Foundation Bearing Capacity Calculator
Calculate the safe bearing capacity of a shallow foundation given soil properties and foundation geometry. Essential for geotechnical engineers sizing footings to prevent shear failure or excessive settlement.
About this calculator
Bearing capacity quantifies how much load per unit area a soil can support before shearing. This calculator uses a simplified Terzaghi-style general shear failure approach. The ultimate bearing capacity combines three terms — cohesion, overburden pressure, and self-weight — scaled by bearing capacity factors derived from the internal friction angle φ. The formula used here is: q_ult = [c × (1 + 0.2·B) × tan²(45 + φ/2)] + [γ × D_f × tan(φ)] + [0.5 × γ × B × tan²(φ)], where c is cohesion (kN/m²), B is foundation width (m), φ is friction angle (°), γ is unit weight (kN/m³), and D_f is foundation depth (m). The safe bearing capacity is q_safe = q_ult / FS, where FS is the factor of safety, typically 2.5–3 for permanent structures.
How to use
Example: c = 20 kN/m², φ = 25°, γ = 18 kN/m³, D_f = 1.2 m, B = 1.5 m, FS = 3. Step 1 — first term: 20 × (1 + 0.2 × 1.5) × tan²(57.5°) = 20 × 1.3 × 2.464 = 64.1 kN/m². Step 2 — second term: 18 × 1.2 × tan(25°) = 21.6 × 0.4663 = 10.1 kN/m². Step 3 — third term: 0.5 × 18 × 1.5 × tan²(25°) = 13.5 × 0.2174 = 2.9 kN/m². Step 4 — q_ult = 64.1 + 10.1 + 2.9 = 77.1 kN/m². Step 5 — q_safe = 77.1 / 3 ≈ 26 kN/m².
Frequently asked questions
What is the difference between ultimate and safe bearing capacity of a foundation?
Ultimate bearing capacity (q_ult) is the maximum load per unit area the soil can sustain before a shear failure mechanism develops. Safe bearing capacity (q_safe) divides q_ult by a factor of safety — typically 2.5 to 3.0 for permanent buildings — to account for uncertainties in soil variability, load estimation, and construction quality. Foundations are always designed using the safe value. Settlement must also be checked independently, as some soils may settle excessively even well below the ultimate capacity.
What factor of safety should I use for foundation bearing capacity design?
A factor of safety (FS) of 3.0 is standard for permanent structures where soil data is limited or variable. FS = 2.5 is acceptable when thorough site investigation data is available and loads are well-defined. Temporary structures or falsework may use FS = 2.0. Higher factors of safety are also warranted near sensitive infrastructure, in seismic zones, or when founding on soft cohesive soils prone to long-term consolidation. Building codes and geotechnical standards such as Eurocode 7 provide country-specific guidance.
How does soil cohesion affect foundation bearing capacity compared to friction angle?
Cohesion (c) contributes a constant resistance to shear that acts independently of overburden stress, making it especially important in clay-dominated soils. Friction angle (φ) captures the interlocking and frictional resistance of granular particles and becomes dominant in sands and gravels. Pure clays may have φ ≈ 0° and rely entirely on cohesion, while clean sands have c = 0 and rely solely on φ. Most real soils are c–φ mixtures. Increasing either parameter raises bearing capacity, but deep footings with wide widths gain proportionally more from friction angle because two of the three bearing capacity terms scale with φ.