The gross internal area of a home or building takes into account the area occupied by the internal walls, partitions and columns. For example, when taking internal measurements, normally you would measure the length and width of each room as measured from wall to wall, without taking into account the space occupied by walls. Gross internal area takes this space into account. However, it excludes the outer wall thickness, since the outer wall is not considered internal area.
Locate the blueprints of the home, building or facility. Find the width of the internal wall space for each internal wall and multiply that width by the height of the wall to get the area of the internal wall space. Do this for each wall.
Calculate the area of each room by multiplying the length by the width. You can get the length and width from the blueprints or you can easily measure it from wall to wall.
Calculate the total internal area by adding the areas of the rooms and the areas of the internal wall space. The sum total is your gross internal area.