Soil Bearing Capacity Calculator (Terzaghi)
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Enter the soil's cohesion, friction angle and unit weight with the footing size, and get the safe bearing capacity by Terzaghi's equation — plus a typical-SBC table to sanity-check it against.
Soil Bearing Capacity (Terzaghi)
qu = c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ · safe = ultimate ÷ FoS
Strip footing on soil
Safe 148 kN/m² · Nc 17.7, Nq 7.4, Nγ 5
0 for clean sand
Safe bearing capacity
148 kN/m²
ultimate 444 kN/m² ÷ FoS 3 · φ 20° (Nc 17.7, Nq 7.4, Nγ 5)
Safe SBC
148 kN/m²
Ultimate qu
444 kN/m²
c·Nc
177 kN/m²
Surcharge γ·Df·Nq
200 kN/m²
Typical safe bearing capacity (IS 1904, kN/m²)
Soft clay
50–100
Medium clay
100–150
Stiff clay
150–300
Loose sand
100–150
Dense sand
250–450
Gravel
450–900
Hard rock
1,650+
This is an estimate, not a substitute for a soil test. Terzaghi's general shear applies to firm ground; loose soil fails by local shear (use reduced c and φ). Get the real c, φ and γ from a geotechnical report before designing a footing.
The formula (Terzaghi, strip footing, general shear)
qu = c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ
- c = cohesion, γ = soil unit weight, Df = founding depth, B = footing width.
- Nc, Nq, Nγ = bearing capacity factors, which rise steeply with the friction angle φ.
- Safe bearing capacity = ultimate ÷ factor of safety (usually 3).
Worked example
Firm soil with c = 10 kN/m², φ = 20° (Nc 17.7, Nq 7.4, Nγ 5.0), γ = 18 kN/m³, B = 1.5 m, Df = 1.5 m:
- qu = 10×17.7 + 18×1.5×7.4 + 0.5×18×1.5×5 = 177 + 199.8 + 67.5 = 444 kN/m²
- Safe (FoS 3) = 148 kN/m²
Typical safe bearing capacity (IS 1904)
| Soil | SBC (kN/m²) |
|---|---|
| Soft clay | 50–100 |
| Medium clay | 100–150 |
| Stiff clay | 150–300 |
| Dense sand | 250–450 |
| Gravel | 450–900 |
| Hard rock | 1,650+ |
Frequently asked questions
How do I calculate safe bearing capacity? Work out the ultimate capacity with Terzaghi's equation (c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ) and divide by a factor of safety of about 3. For firm soil this is often 120–200 kN/m².
What is a good bearing capacity for a house foundation? Around 150–200 kN/m² for medium soils is comfortable for a low-rise house; below ~100 you may need wider or deeper footings, or a raft.
Do I still need a soil test? Yes. This is an estimate from assumed soil properties — the real cohesion, friction angle and unit weight come from a geotechnical investigation, and loose soils fail differently (local shear).
Why does the safe capacity rise with depth? Deeper founding adds the surcharge term γ·Df·Nq, and the soil at depth is usually firmer — so a footing carries more the deeper it sits.
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CivilSite Editorial Team✓ Engineer reviewed
Written and reviewed by practising civil engineers with 10+ years of Indian residential construction experience.