Owner reading an existing design
Check entered dimensions without the tool silently solving a pass.
Enter both dimensions and sourced matched gross loads/limit.
Actual excess pressure is visible; no auto-sized safe footing.
Footing Size Calculator
Review an independently proposed rectangular footing under gross service-load cases. Inspect centered or one-axis eccentric contact pressure against a sourced applicable limit.
Use this result well
- Inputs that matter
- Independent proposed B and L, reviewed gross service-load cases and an applicable allowable gross contact-pressure limit
- Output to expect
- Per-case uniform/linear/triangular pressures, unsupported-resultant warnings and limited pressure comparison
- How it works
- Uses e=|M|/N for one-axis eccentricity; full contact through D/6 and triangular partial contact before D/2
- Do not pair factored, ultimate or unconverted net values with this gross allowable model.
- Pressure comparison is not foundation sizing or safety approval; settlement and all structural/site limit states remain unchecked.
Choose your path
Built around the job you need to finish
Review independent proposed dimensions against limited centered or one-axis gross-service contact-pressure cases without declaring a design adequate.
Engineering reviewer
Distinguish average pressure, eccentric peak and unsupported contact.
Compare centered/full/partial one-axis cases and resultant at the base edge.
Each case is independent, no biaxial or omitted-limit-state approval.
Phone-based reviewer
Correct basis and unit errors while retaining source traceability.
Select unsupported net/ultimate basis, recover to reviewed gross basis, switch units and export.
Invalid inputs block stale records; original force/moment units and limitations remain explicit.
Authoritative checks for this tool
Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.
- Shallow Foundations, GEC6FHWA · 2002 professional guidance explains linear and triangular contact-pressure models. Centered and one-axis eccentric contact are limited models; this guidance explicitly is not a standard or regulation and does not replace site-specific design.
- SI and customary conversion factorsNIST · Exact dimensional conversions preserve quantity units, not measurement accuracy, current product specifications or structural suitability.
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Reference & details
How it works
Updated August 2026
How it works
Updated August 2026Use matched load and pressure bases
Use reviewed gross service loads including applicable self-weight and backfill, paired with the appropriate allowable gross pressure. Unconverted net, ultimate or factored values are blocked. Each load case is checked independently, not summed.
Area = B × L; average gross pressure = N / area; centered-only area reference = N / entered limitEvaluate one-axis eccentric contact
D is the selected direction of pressure variation, B or L; the moment acts about the perpendicular axis. For D/6 < e < D/2, compression contact is 3(D/2−e) and peak pressure is 2N/(transverse width × contact length). At or beyond D/2, no supported contact solution is reported.
e = |M|/N; for e ≤ D/6, qmin/max = N/(B×L) × (1 ∓ 6e/D)Keep comparison separate from approval
Confirm that the source limit permits this particular peak-pressure comparison. Passing this one comparison does not verify settlement, sliding, overturning, biaxial loading, shear, flexure, reinforcement, depth, frost, scour or any permit requirement.
Comparison ratio = modeled peak contact pressure / applicable entered limitUpdated: August 2026
Example Scenarios
A purely illustrative 20,000 lbf centered gross load on an entered 2 × 2 ft rectangle produces 5,000 psf. Against an entered applicable 2,000 psf limit it exceeds that limit; the tool does not silently lengthen the rectangle.
For a 4 × 4 ft illustration with 20,000 lbf and a one-axis 10,000 lbf·ft moment, e = 0.5 ft. Full-contact pressure ranges from 312.5 to 2,187.5 psf even though the average is 1,250 psf.
With the same illustration and a 20,000 lbf·ft moment, e = 1 ft, contact length is 3 ft and peak pressure is about 3,333.33 psf. At a 40,000 lbf·ft moment the resultant reaches the edge and the tool declines a pressure solution.
Common Mistakes to Avoid
Common Mistakes to Avoid
Combining factored loads with an unrelated allowable net pressure
Establish a matched gross service/allowable gross basis before using this limited model.
Using average pressure or centered area as proof of a complete design
Review eccentric contact and all independent geotechnical, structural and site limit states with the responsible professional.
FAQ
Only the applicable reviewed site/geotechnical pressure limit and its basis. No universal value by soil appearance or classification is supplied. Confirm whether the source supports the modeled peak comparison.
Not without qualified conversion outside this worksheet. This model accepts matched gross service loads and allowable gross contact pressure only.
No. It means only the modeled peak is within the entered applicable pressure limit. All other geotechnical, structural and site limit states remain unverified.
No. A case supports centered or one-axis eccentric loading along B or L. Biaxial loading needs separate qualified analysis; do not combine two independent one-axis passes into an approval.
No. Obtain responsible design and current local/site requirements. The downloaded record is traceable arithmetic, not a permit, excavation clearance, inspection or design approval.
About Footing Size Calculator
Enter both proposed dimensions independently, matched gross service-load cases and an applicable allowable gross contact-pressure limit. This is a limited pressure worksheet, not an automatic foundation design. It does not assign soil capacity or declare a footing safe, adequate or code-compliant.