Soil Bearing Capacity Calculator: IBC 1806.2 Footing Sizing

How It Works

1. Select Soil Type

   Choose from IBC Table 1806.2: crystalline bedrock (12,000 PSF), sedimentary rock (4,000 PSF), sandy gravel (3,000 PSF), sand/silty sand (2,000 PSF), or clay/silty clay (1,500 PSF). These are presumptive values for footings at least 12 inches deep.

2. Enter Column Load

   Input total column load in pounds (dead + live). The calculator divides load by allowable bearing to find required footing area: A = P / q_allowable.

3. Check Bearing Pressure

   The calculator adds footing self-weight and soil overburden to the applied load, then compares actual bearing pressure to the allowable value. Safety factor is already built into IBC presumptive values.

4. Review Footing Dimensions

   See minimum square footing width or enter a rectangular width to get required length. Results include pass/marginal/fail verdict based on bearing pressure ratio.

Built For

• Residential contractors sizing spread footings for column loads on new construction
• Civil engineers doing preliminary foundation design before a geotechnical report is available
• Building inspectors verifying that footing sizes match the soil type at the job site
• Pole barn builders calculating pad footings for post-frame structures
• Deck builders sizing footings for deck posts based on tributary area loading
• DIY builders checking footing requirements for small outbuildings and sheds

Features & Capabilities

IBC Table 1806.2 Values

Built-in presumptive bearing capacities for 5 soil types per IBC. No need to look up the code table separately.

Footing Self-Weight Adjustment

Adds concrete footing weight and soil overburden to the applied load before checking bearing. Prevents undersizing from forgotten dead loads.

Pass/Marginal/Fail Verdict

Clear color-coded verdict based on bearing pressure ratio. Marginal flags footings within 10% of allowable capacity.

Square and Rectangular Options

Calculates minimum square footing width or lets you specify one dimension and solves for the other.

Water Table Adjustment

Accounts for reduced effective bearing when groundwater is near the footing level. Saturated soils lose roughly 50% of bearing capacity.

PDF Export

Export footing calculations as a branded PDF for permit applications or construction documents.

Assumptions

• Allowable bearing values from IBC Table 1806.2 presumptive capacities — already include a safety factor of approximately 3 against ultimate bearing failure
• Footing assumed to be at least 12 inches below undisturbed ground surface per IBC minimum depth requirement
• Soil type assumed to be uniform and consistent beneath the footing — no layered or variable conditions modeled
• Footing self-weight included in total load using concrete unit weight of 150 pcf
• Load assumed concentric on the footing — no moment or eccentric loading considered
• Water table assumed below the footing base unless user specifies high water table condition (50% reduction applied)

Limitations

• Presumptive values do not account for site-specific conditions — fill, organic material, expansive clay, or disturbed soil require geotechnical investigation
• Does not perform settlement analysis — bearing capacity may be adequate but settlement could exceed structural tolerances
• Does not evaluate lateral soil pressure on foundation walls or deep footings
• Does not check footing structural design (concrete thickness, rebar, punching shear) per ACI 318
• Eccentric loads, moment loads, and combined loading conditions require more advanced foundation analysis
• Does not address frost-protected shallow foundations or insulated foundation systems per IRC R403.3
• Adjacent footings and surcharge loads from neighboring structures are not considered

References

• IBC Table 1806.2 — Presumptive Load-Bearing Values of Foundation Materials
• IBC Section 1806 — Presumptive Load-Bearing Values (footing depth and soil classification requirements)
• ACI 318 Chapter 13 — Foundations (structural design of footings)
• ASCE 7 — Minimum Design Loads (load combinations for foundation design)
• Terzaghi, Peck, and Mesri — Soil Mechanics in Engineering Practice (bearing capacity theory)
• Coduto — Foundation Design: Principles and Practices (presumptive values and preliminary sizing methods)

Frequently Asked Questions