Calculating the occupant load of a room or building requires first calculating the area of the space in question in square feet, and then dividing the area by the occupant load factor specified by the International Building Code. The resulting number from this calculation determines how many occupants can legally use the space at one time.
The occupant load factor is used to specify the minimum amount of space needed per person, and it varies according to what the space is being used for. For instance, classrooms only require 20 net square feet per occupant, whereas offices require at least 100 gross square feet per occupant, and residences need at least 200 gross. The most recent occupant load factors can be found in Table 1004.1.2 of the 2015 IBC.
It's important to consider that some occupant load factors are determined in gross square feet per occupant, but others are determined in net square feet. The gross area of a space is defined as everything within its walls, including occupiable spaces and non-occupiable spaces, such as closets, bathrooms, hallways and stairs. Determine net area by measuring all of the non-occupiable spaces, and then subtracting this area from the gross area.