Occupant Load Is the Starting Point for Every Egress Design Decision
The number of people that a building is designed to accommodate - the occupant load - is not just a planning statistic. It is the basis for calculating required egress width, the required number of exits, plumbing fixture counts, and, in assembly occupancies, posted occupancy limits. The IBC provides occupant load factors (in square feet per person) for each use type in IBC Table 1004.5; dividing the net or gross floor area of a space by the occupant load factor gives the design occupant load for that space. The ARE PA exam tests occupant load calculation because it is the foundational calculation from which most life safety code requirements flow, and every practicing architect must be able to perform it correctly.
IBC Table 1004.5 Occupant Load Factors
IBC Table 1004.5 provides occupant load factors for common use types. Selected values:
| Use Type | Occupant Load Factor (sq ft/person) | Measured By |
|---|---|---|
| Assembly - fixed seating | Actual number of seats | Actual |
| Assembly - unconcentrated (tables) | 15 | Net |
| Assembly - concentrated (chairs) | 7 | Net |
| Assembly - standing | 5 | Net |
| Business (office) | 150 | Gross |
| Educational (classroom) | 20 | Net |
| Mercantile (retail) | 60 | Gross |
| Storage | 300 | Gross |
| Industrial (general) | 100 | Gross |
Net vs. Gross Area
Some occupant load factors are applied to net area (the actual floor area of the space, excluding walls, columns, and fixed elements) and others to gross area (total floor area including walls and structural elements). Net-based calculations are used where actual usable floor area is more relevant (classrooms, restaurant dining rooms); gross-based calculations are used where a quick aggregate calculation of a large area is more appropriate (office floors, retail buildings). Knowing which factor type applies to which use is important for accurate calculation.
Using Occupant Load for Egress Design
After calculating the occupant load, the architect uses it to determine: required number of exits (IBC Section 1006 specifies minimum exits based on occupant load - one exit permitted for OL ≤ 49 in most occupancies; two exits required for OL 50-499; three exits for OL 500-999; four exits for OL 1,000+); required egress width (IBC Section 1005 specifies capacity factors - inches of width per occupant - for horizontal exits, stairways, and corridors); and plumbing fixture counts (IBC Table 2902.1 uses occupant load to determine minimum fixture counts).
Key Exam Points
- Occupant load: floor area / occupant load factor (IBC Table 1004.5).
- Factor variation by use: office 150 gross; classroom 20 net; restaurant tables 15 net; restaurant standing 5 net.
- Net vs. gross: net excludes walls and fixed elements; gross includes everything.
- Number of exits: based on occupant load - 1 exit for OL ≤ 49; 2 exits for OL 50–499.
- Egress width: occupant load × capacity factor (IBC Table 1005.1) = minimum clear width.
AREprep's PA code content covers occupant load calculation, egress design requirements, exit capacity, and the complete means of egress system - giving ARE PA candidates the IBC knowledge they need to perform code calculations accurately and to answer the code compliance questions and building planning scenarios the PA exam tests across its regulatory and planning content areas.
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