How to use the allowable bearing capacity calculator
Sizing a shallow footing starts with the pressure the soil can carry. This calculator uses the classic three-term Terzaghi-form bearing capacity equation with shape factors to get the ultimate capacity qu, then divides by a factor of safety for the allowable capacity qa. The capacity factors are Nq = e^(π·tanφ)·tan²(45+φ/2) (Prandtl–Reissner), Nc = (Nq−1)·cotφ and Nγ = 2(Nq+1)·tanφ (Vesic), with shape factors of 1.0/0.5 for strip, 1.3/0.4 for square and 1.3/0.3 for circular footings.
Friction angle and cohesion should come from a geotechnical report (triaxial or direct shear testing), not from memory. For clean sand, leave cohesion at 0 and enter only the friction angle. For a short-term undrained check in saturated clay, set the friction angle to 0 and enter the undrained shear strength as cohesion. Deeper embedment raises the surcharge term quickly, so lowering a footing slightly often lets you shrink its plan size.
A factor of safety of 3 is typical, with larger values where subsurface data is thin or loads vary widely. When groundwater sits near the footing base, use the submerged unit weight (total minus 62.4 pcf) and re-run the numbers. Passing the bearing check is only half the job: settlement must also stay within the tolerance of the structure, so pair this with a settlement estimate before fixing footing sizes.
These numbers are a screening estimate that assumes uniform soil. Final foundation design should be confirmed by a licensed geotechnical engineer using site-specific boring data.
Frequently asked questions
Not exactly. Allowable bearing capacity applies a factor of safety against shear failure only. The design bearing pressure must also satisfy a settlement limit, so the governing value is often smaller than the shear-based number.
A factor of safety of 3 against shear failure is the common practice for shallow footings. Use a larger value when subsurface data is limited, loads vary widely, or the structure is high risk.
Soil below the water table is buoyant, so the effective unit weight drops to roughly the submerged value (total unit weight minus 62.4 pcf). Enter that submerged unit weight to see how much the capacity falls.