What Is Occupant Load in Building Code?

Divide the area of each space by the factor for how it is used, then round up. A 3,000 square foot business area at 150 gross square feet per occupant gives 20 occupants. A conference room inside that suite may be calculated as assembly, with a much smaller factor and a much higher load.

The factor follows the function of the space, not the occupancy classification of the whole building. An office building can contain assembly, storage, and kitchen areas, and each gets its own factor.

Gross floor area is measured to the inside of the exterior walls and includes corridors, stairs, toilet rooms, and other support space. Net floor area is the occupied area only, excluding those spaces.

Using gross area where the code asks for net overstates the load, and using net where it asks for gross understates it. Check which basis the adopted edition applies to each use before the numbers go on the code plan.

The required width of stairs, doors, and corridors is the occupant load served multiplied by a width factor per occupant, and stairs take a larger factor than other components. The number of exits steps up as the load on a story grows: under the IBC, more than 500 occupants requires at least three exits and more than 1,000 requires at least four.

Loads accumulate. A stair that serves three floors and a door that serves a whole suite have to be sized for everyone who reaches them, not only the room next to them.

Turning storage into offices, or an office into a training room, can multiply the occupant load without moving a wall. The egress, the plumbing fixture count, and sometimes the occupancy classification have to be checked again.

This is a common reason a tenant improvement fails plan check. The new use is drawn, but the code plan still carries the old load.

The IBC requires assembly rooms with an occupant load of 50 or more to post that number in a conspicuous place near the main exit or exit access doorway. The posted number is what the fire code official enforces once the building is in use.

The designer can design for a load above the calculated number, but the egress then has to be sized for the higher load, and the building official can limit it.