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Calcimator

Soil Bearing Capacity Calculator

Calculate ultimate and allowable soil bearing capacity using Terzaghi's equation. Inputs include soil cohesion, friction angle, and foundation geometry.

Inputs

psf

Soft clay: 200–500 psf; stiff clay: 1,000–2,000 psf; hard clay: 2,000–4,000 psf

°

ASTM D3080: loose sand 28–32°; dense sand 36–40°; gravel 40–45°; clay 0–15°

ft
ft
pcf

Results

Ultimate Bearing Capacity

18,029 psf

Allowable Bearing (FS=3)

6,010 psf

Net Allowable Bearing5,890 psf
Bearing Factor Nc30.14
Bearing Factor Nq18.4
Bearing Factor Nγ22.4
How to Use This Calculator
  1. Enter soil cohesion (psf), friction angle (°), foundation width (B), foundation depth (D), and soil unit weight.
  2. Review Ultimate Bearing Capacity and Allowable Bearing Capacity (FS=3) in psf.
  3. Compare allowable bearing to your design footing pressure — reduce footing load or increase width if needed.

How the result changes with Friction Angle (φ)

Friction Angle (φ)Ultimate Bearing CapacityAllowable Bearing (FS=3)
51,970 psf657 psf
185,489 psf1,830 psf
3325,566 psf8,522 psf
45140,549 psf46,850 psf

What each input means

Soil Cohesion (c)
Undrained shear strength of cohesive soil per ASCE 7 §12.13 and geotechnical reports. Sand/gravel ≈ 0 (frictional only); soft clay ≈ 200-500 psf; stiff clay ≈ 1,000-2,000 psf; hard clay ≈ 2,000-4,000 psf.
Friction Angle (φ)
Internal friction angle from soil testing (ASTM D3080 or SPT correlation). Loose sand ≈ 28–32°; medium sand ≈ 32–36°; dense sand ≈ 36–40°; clay ≈ 0–15°. Per ASCE 7 and geotechnical practice.
Foundation Width (B)
Width of the strip or square footing. Wider foundations have higher total capacity but lower unit pressure.
Foundation Depth (D)
Depth of the foundation below ground surface. Deeper embedment increases bearing capacity via overburden.
Soil Unit Weight (γ)
Unit weight of soil. Loose sand ≈ 100 pcf; dense sand ≈ 130 pcf; clay ≈ 110-125 pcf.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Soil Cohesion (c) = 200, Friction Angle (φ) = 30, Foundation Width (B) = 4, Foundation Depth (D) = 3 = 5 input(s) provided
  2. Calculate Ultimate Bearing Capacity
    Ultimate Bearing Capacity
    18029 = 18029
  3. Calculate Allowable Bearing
    Allowable Bearing
    6010 = 6010
  4. Calculate Net Allowable Bearing
    Net Allowable Bearing
    5890 = 5890
  5. Calculate Bearing Factor Nc
    Bearing Factor Nc
    30.14 = 30.14

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