When calculating the square footage of a home, do you consider only the net square footage or is it the gross square footage?

In other words, if the footprint is say. . . 25' by 80', that would be 2,000 gross square feet.

If you subtract out all the wall cavities, all the cabinets, and all the closets, which total say. . . 150 square feet, then you have a total of 1,850 net square feet.

Which square footage do you use?