Soil Bearing Capacity Calculator helps you estimate the ultimate and allowable bearing capacity of shallow foundations using Terzaghi’s method. It supports different soil types and footing shapes for preliminary geotechnical checks.
About soil bearing capacity. Soil bearing capacity is the ability of soil to support the loads applied to the ground. It is usually expressed in kilopascals (kPa) or kips per square foot (ksf). Estimating it helps ensure foundations are safe and do not experience shear failure or excessive settlement.
About soil parameters. This tool uses three main soil properties: cohesion (c), friction angle (φ), and unit weight (γ). Cohesion represents the soil’s inherent strength without confinement (e.g. clay), while friction angle represents the resistance to sliding (e.g. sand). Unit weight represents the density of the soil, expressed as force per volume.
About footing types. The calculator offers three footing shapes — strip, square, and circular. Terzaghi’s method uses different shape factors (sc, sq, sγ) for each footing type to adjust the bearing capacity. Strip footings are long and narrow, square footings are compact, and circular footings are often used for isolated columns.
About units. The tool works in a single SI-based state and lets you switch between Metric (kPa, m, kN/m³) and Imperial (ksf, ft, pcf) units. Input values are converted automatically when switching, ensuring identical results in both systems.
Calculation. The calculator uses Terzaghi’s general shear failure equation to estimate the ultimate bearing capacity (qult), then divides it by the chosen factor of safety (FS) to get the allowable bearing capacity (qallow):
Formulas:
- qult = c·Nc·sc + q·Nq·sq + 0.5·γ·B·Nγ·sγ (ultimate bearing capacity)
- q = γ·Df (overburden pressure at foundation level)
- qallow = qult / FS (allowable bearing capacity)
Examples:
- Example 1: c = 25 kPa, φ = 30°, γ = 18 kN/m³, Df = 1.5 m, B = 2.0 m (strip) → qult ≈ 680 kPa → qallow (FS 3.0) ≈ 227 kPa
- Example 2: c = 0, φ = 35°, γ = 19 kN/m³, Df = 1.2 m, B = 1.5 m (square) → qult ≈ 510 kPa → qallow (FS 3.0) ≈ 170 kPa
- Example 3: c = 50 kPa, φ = 0°, γ = 17 kN/m³, Df = 2.0 m, B = 2.5 m (circular) → qult ≈ 950 kPa → qallow (FS 3.0) ≈ 317 kPa
Corresponding tools. For estimating soil settlement, use the Soil Settlement Calculator. For calculating loads on isolated footings, use the Isolated Footing Load Calculator.
