Commercial office floor area calculation methods.
GROSS FLOOR AREA
The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vent shafts and courts, without deduction for corridors, stairways, closets, the thickness of interior walls, columns or other features. The floor area of a building, or portion thereof, not provided with surrounding exterior walls shall be the usable area under the horizontal projection of the roof or floor above. The gross floor area shall not include shafts with no openings or interior courts.
NET FLOOR AREA
The actual occupied area not including unoccupied accessory areas such as corridors, stairways, toilet rooms, mechanical rooms and closets.
Floor Area definitions vary depending on the defining agency. Building owners or AHJ (authorities having jurisdiction) usually adopt the definition by one of these agencies:
International Code Council (ICC)
Commercial Real Estate Development Association (NAIOP)
Building Owners and Managers Association (BOMA)