Bearing Capacity of Soil: How Much Load a Footing Can Really Carry

Every building you have ever walked into transfers its entire weight into the ground through a handful of footings. The soil under those footings is not bedrock — it is a granular or clayey material that can shear, flow, and settle. Push a footing too hard and the soil beneath it gives way along a curved failure surface, the footing punches down, and the structure above tilts or cracks. The question every foundation engineer answers first is deceptively simple: how much pressure can this soil take before that happens?

This article explains what bearing capacity means, how the classic Terzaghi method estimates it, works through a full example on saturated clay, and points out the mistakes that most often turn a safe-looking number into an unsafe foundation.

Why this calculation matters

Bearing capacity is the dividing line between a foundation that performs and one that fails. It governs the footing dimensions on almost every shallow-foundation project — strip footings under walls, pad footings under columns, raft slabs under whole buildings. Get it wrong on the unsafe side and you risk a shear failure of the ground; get it wrong on the conservative side and you pour far more concrete and excavate far more soil than the job needs.

There are really two limits to respect. The first is the ultimate bearing capacity, the pressure at which the soil shears and the footing fails outright. The second is settlement — the footing may be nowhere near shear failure yet still sink enough to damage the structure. A complete check looks at both. This article focuses on the strength side, the ultimate capacity and the allowable pressure derived from it, because that is the calculation engineers reach for first.

The core formula

In 1943 Karl Terzaghi published a bearing-capacity equation that remains the backbone of shallow-foundation design. For a strip footing it reads:

qu = c*Nc + q*Nq + 0.5*gamma*B*Ngamma

Each term represents a different source of resistance. The first, c*Nc, is the contribution of soil cohesion. The second, q*Nq, is the contribution of the surcharge — the weight of soil sitting alongside the footing at its base level, which confines the failure zone. The third, 0.5*gamma*B*Ngamma, is the contribution of the self-weight of the soil within the failure wedge.

The terms Nc, Nq and Ngamma are the bearing capacity factors. They are dimensionless numbers that depend only on the soil friction angle phi, and they rise steeply as phi increases. The surcharge q is computed from the unit weight of the soil above the footing base and the embedment depth:

q = gamma*Df

A special and very common case is saturated clay loaded quickly, the undrained condition. Here the friction angle phi is taken as zero, and the factors collapse to fixed values:

phi = 0  ->  Nc = 5.14,  Nq = 1.0,  Ngamma = 0

The Ngamma term vanishes, so the footing width B drops out of the strength calculation entirely. Once the ultimate capacity is known, a factor of safety converts it into a pressure you can actually design to:

q_allow = qu / FS

A factor of safety of about 3 is widely used for shallow foundations, reflecting the scatter in soil properties and the consequences of a bearing failure.

A worked example

Consider a shallow strip footing resting on saturated clay, loaded fast enough that the undrained condition applies, so phi = 0. The undrained shear strength is c = 50 kPa, the soil unit weight is gamma = 18 kN/m^3, and the footing base sits at a depth Df = 1.0 m.

Step 1 — pick the bearing capacity factors. With phi = 0:

Nc = 5.14,  Nq = 1.0,  Ngamma = 0

Step 2 — compute the surcharge at footing base level.

q = gamma*Df = 18 * 1.0 = 18 kPa

Step 3 — compute the ultimate bearing capacity. Because Ngamma is zero, only the first two terms survive:

qu = c*Nc + q*Nq
qu = 50*5.14 + 18*1.0
qu = 257 + 18 = 275 kPa

Step 4 — apply the factor of safety. Using FS = 3:

q_allow = qu / FS = 275 / 3 = 92 kPa

So this footing can safely carry about 92 kPa of net applied pressure. Notice how the structure of the answer reflects the physics: the cohesion term, 257 kPa, dominates, while the surcharge contributes a modest 18 kPa. On undrained clay the strength of the foundation is essentially the strength of the clay itself.

Common mistakes

Forgetting that B vanishes in undrained clay. Engineers sometimes expect a wider footing to have a higher ultimate bearing capacity. When phi = 0, Ngamma = 0 and the width B drops out of the strength equation. A wider footing carries more total load only because it spreads the same allowable pressure over more area — not because qu rises.

Confusing gross and net pressure. The qu from Terzaghi's equation is a gross value. The pressure the structure actually adds is the net pressure, after accounting for the soil removed during excavation. Comparing a gross capacity with a net demand, or the reverse, can quietly bias a design.

Mixing up drained and undrained parameters. Clay behaves very differently in the short term and the long term. Using the undrained strength c with a non-zero friction angle, or pairing drained parameters with the phi = 0 factors, produces numbers that look reasonable but describe no real soil condition.

Checking strength but skipping settlement. A footing can satisfy bearing capacity with a comfortable factor of safety and still settle too much for the structure above. On soft or compressible soils, settlement — not shear failure — often governs the design.

Ignoring the groundwater table. A water table near the footing reduces the effective unit weight of the soil and lowers both the surcharge and self-weight terms. Designing with a dry unit weight when the soil is in fact submerged overstates the capacity.

Try the interactive NovaSolver calculator

Working one case by hand is the right way to learn the method, but real projects need fast iteration across footing sizes, depths and soil types. The Bearing Capacity Calculator (Terzaghi & Meyerhof) on NovaSolver does exactly that — choose the foundation type, set cohesion, friction angle, unit weight, footing width and length, embedment depth, factor of safety and load eccentricity, and it returns the ultimate bearing capacity q_ult, the allowable pressure q_allow, the bearing capacity factors Nc, Nq and Ngamma, and an estimate of elastic settlement.

Related calculators

• Soil bearing capacity calculator — a focused tool for working bearing capacity directly from soil strength parameters and footing geometry.
• Slope stability analysis — when a footing sits near a slope, the same shear strength that governs bearing must also resist a sliding mass.
• Earth pressure simulator — for the lateral pressures a footing or basement wall must also carry alongside the vertical load.

You can explore the full set in the geotechnical tools hub.

Closing note

Bearing capacity is one of the oldest calculations in geotechnical engineering and still one of the most consequential. The method is compact: sum the cohesion, surcharge and self-weight contributions, lean on the bearing capacity factors that depend on friction angle, and divide by a sensible factor of safety. The hard part is not the arithmetic — it is choosing soil parameters that honestly represent the ground, deciding whether the drained or undrained case applies, and remembering that a footing must satisfy settlement as well as strength. Get the soil model right, and the equation will give you a number you can defend.