Soil Bearing Capacity Calculator (Terzaghi Method)
Calculate ultimate and allowable soil bearing capacity for a shallow footing using Terzaghi's bearing capacity equation, soil cohesion, internal friction angle, and unit weight. Use the result alongside the foundation depth calculator and foundation settlement calculator to check a footing design against both strength and settlement limits.
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Set Soil Parameters
Pick a soil preset or enter cohesion, friction angle, and unit weight from a boring log.
Define Footing Geometry
Enter width, length if applicable, and embedment depth below finished grade.
Apply Terzaghi's Equation
The tool applies shape factors and bearing capacity factors Nc, Nq, Nγ for your inputs.
Compare Against Allowable
Ultimate capacity is divided by your factor of safety to produce an allowable bearing pressure.
Typical Bearing Capacity Factors by Friction Angle
Terzaghi's bearing capacity factors Nc, Nq, and Nγ depend only on the soil's internal friction angle φ. Use this lookup table to sanity-check the factors the calculator applies to your inputs.
| Friction Angle φ (deg) | Nc | Nq | Nγ |
|---|---|---|---|
| 0 (pure clay) | 5.7 | 1.0 | 0.0 |
| 10 | 9.6 | 2.7 | 1.2 |
| 20 | 17.7 | 7.4 | 5.0 |
| 28 | 31.6 | 17.7 | 15.7 |
| 32 | 44.0 | 28.5 | 26.9 |
| 36 | 63.5 | 48.9 | 50.5 |
| 40 | 95.7 | 81.3 | 100.4 |
Source: Terzaghi (1943), Theoretical Soil Mechanics, as tabulated in NAVFAC DM-7.01 Chapter 4, Figure 4.
Why Bearing Capacity Governs Footing Size
Every shallow footing transfers building load into soil as a bearing pressure measured in psf. If that pressure exceeds the soil's ultimate bearing capacity, the soil shears and the footing punches downward or rotates. Terzaghi's 1943 bearing capacity equation remains the most widely referenced method for estimating this failure threshold for strip, square, rectangular, and circular footings.
The equation combines three failure mechanisms into one formula: cohesion resistance (Nc term), surcharge resistance from soil above the footing (Nq term), and friction resistance from soil unit weight below the footing (Nγ term). NAVFAC DM-7.01 Chapter 4 and Bowles' Foundation Analysis and Design, 5th edition, both use this same three-term structure with shape correction factors layered on top.
Ultimate bearing capacity is a theoretical failure point, not a design value. Every geotechnical report divides it by a factor of safety, typically 3.0 per NAVFAC DM-7.01, to reach an allowable bearing pressure the footing must not exceed under service loads.
Example Scenario: Sizing a Column Footing on Stiff Clay
Project Inputs
Footing shape: Square, 5 ft x 5 ft
Embedment depth: 3 ft
Soil: Stiff clay, c = 2,000 psf, φ = 0°
Unit weight: 120 pcf
Factor of safety: 3.0
A column carrying 100 kips on this footing produces a bearing pressure of 4,000 psf (100,000 lb divided by 25 sq ft), which falls under the 5,180 psf allowable limit, indicating the footing size is adequate for bearing capacity, subject to a full settlement check.
Mistakes That Skew Bearing Capacity Results
Using Total Load Instead of Bearing Pressure
Bearing capacity is a pressure (psf), not a load (lb). Divide total footing load by footing area before comparing it to the allowable bearing capacity result.
Ignoring the Groundwater Table
A shallow water table reduces the effective unit weight of soil below the footing, which lowers the Nγ term. Skipping this correction overstates capacity in sites with high groundwater, per NAVFAC DM-7.01 Section 4.3.
Applying General Shear Factors to Loose Soil
Loose sand and soft clay fail in local shear, not general shear. Terzaghi (1943) recommends reducing cohesion to two-thirds and tan(φ) to two-thirds for local shear cases; using general shear factors on loose soil overestimates capacity.
Confusing Ultimate and Allowable Capacity
Ultimate bearing capacity is a failure threshold with zero safety margin. Design footings using the allowable value only, after dividing by an appropriate factor of safety.
