There are some instances when gross and net square footage are calculated. Gross square footage generally refers to all of the square feet, whereas the net square footage accounts for some subtractions. An example for this could pertain to cost estimating, where gross square feet would include the interior floor space as well as interior and exterior walls. It would account for "all" of it, with the exception of open courts within a building, walls that extend outside of the footprint of the building/roof overhang at ground level, and so on.
With net square footage, there is typically an accounting of some loss. Again, considering cost estimating, net square footage accounts for floor space but does not account for exterior walls and other elements such as corridors, toilets, mechanical rooms.
With gross square footage and how it pertains to code... Well, this is where it gets a little tricky because it does not follow that typical definition and tends to trip up a lot of people in application.
For measuring gross square feet per the code-and, again, this pertains to this particular instance- the gross square footage does not include the exterior walls. The gross square footage regarding code is measured to the interior face of the exterior walls.
Don't believe it? Let's reference it.
The International Building Code (IBC) gives definitions to certain terminology that can be found in Chapter 2-the chapter designated for definitions. If a term is not found in this chapter, it is assumed that the definition of that term is consistent with practice and the industry and is well understood making further and/or specific definition unnecessary. The terminology for these definitions is italicized throughout the code indicating that there is a specific meaning for each italicized term. Per the definition in Chapter 2 of the IBC, the gross floor area is listed as "Floor Area, Gross". This definition states that the gross floor area, per code, is, "The floor area within the inside perimeter of the exterior walls of the building..."
This is tricky, but again, it's a matter of definition and can be especially tricky compared with all the ways to measure space. What is important to know is that this definition differs and that the code defines it in a way that it does not include the exterior walls. So, when determining occupancy, for example, the code allots a certain area for certain occupancies. Some are measured in net square feet and others in gross square feet. For the latter, that measurement would be the gross square feet; the entirety-excluding vent shafts and courts but including corridors, closets, ramps, stairways, and so on-to the interior perimeter of the exterior walls. It may not be consistent in the way that it is measured with other instances and with other tasks, but concerning code, gross square feet is to the interior of the exterior walls.