Soil Bearing Capacity Calculator

Bearing capacity is the maximum pressure soil can support beneath a foundation without shear failure. Engineers use it to size footings: the load divided by the footing area must stay below the safe bearing capacity. This calculator applies Terzaghi's classic bearing capacity equation to give the ultimate and safe values.

How to use it

1. Enter the soil cohesion c (kPa), angle of internal friction φ and unit weight γ, ideally from a geotechnical report.
2. Enter the footing width B and depth of foundation Df.
3. Choose the footing shape and a factor of safety, typically 2.5 to 3.
4. Read the ultimate and safe bearing capacity.

Terzaghi's equation (strip footing)

qu = c Nc + q Nq + 0.5 γ B Nγ, where q = γ × Df is the overburden pressure and Nc, Nq, Nγ are bearing capacity factors that depend on φ. For square and circular footings, shape factors modify the terms, for example 1.3 c Nc and 0.4 γ B Nγ for a square footing.

Worked example

Sand with c = 0, φ = 30°, γ = 18 kN/m³; strip footing B = 2 m at Df = 1.5 m. Terzaghi factors for 30° are Nq = 22.5 and Nγ = 19.7. q = 18 × 1.5 = 27 kPa. qu = 27 × 22.5 + 0.5 × 18 × 2 × 19.7 = 607.5 + 354.6 = 962 kPa. With a factor of safety of 3, the safe bearing capacity is about 321 kPa.

Important limitations

• Bearing capacity checks shear failure only. Settlement often governs design, especially on clay; check allowable settlement separately.
• A high water table reduces the effective unit weight and can roughly halve the γ terms.
• Soil parameters must come from site investigation and laboratory tests. Never rely on assumed values for real foundations.
• Codes such as IS 6403, Eurocode 7 and others use refined factors; final design must follow your local code and be checked by a qualified engineer.