Using Assumed Soil Values Without Verification
Preset soil values are planning-level ranges. Permitted construction requires cohesion, friction angle, and unit weight from an actual geotechnical boring log, not visual soil classification.
Where This Estimate Fits in a Foundation Design
IBC 2024 Table 1806.2 lists presumptive load-bearing values by soil class for cases where a full geotechnical investigation is not required by the local building official, ranging from 1,500 psf for clay to 12,000 psf for sedimentary rock. Most jurisdictions still require site-specific testing for anything beyond single-story residential construction on undisturbed soil.
A bearing capacity check answers one question: will the soil shear under this pressure? It says nothing about how much the footing will settle over time. Pair this tool with the foundation settlement calculator for a settlement check, and the concrete bearing pressure calculator to verify the pressure your specific footing dimensions actually generate.
Practical Tip
For DIY sheds, decks, and small residential footings, compare your result against the foundation depth calculator to confirm your footing sits below the local frost line in addition to meeting bearing capacity.
Bearing Capacity Questions Answered
Soil bearing capacity is the maximum pressure a soil layer can support without shear failure or excessive settlement. Terzaghi's bearing capacity theory (1943) separates ultimate bearing capacity, the theoretical failure load, from allowable bearing capacity, which applies a factor of safety per NAVFAC DM-7.01.
NAVFAC DM-7.01 and standard geotechnical practice commonly use a factor of safety of 3.0 for shallow foundations under static loads. A value of 2.5 is sometimes accepted with well-established soil data, while 4.0 or higher applies when soil investigation data is limited.
Terzaghi's equation applies shape correction factors. A square footing uses 1.3 times the cohesion term and 0.8 times the unit weight term relative to a strip footing of the same width, per Terzaghi (1943) and Bowles Foundation Analysis and Design, 5th edition.
General shear failure occurs in dense sand and stiff clay with a well-defined failure surface reaching the ground surface. Local shear failure occurs in loose sand and soft clay where the surface does not fully develop. Terzaghi (1943) recommends reducing cohesion to two-thirds and tan(φ) to two-thirds when local shear governs.
No. This tool applies Terzaghi's simplified formula for planning-level estimates only. IBC 2024 Table 1806.2 lists presumptive load-bearing values that require verification, and any permitted foundation must be designed from actual boring log data reviewed by a licensed engineer.
Deeper footings gain a surcharge term (q times Nq) from the weight of soil above the footing base, which confines the soil and resists shear failure. Terzaghi's equation includes embedment depth as a direct multiplier in the surcharge component.
IBC 2024 Table 1806.2 lists presumptive bearing values ranging from 1,500 psf for clay to 12,000 psf for sedimentary rock, usable only when a geotechnical investigation is not required by the local building official and site conditions match the table's soil class description.
Sources and Calculation Method
- Terzaghi, K. (1943). Theoretical Soil Mechanics. John Wiley & Sons. Origin of the bearing capacity equation and shape factors used in this calculator.
- Naval Facilities Engineering Command, NAVFAC DM-7.01, Soil Mechanics, Chapter 4, bearing capacity factors and factor of safety guidance.
- Bowles, J. E. Foundation Analysis and Design, 5th Edition. Shape correction factors and worked examples for Terzaghi's method.
- International Code Council, IBC 2024, Table 1806.2, presumptive load-bearing values, and Section 1604, professional design requirements.
- American Concrete Institute, ACI 336.3R-93, Design and Construction of Drilled Piers, referenced for soil-structure interaction context on deep foundation comparisons.
Engineering Disclaimer
This calculator provides estimates for planning purposes. For permitted structural work, foundations, multi-story construction, retaining walls over 4 feet, and commercial projects, calculations must be verified by a licensed structural engineer per IBC 2024 §1604. ConcreteCalculate.com is not liable for structural decisions made from these estimates.
Privacy note: all calculations run in your browser and on our server only for the duration of the request. No project inputs, soil data, or results are stored or transmitted to third parties.
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