Bearing Capacity Calculator (Terzaghi)
The ultimate and allowable bearing capacity of a shallow foundation — the general bearing-capacity equation with shape factors — so you can size a footing for the load it must carry.
How Much Load the Ground Can Take
Every footing pushes down on the soil beneath it, and the soil can only resist so much before it shears and the foundation punches in. Bearing capacity is that limit. The classic Terzaghi-style equation builds it from three sources of resistance: the soil’s cohesion, the surcharge from soil alongside the footing, and the weight of the soil in the failure wedge.
The N factors grow rapidly with the friction angle φ, so a dense sand carries far more than a soft clay. Embedding the footing deeper adds surcharge (the q Nq term) and helps. A wider footing helps through the last term. Divide the ultimate capacity by a factor of safety — commonly 3 — to get the allowable bearing pressure you actually design to.
Why Settlement Often Wins
A footing can be perfectly safe against a bearing failure and still settle too much to be usable. On sands and stiff clays, the allowable pressure is frequently set by settlement limits, not by this capacity equation. Treat the number here as the strength check, then verify settlement separately.
Frequently Asked Questions
Why a factor of safety of 3?
Bearing failure is sudden and serious, soil properties scatter widely, and the theory simplifies real behaviour – so geotechnical practice uses a large margin, typically 2.5 to 3 on ultimate capacity.
How does the water table affect it?
Groundwater at or above the footing reduces the effective weight of the soil to its buoyant value, which cuts the weight term roughly in half. The dry-versus-saturated switch here shows the drop.
Clay or sand – what changes?
Clay carries load mainly through cohesion (the c term), so undrained clay uses phi near zero and Nc around 5.14. Sand carries it through friction (the Nq and N-gamma terms), which climb steeply with phi.
Related calculators
- Slope Stability Calculator — the other half of shallow geotechnics.
- Retaining Wall Calculator — walls that also bear on soil.
- Excavation Calculator — the dig that exposes the bearing soil.
- Engineering Unit Converter — pressure and unit-weight units.