Allowable Building Area per Story (IBC Chapter 5)
The first feasibility number on a commercial project.
Example
You enter
- Tabular area At (ft², correct column) 27000
- Nonsprinklered area NS (ft²) 9000
- Open-frontage length F (ft) 200
- Total perimeter P (ft) 400
- Open-space width W (ft) 30
- Proposed story area (ft²) 25000
You get
- Frontage factor If 0.250
- Allowable area / story 29250 ft^2
- Pass Yes
Details, formula, and sources
How many square feet per floor the building may be, Aa = At + NS x If, where the frontage increase If = [F/P - 0.25] x W/30 rewards perimeter on open space and floors at zero below a quarter of the perimeter. The tabular areas come from Table 506.2 in the correct sprinkler column, and adding a sprinkler system can triple the area for a single-story building - the first lever a developer reaches for when the area is tight. A feasibility aid, not a code-official determination.
w_eff = min(open_width_ft, 30); frontage_if = (F/P) < 0.25 ? 0 : (F/P - 0.25) x (w_eff / 30); allowable = tabular_area + ns_area x frontage_if; pass = actual_area <= allowable.
IBC 2021 §506.2 (Aa = At + NS x If), §506.3.1, §506.3.2 (If = [F/P - 0.25] x W/30), and §506.3.3 (W capped at 30 ft), by name; Table 506.2 supplies the tabular areas.
The allowable-area relation is stated in the published IBC §506.2 / §506.3; the arithmetic is public. The tabular areas are the Table 506.2 values.
Estimate. AHJ and licensed professional govern.
Field names used by the API: tabular_area, ns_area, frontage_ft, perimeter_ft, open_width_ft, actual_area, frontage_if, allowable, pass
- Frontage increase If = [F/P - 0.25] x W/30, floored at 0 below a quarter of the perimeter on open spaceIBC 2021 §506.3
- Open-width cap W is capped at 30 ft in the equation absent the §506.3.3 exceptionIBC 2021 §506.3.3
- Tabular source At and NS come from Table 506.2 in the correct sprinkler column; a sprinkler system can triple the single-story areaIBC 2021 Table 506.2