| Function of space | Factor (sq ft/occ.) | Basis |
|---|
| Requirement | Minimum | Note |
|---|---|---|
| Net clear opening area | 5.7 sq ft | 5.0 sq ft at grade-floor / below-grade openings |
| Clear opening height | 24 in | Actual free height when open |
| Clear opening width | 20 in | Actual free width when open |
| Sill height above floor | 44 in max | Finished floor to bottom of clear opening |
| Window well (below grade) | 9 sq ft | 36 in min horizontal projection & width |
Divide the floor area by the occupant load factor for that use, then round up. IBC Table 1004.5 sets the factors: business areas are 150 sq ft gross per occupant, unconcentrated assembly (tables and chairs) is 15 net, mercantile is 60 gross. A 3,000 sq ft office is 3,000 ÷ 150 = 20 occupants. Gross factors use the whole area; net factors use only the actual occupied space.
Under the IBC, one exit or exit access doorway is permitted where the occupant load is 49 or fewer (subject to occupancy and travel-distance limits), two are required from 50 to 500 occupants, three from 501 to 1,000, and four above 1,000. Assembly and hazardous occupancies have tighter single-exit limits, so confirm the specific requirement for your use group.
IRC R310 requires every sleeping room and basement with habitable space to have an emergency escape and rescue opening with at least 5.7 sq ft of clear opening (5.0 sq ft at grade-floor or below-grade openings), a minimum clear height of 24 inches, a minimum clear width of 20 inches, and a sill no higher than 44 inches above the floor. Meeting the minimum height and width alone is not enough: 24 × 20 inches is only 3.3 sq ft, so the opening must be larger to reach 5.7.
Egress width is the occupant load times a per-occupant factor: 0.3 inch for stairways and 0.2 inch for doors, ramps and level components without sprinklers, reduced to 0.2 and 0.15 inch in a fully sprinklered building with an alarm. Corridors are at least 44 inches wide (36 inches where they serve fewer than 50 occupants), and each exit door must give at least 32 inches of clear width.
A gross factor is applied to the entire floor area within the exterior or tenant walls, including corridors, closets and mechanical space, used for offices, mercantile and storage. A net factor is applied only to the actual occupied area of the specific function, excluding accessory spaces, used for assembly seating, classrooms and kitchens. The table tells you which basis each use takes.