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 if 0.5
- Allowable area / story 31,500 ft^2
- Pass Yes
Checked against published worked examples from: American Wood Council / International Code Council.
Details, formula, and sources
ScopeHow many square feet per floor the building may be, Aa = At + NS x If, where the frontage increase If from IBC Table 506.3.3 (up to 0.75) rewards perimeter on open space at least 20 ft wide and is 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.
Formula or tablefrontage_if from IBC Table 506.3.3 (values identical in 2021, 2024 and 2027): 0 below 25% of the perimeter or 20 ft of open space; otherwise the row for 25-50% / 50-75% / 75-100% of the perimeter (0.17-0.25 / 0.33-0.50 / 0.50-0.75) read from the printed 20 / 25 / 30+ ft open-space column under IBC 2027, or interpolated across those columns under IBC 2021 / 2024 (footnote a, deleted in 2027); allowable = tabular_area + ns_area x frontage_if; pass = actual_area <= allowable.
Source and editionIBC 2027 §506.2 (Aa = At + NS x If), §506.3.1 and Table 506.3.3 (frontage increase factor If; interpolation permitted in 2021 and 2024, not in 2027), by name; Table 506.2 supplies the tabular areas.
Free accessThe 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.
What governsEstimate. AHJ and licensed professional govern.
API field namestabular_area, ns_area, frontage_ft, perimeter_ft, open_width_ft, actual_area, frontage_if, allowable, pass
- Frontage increase If from Table 506.3.3 by perimeter bracket and open space, interpolated across the open-space columns under 2021 / 2024, the printed column under 2027; 0 below a quarter of the perimeter or 20 ftIBC 2027 Table 506.3.3 (footnote a in 2021 / 2024)
- Open-width cap open space 30 ft or more takes the last column of Table 506.3.3IBC 2027 Table 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 2027 Table 506.2