Soil Bearing Capacity Calculator
Estimates the allowable load a shallow foundation can safely impose on soil using a simplified Terzaghi bearing capacity approach. Used by geotechnical engineers and structural designers when sizing spread footings or mat foundations.
About this calculator
Terzaghi's general bearing capacity equation for a strip footing is: q_ult = c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ, where c is cohesion, γ is soil unit weight, Df is foundation depth, B is foundation width, and Nc, Nq, Nγ are dimensionless bearing capacity factors that depend on friction angle φ. This calculator uses simplified Terzaghi coefficients for general shear failure: Nc ≈ 5.7 for a purely cohesive soil (φ = 0), Nq ≈ 1.0, and Nγ ≈ 0.4 as representative values. Allowable bearing capacity q_allow = q_ult / FS, where FS is the factor of safety, typically 2.5–3.0. The result tells designers the maximum pressure (psf) a footing can safely apply to the soil without risk of shear failure or excessive settlement.
How to use
Soil has cohesion c = 500 psf, unit weight γ = 110 pcf, foundation depth Df = 3 ft, width B = 4 ft, and a factor of safety FS = 3.0. q_ult = (500 × 5.7) + (110 × 3 × 1.0) + (0.5 × 110 × 4 × 0.4) = 2,850 + 330 + 88 = 3,268 psf. q_allow = 3,268 / 3.0 = 1,089.3 psf ≈ 1,089 psf. This means each square foot of the footing base can safely carry about 1,089 pounds of load.
Frequently asked questions
What is the difference between ultimate bearing capacity and allowable bearing capacity?
Ultimate bearing capacity (q_ult) is the maximum pressure at which the soil beneath a footing fails in shear—the footing punches into or causes the soil to heave. Allowable bearing capacity (q_allow) divides q_ult by a factor of safety, typically 2.5 to 3.0, to account for uncertainties in soil properties, load estimates, and construction variability. Structural designs must keep applied footing pressures below q_allow to ensure both shear stability and acceptably small settlements under service loads.
How does friction angle affect the soil bearing capacity of a foundation?
The friction angle φ governs the bearing capacity factors Nq and Nγ in Terzaghi's equation. As φ increases, both factors rise rapidly—Nγ for a 30° friction angle is roughly 20 times larger than for 10°. Sandy and gravelly soils typically have φ between 30° and 40° and rely heavily on the friction and surcharge terms for their strength. Cohesive clays often have φ near zero under undrained conditions, so their bearing capacity depends almost entirely on cohesion and the Nc term. Accurate laboratory or field testing of φ is critical for reliable capacity predictions.
Why is a factor of safety of 3 commonly used in shallow foundation design?
A factor of safety of 3 is the traditional geotechnical standard because soil properties are inherently variable and difficult to characterize with precision from a limited number of borings or tests. The factor guards against uncertainties in cohesion and friction angle measurements, spatial variability across the footing area, unanticipated load increases, and the difference between laboratory sample behavior and in-situ conditions. Lower factors (2.5) may be acceptable when extensive site investigation data is available, while higher factors are used for critical structures or highly variable soil conditions.