The International Building Code (IBC) determines the occupant load of a space within a building. This figure is used for various other requirements found in the code.
Occupant load factors are found in table 1004.5 of the 2021 IBC, shown below. Certain functions of space, like warehouses have a very high square footage per occupant (500 sf). Other functions, like Assembly- standing space, have a very low square footage per occupant (5 sf).
In other words, these figures are stating that for every 5 sf of area, account for 1 person (assembly -standing space)
It is important to note that the “functions of space” listed in the table below is different then the building occupancy. The names are sometimes similar or the same but they are two separate concepts. Sometimes they coincide, for example, an assembly function within an assembly occupancy. But, sometimes they are different, for example, an assembly function within a business occupancy.
Gross SF includes the thicknesses of walls, hallways, closets- any and all space. Whereas net SF does not count these unoccupied areas.
Different parts of rooms may have different occupant load factors, not necessarily split up by walls. For instance, a restaurant might have a seating area, and a merchandise area next to it, with a different occupant load. On a code floor plan, you might just draw a dashed “imaginary line” to delineate these two areas during plan review.
Below is an example of an occupant load calculation for a simple, open 45’x20′ room, used as assembly- un-concentrated tables and chairs. This has an occupant load factor of 15 net sf per person.
The occupant load is the square footage: 45×20 = 900 sf, divided by 15 sf per person = 60 people.
This room only has 1 exit, and per the IBC, assembly areas exceeding 49 occupants must have 2 exits. So, it is technically not compliant as shown. So, what can we do? We can add a 2nd exit, or we can adjust the layout within the room, as the example following shows:
This second example shown above now adds a bar. The front area, including the bar, has an different occupant load factor.
The bar is 200 sf/ 100 sf per person = 2 people at the bar area
The tables and chairs area is 700 sf/ 15 sf per person = 47 people
Total = 49 occupants
49 occupants is permissible to have 1 exit and is compliant.
In summary, occupant loads are a general way for the code to calculate how many occupants are within a space. This then determines other things regarding egress, door sizing, sprinklers, etc.
The occupant loads can be somewhat subjective as the determination of what function is used in spaces. It is ultimately up to the authority having jurisdiction, like your city building department to agree with the occupant load analysis.
