Architecture
How do I calculate occupant load?
Short answer
Divide the floor area of each space by the occupant load factor for its use in IBC 2021 Table 1004.5, which lists square feet per person as either gross or net area. For example, business areas use 150 gross, so 4,200 square feet gives 28 occupants. Spaces with fixed seating are counted by seats. Always confirm the factors in the code your jurisdiction has adopted.
Occupant load is the number of people a space must be designed to hold. It is not a prediction of how many people will show up. Building codes use it to size exits, corridors and stairs, and to decide how many exits a space needs. In the International Building Code (IBC), the calculation for spaces without fixed seating is in Section 1004.5.
The calculation
For each space, use this formula:
Occupant load = floor area / occupant load factor
The factor comes from IBC 2021 Table 1004.5, in square feet per occupant, and is marked gross or net:
- Gross floor area is measured within the inside perimeter of the exterior walls, without deducting corridors, stairs, closets, interior wall thickness or columns.
- Net floor area is the actual occupied area, leaving out unoccupied accessory areas such as corridors, stairs, toilet rooms, mechanical rooms and closets.
Selected factors
These are common entries from IBC 2021 Table 1004.5. The table has many more lines.
| Use | Factor (sq ft per person) |
|---|---|
| Assembly, standing space | 5 net |
| Assembly, concentrated (chairs only, not fixed) | 7 net |
| Assembly, unconcentrated (tables and chairs) | 15 net |
| Classroom area | 20 net |
| Exercise rooms | 50 gross |
| Mercantile (retail sales) | 60 gross |
| Industrial areas | 100 gross |
| Business areas | 150 gross |
| Commercial kitchens, residential | 200 gross |
| Accessory storage, mechanical rooms | 300 gross |
| Warehouses | 500 gross |
Some uses refer elsewhere instead of giving a factor. Assembly spaces with fixed seating are counted from the actual number of seats (Section 1004.6), and concentrated business use areas such as call centers have their own rule in Section 1004.8, with a minimum of one occupant per 50 square feet of gross area.
Worked example
A tenant suite has three non-overlapping spaces:
| Space | Area | Factor | Occupant load |
|---|---|---|---|
| Open office | 4,200 sq ft gross | 150 gross | 28 |
| Conference room, 20 ft by 30 ft | 600 sq ft net | 15 net | 40 |
| Storage room | 300 sq ft gross | 300 gross | 1 |
The suite totals 69 occupants. Notice that the conference room has a higher load than the office floor around it, even though it is much smaller. In a spreadsheet, each row is =ROUNDUP(area / factor, 0), with the factor pulled from a table of uses by a lookup such as =XLOOKUP(B2, UseList, FactorList).
Points to check
- Adopted code and edition. Jurisdictions adopt and amend the IBC on their own schedules, so the factors that apply to your project may differ from IBC 2021. Confirm with your building official.
- Rounding. The table gives a minimum number, not a rounding rule. Many designers round fractions up, and some jurisdictions say so explicitly.
- Unlisted uses. If a function is not in the table, the building official determines the closest similar use.
- Actual numbers. With the official's approval, the actual number of people a space is designed for can be used even when it is lower than the calculated load. A higher load than the table's is permitted only if the other code requirements are met for it, up to a maximum of one person per 7 square feet.
- Egress. The result drives the number of exits (IBC Section 1006) and the required egress width (Section 1005).
This answer is a calculation guide, not a code review. The adopted code and your building official decide what applies.
The occupant load and egress width calculator sheet covers this calculation and the egress width that follows from it. The blog post on room and area schedules for architects helps with tabulating the areas the calculation starts from.