Stax
Tools

Bearing Capacity Calculator

Calculate safe bearing capacity of soil using Terzaghi's theory.

Nc
14.835
Nq
6.399
Nγ
1.435
qu (ultimate)
563.03 kN/m²
q_net
536.03 kN/m²
q_allow (safe)
205.68 kN/m²
q_allow (t/m²)
20.97 t/m²
FOS applied
3
qu = c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ
q_allow = (qu - γ·Df) / FOS + γ·Df

How the Bearing Capacity Calculator works

The bearing capacity calculator computes the ultimate and allowable bearing capacity of shallow foundations using Terzaghi's equation as codified in IS 6403. Enter soil cohesion, friction angle, unit weight, and foundation dimensions to get the safe bearing capacity — the key geotechnical parameter for sizing footings and raft foundations in any soil condition.

Terzaghi's general bearing capacity equation

Terzaghi's equation is qu = c·Nc + q·Nq + 0.5·γ·B·Nγ, where c is cohesion, q is overburden pressure, γ is soil unit weight, B is footing width, and Nc, Nq, Nγ are dimensionless bearing capacity factors that depend on the soil friction angle φ. For φ = 30°, these factors are approximately Nc = 37.2, Nq = 22.5, Nγ = 19.7 — values the calculator looks up automatically from the standard tables.

Cohesive vs granular soils

For purely cohesive soils (clay, φ = 0), the Terzaghi equation simplifies to qu = 5.14c + q. Safe bearing capacity for soft clay may be as low as 50 kPa, while stiff clays can reach 200–300 kPa. Granular soils (sand, gravel) have φ values of 28–40° and zero cohesion; their bearing capacity increases significantly with foundation depth and width due to the Nq and Nγ terms.

Factor of safety and allowable bearing capacity

IS 6403 recommends a factor of safety of 3.0 applied to the net ultimate bearing capacity to obtain the net safe bearing capacity: q_safe = (qu − γDf)/FOS + γDf. This FOS of 3 accounts for soil variability, load uncertainties, and long-term settlement. The calculator applies this factor automatically and distinguishes between gross and net safe bearing capacity in the output.

Foundation depth and shape corrections

Bearing capacity increases with depth as overburden pressure q = γDf grows. Shape correction factors (sc, sq, sγ) adjust the equation for rectangular, square, and circular footings. A square footing has sc = 1.3 compared to sc = 1.0 for a strip footing, giving approximately 30% higher capacity. The tool applies Hansen or IS 6403 shape factors based on your selected footing geometry.

Frequently asked questions

What is bearing capacity of soil?
The bearing capacity of soil is its ability to support the loads applied to it from a foundation without experiencing shear failure or excessive settlement. The ultimate bearing capacity (qu) is the maximum pressure the soil can sustain before sudden shear failure. The safe or allowable bearing capacity is calculated by dividing qu by a factor of safety, typically 2.5 to 3.0 for permanent structures.
What factor of safety should I use?
IS 6403 recommends a factor of safety of 3.0 for general shear failure conditions in permanent structures where the shear strength parameters have moderate uncertainty. A factor of safety of 2.5 is acceptable when the shear parameters are well-established from multiple tests. A factor of safety of 2.0 may be used for temporary structures or when very thorough site investigation has been done.
What is the difference between general and local shear failure?
General shear failure occurs in dense or stiff soils — a sudden, well-defined failure surface develops and the soil collapses abruptly. Local shear failure occurs in loose or soft soils — the soil compresses gradually under load without a clear failure plane, and the failure is progressive rather than sudden. Terzaghi's standard bearing capacity formula is derived for general shear conditions. For local shear, reduced shear parameters (c' = 2c/3 and tan φ' = 2/3 tan φ) are used.

Related tools