geotechnical engineering

Ultimate vs Allowable Bearing Capacity: Formula and Example

Compare ultimate, net ultimate, gross allowable, and net allowable bearing capacity with factor-of-safety formulas, settlement limits, and a worked footing example.

9 minUpdated 2026-08-17Technical editorial review

Practical Overview

Ultimate bearing capacity is a failure-state estimate. Allowable bearing capacity is a working value selected for design or preliminary screening after applying safety and serviceability judgment. Confusing the two is one of the easiest ways to overstate foundation capacity.

This article is written for preliminary engineering understanding, study, field review, and calculation checking. It should not be used as a substitute for project-specific subsurface exploration, local code requirements, or review by a licensed professional engineer.

Term	Meaning	Typical use
Gross ultimate capacity, $q_{ult}$	Total contact pressure at the interpreted bearing failure state	Strength model output
Net ultimate capacity, $q_{net,ult}$	Ultimate capacity above the pre-existing overburden pressure at footing level	Factor-of-safety calculation
Net allowable capacity, $q_{net,all}$	Net ultimate capacity divided by the selected factor of safety	Preliminary design comparison
Gross allowable capacity, $q_{gross,all}$	Net allowable capacity plus the overburden pressure at footing level	Comparison with gross footing pressure
Serviceable bearing pressure	Pressure limited by settlement, distortion, code, or performance criteria	Often the governing design value

Soil Mechanics Or Design Basis

A common strength-based relationship is:

q_{net,all}=\frac{q_{net,ult}}{FS}

For a footing embedded a depth $D_f$ below grade in soil with total unit weight $\gamma$ , the initial overburden pressure is often represented as:

q_0=\gamma D_f

Using consistent gross and net definitions:

q_{net,ult}=q_{ult}-q_0

q_{gross,all}=q_{net,all}+q_0

These relationships are useful but incomplete. The design bearing pressure should not exceed the lower of the strength-based allowable pressure and the pressure that satisfies settlement and differential-movement criteria.

q_{design}\leq\min\left(q_{allowable,strength},q_{allowable,settlement}\right)

The important habit is to name the soil model before selecting numbers. For geotechnical topics, that usually means asking whether the problem is drained or undrained, total stress or effective stress, short-term or long-term, and whether the soil profile is uniform enough for the simplified method being used.

Worked Mini Example

Consider a footing embedded 3 ft in soil with unit weight 120 pcf. Suppose the calculated gross ultimate bearing capacity is 9,360 psf and the selected factor of safety is 3.

First calculate overburden pressure at footing level:

q_0=\gamma D_f=(120)(3)=360\ \text{psf}

Then calculate net ultimate capacity:

q_{net,ult}=9{,}360-360=9{,}000\ \text{psf}

The net allowable capacity is:

q_{net,all}=\frac{9{,}000}{3}=3{,}000\ \text{psf}

Convert back to gross allowable pressure for comparison with a gross applied contact pressure:

q_{gross,all}=3{,}000+360=3{,}360\ \text{psf}

A settlement analysis might still limit the footing to 2,000 psf if compressible soil extends through the stress influence zone. In that case, 2,000 psf governs even though the strength-based value is higher.

The purpose of the example is not to create a universal design value. It shows how to organize the calculation, keep units visible, and interpret whether the result is controlled by strength, serviceability, water, construction, or uncertainty.

Field Checks And Practical Clues

Ask whether the allowable value is governed by shear, settlement, or prescriptive code limits.
Check whether the factor of safety is stated explicitly.
Confirm whether both the applied pressure and capacity are expressed on a net or gross basis.
Confirm the load case: dead plus live load may not be the same as transient construction or wind load combinations.
Look for different values for isolated footings, strip footings, mats, and slabs-on-grade.

Common Mistakes

Calling an ultimate pressure an allowable design pressure.
Using one factor of safety for all soil conditions without judgment.
Ignoring settlement because the shear capacity appears adequate.
Mixing net and gross bearing pressures without stating which is used.
Comparing service-level allowable pressure with factored structural loads without reconciling the design framework.
Treating a tabulated presumptive pressure as though it came from a site-specific bearing and settlement analysis.

How To Use This On Civil Geo Tools

The related calculator, /calculators/bearing-capacity-calculator, can be used as a transparent worksheet after the assumptions are understood. Start with the sketch or geometry, enter conservative but realistic parameters, read the step-by-step output, and compare the result against the limitations on the page.

Engineering Interpretation

For real projects, the calculation is only one part of the decision. Review the boring logs, groundwater observations, lab data, construction sequence, drainage, loading, and consequence of poor performance. Where uncertainty is high, sensitivity checks are often more useful than a single polished number.

References And Further Reading

Das, B. M. Principles of Foundation Engineering.
Coduto, Yeung, and Kitch. Geotechnical Engineering: Principles and Practices.
FHWA geotechnical engineering manuals and design circulars.
ASTM and AASHTO test standards where project specifications require them.

FAQ

Can this article be used for final design?

No. It is educational and useful for preliminary screening, but final design requires project-specific data, applicable standards, and professional judgment.

What is the most important input to verify?

Usually the soil or water condition that controls the mechanism. For many geotechnical problems, groundwater, drainage, weak layers, and construction disturbance matter as much as the headline formula.

Why do different engineers sometimes get different answers?

They may be checking different failure modes, using different drainage assumptions, selecting different strength parameters, or applying different safety and serviceability criteria. The assumptions should be compared before the final numbers are compared.

Related calculators

Bearing Capacity CalculatorScreen shallow foundation bearing capacity using a Terzaghi-style calculation.

References

Das, B. M. Principles of Foundation Engineering.
FHWA geotechnical engineering circulars and technical references.
ASTM and AASHTO standards where applicable.