Gross area is the total area within the walls of a building structure, including the walls themselves and unlivable space. When you see buildings listed as having a certain square footage, this number refers to net area, which is the gross area minus the structural area. Thus, gross area includes the outer walls, interior walls, internal ducts and other internal spaces that would not generally be considered "living areas." Calculating gross area is a fairly simple process.
View the blueprints or plans of the building for which you wish to calculate the gross area.
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Cordon the drawing off into geometric sections by cross-sectioning the blueprint into squares and rectangles.
Calculate the area of each individual geometric shape based upon the outer wall blueprint measurements. That is, look for the measurements of each outer wall on the blueprint. Calculate the area of each square or rectangle by multiplying the length by height. Add up the area of each square or rectangle to obtain the total gross area. This process must be repeated for each floor of the building.