there are two in this exercise): 15 sq. ft. x 2 = 30 sq. ft.