Soil Bearing Capacity Calculator
Every Foundation Rests on a Single Question: How Much Can This Soil Hold?
Before a single footing is poured, geotechnical engineers need an answer to a deceptively simple question: at what pressure does the soil beneath a foundation fail? Karl Terzaghi's bearing capacity equation, published in the 1940s, is still the conceptual foundation (so to speak) of shallow-foundation design, combining soil cohesion, overburden pressure, and unit weight into a single ultimate bearing pressure.
The Formula
Where c is soil cohesion, q is surcharge (overburden) pressure, γ is soil unit weight, B is footing width, and Nc, Nq, Nγ are dimensionless bearing capacity factors that depend entirely on the soil's internal friction angle φ. This calculator interpolates those factors from the standard published Terzaghi table.
Why This Simplified Version Exists
- Early-stage feasibility checks — before commissioning a full geotechnical investigation, a simplified bearing capacity estimate helps gauge whether a site is even reasonably viable for a planned structure.
- Teaching and coursework — the Terzaghi equation is a standard entry point in geotechnical engineering education, precisely because it isolates the three physical contributions (cohesion, surcharge, and friction/weight) clearly.
- Sanity-checking software output — engineers running full geotechnical software packages sometimes hand-check outputs against the simplified Terzaghi form to catch input errors.
- Comparing soil types conceptually — adjusting friction angle and cohesion shows directly how sand-like versus clay-like soils contribute to bearing capacity in fundamentally different ways.
Standard Terzaghi Bearing Capacity Factors
| Friction Angle φ | Nc | Nq | Nγ |
|---|---|---|---|
| 0° | 5.7 | 1.0 | 0.0 |
| 20° | 17.7 | 7.4 | 5.0 |
| 30° | 37.2 | 22.5 | 19.7 |
| 40° | 95.7 | 81.3 | 100.4 |
This calculator linearly interpolates between these published table points for friction angles that fall between them.
How to Use This Calculator
- Enter Soil Cohesion c (e.g. kPa).
- Enter Surcharge / Overburden Pressure q (e.g. kPa).
- Enter Soil Unit Weight γ (e.g. kN/m³).
- Enter Footing Width B (e.g. m).
- Enter Soil Friction Angle φ in degrees, between 0 and 45.
- Select Calculate to see the ultimate bearing capacity qu and the interpolated factors used.
Related Calculations
Check subsurface water movement with the Permeability Calculator, or estimate void space with the Porosity Calculator.