A fundamental concept of model building codes, fire codes, and life safety codes is that a means of egress is designed to accommodate all occupants of a building. Knowing how to determine the total occupant load of a building is an integral part in determining if the building meets that basic concept. It can be difficult to estimate how many people are going to use a space within a building so most model codes that address egress design will provide requirements for how to estimate this number.
If you are working with NFPA 101, Life Safety Code, Table 22.214.171.124 provides occupant load factors for different uses found in a building. Occupant load factors are chosen based on how the space is used and not the occupancy classification of the space. For example, it isn’t uncommon for a business occupancy to have spaces that would fall under “business use”, as there will almost always be spaces used for non-business purposes also within the building. A conference room within the business occupancy wouldn’t be considered an assembly occupancy unless it was determined to have an occupant load of 50 or more people. For the purposes of determining the occupant load, that conference room has an assembly use. Once the occupant load factor has been determined based on the use of the space, it is then used to calculate the occupant load of that space. Calculating occupant load can be thought of in three steps:
- Select an occupant load factor
- Determine the size of the room
- Apply the occupant load factor to the space
There is a common misconception that the calculated occupant load is the maximum number of occupants the space can contain. Instead, the calculated occupant load is actually the minimum number of expected occupants. If the designer, building owner, or other involved party knows the expected number of occupants may be higher than the calculated number of occupants, then that number should be used as the occupant load. If, for example, the building owner knows there will be 5 people working in a storage room that has a calculated occupancy of 3 people, the design needs to be based off of the expected occupant load (5 people). Now, if the building owner says there will only be 1 person in the storage room that has a calculated occupancy of 3 people, the design needs to be based off of the calculated occupant load (3 people).
For a detailed step-by-step explanation of calculating occupant load and to learn about changes to some of the occupant load factors for the current edition (2018), download your free fact sheet!