When to use: Quick sizing of an isolated shallow (spread) footing using presumptive allowable bearing pressures from IBC Table 1806.2 — no geotechnical report required for preliminary work. The required bearing area is A = P / qallow, and for a square footing the side dimension is B = √A. Compaction targets are expressed as a percent of maximum Proctor dry density: structural fill is typically ≥95%, pavement subgrade 95–98%, and non-structural fill ~90%.
This calculator sizes a shallow spread footing using presumptive allowable bearing pressures from IBC Table 1806.2 and reports compaction requirements as a percentage of Proctor maximum dry density — the two soil engineering metrics most commonly specified on civil and structural grading plans.
The required bearing area for a shallow footing is A_req = P / q_allow, where P is the applied service load (pounds) and q_allow is the allowable bearing pressure (psf) from IBC Table 1806.2. For a square isolated spread footing, the minimum side dimension is B = sqrt(A_req). This sizing approach is permissible under IBC Section 1806.1 for preliminary design without a geotechnical report, using tabulated presumptive values.
Presumptive bearing values from IBC Table 1806.2 range from 1,500 psf for clays up to 12,000 psf for crystalline bedrock. These are conservative working-stress (ASD) allowable values with an implied factor of safety of approximately 3 against shear failure. For buildings over two stories or where unusual soil conditions exist, IBC 1803.2 requires a geotechnical investigation with site-specific bearing capacity analysis per ASCE 7.
Soil bearing capacity and footing design are governed by IBC Chapter 18 (Soils and Foundations) and ASCE 7 Chapter 18. Foundation design for structures subject to seismic loading must also comply with ASCE 7 Chapter 12 and 13 requirements for geotechnical hazard investigation. Compaction testing standards include ASTM D698 (Standard Proctor, 12,400 ft-lb/ft³) and ASTM D1557 (Modified Proctor, 56,000 ft-lb/ft³). Structural fill under building slabs and footings is typically specified to Modified Proctor, while roadway embankment may specify Standard Proctor.
Field compaction is verified by ASTM D6938 (nuclear density gauge) or ASTM D1556 (sand cone) test methods. OSHA 29 CFR 1926 Subpart P governs excavation and trenching safety, which depends on soil classification affecting allowable slopes and shoring requirements.
For sandy and gravelly soils, bearing capacity is primarily controlled by relative density and the allowable settlement. For fine-grained soils (clays and silts), both shear strength and settlement (especially consolidation of soft clays) govern. Expansive clays (CH and CL soils with high plasticity) require special attention: swelling pressures can exceed 5,000–15,000 psf, far exceeding typical structural loads and causing slab heave and foundation distress.
Compaction moisture conditioning is critical for expansive clays: compacting 2–3% wet of optimum (above optimum moisture content) reduces swell potential. Over-excavation (removing 12–24 in of native expansive clay and replacing with non-expansive engineered fill) is common in high-shrink/swell regions like Texas, Oklahoma, and Colorado.
Select the soil type from the IBC 1806.2 categories — if you have a geotechnical report, use the recommended bearing capacity from that report instead of the presumptive values. Enter the service column or footing load P (kips). Select the required compaction level from the drop-down based on your structural specifications: 95% Modified Proctor is standard for structural fill, 98% for pavement subgrade, 90% for non-structural embankment. The calculator returns required bearing area and minimum square footing dimension.
IBC Section 1803.2 requires a geotechnical investigation for: all structures in Seismic Design Category C or higher; all structures taller than three stories; buildings with more than 5,000 sf on ground floor; sites with known or suspected problematic soils (expansive clays, fill, karst, liquefiable sands); and any structure where settlement or lateral movement could affect performance. For most commercial projects a geotechnical report is standard practice regardless of IBC minimums.
Standard Proctor (ASTM D698) uses 12,400 ft-lb/ft³ of compaction energy, representing typical earthwork equipment for general embankment and fill. Modified Proctor (ASTM D1557) uses 56,000 ft-lb/ft³ — approximately 4.5 times more energy — producing a higher maximum dry density and lower optimum moisture content. Structural fill under buildings is typically specified to 95% of Modified Proctor because it better represents the long-term load-bearing performance needed.
Most residential lots in non-expansive soil regions have presumptive bearing values of 1,500–3,000 psf for sandy clays and sand. A typical one-story house with spread footings exerts 500–1,500 psf on the soil, well within presumptive limits. Problems arise with soft clays (< 1,000 psf bearing) or undocumented fill, where settlement rather than shear failure governs.
Liquefaction assessment requires a subsurface investigation with Standard Penetration Test (SPT) or Cone Penetration Test (CPT) data. ASCE 7 Section 11.8 requires liquefaction assessment for structures in Seismic Design Categories B through F when the site has liquefiable conditions (saturated loose sand with depth to groundwater < 50 ft). NCEER (1997) and Youd et al. (2001) procedures are the standard screening methods.
Structural slabs (supported slabs, warehouse floors) typically specify 95% Modified Proctor for the top 12–24 in of subgrade and the engineered granular base course. Non-structural slabs (sidewalks, patios) may accept 90% Standard Proctor. Post-construction settlement of poorly compacted fill under slabs is the most common cause of slab cracking and drainage problems on commercial sites.