Structural Footing Size Calculator
Size concrete footings from column load and soil bearing capacity. Get minimum footing area, dimensions, and bearing pressure for residential foundations.
Built for Structural Engineering1 documented formulaFocused single-task workflowPrivate in your browser
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Inputs that matter
Column Load, Allowable Bearing
Output to expect
Footing Size Calculator
How it works
1 documented formula across 1 calculation mode
- Keep 2 bounded inputs within the limits shown beside the fields.
- Confirm that every value uses the unit and time basis shown before comparing scenarios.
- Use the result for planning only and confirm consequential decisions with a qualified source.
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Reference & details
How it works
Updated July 2026
How it works
Updated July 2026Required Footing Area
Divide the total service or factored load on the footing by the allowable net bearing capacity of the soil to get minimum contact area.
Required Area (sq ft) = Load (lbs) ÷ Bearing Capacity (psf)Square Footing Side Length
For a square spread footing, take the square root of required area for equal length and width. Rectangular footings divide area by the fixed dimension.
Side (ft) = √(Required Area)Actual Bearing Pressure Check
Verify the proposed footing dimensions produce bearing pressure at or below the allowable soil capacity. If pressure exceeds capacity, the footing is undersized.
Actual Pressure (psf) = Load ÷ (Width × Length)Updated: July 2026
Example Scenarios
A deck column carrying 4,000 lb on clay at 1,500 psf needs 2.67 sq ft minimum — a 20″×20″ pad is marginal; 24″×24″ (4 sq ft) is safer.
A 20,000 lb load on firm soil at 2,000 psf requires 10 sq ft — a 3×3.3 ft rectangular footing or 3.16 ft square. Confirm with local code and frost depth.
Enter 2 ft width with 15,000 lb load to verify actual pressure of 3,750 psf against 2,000 psf capacity — result flags undersized footing for redesign.
FAQ
Typical allowable values: loose sand 1,500 psf, firm clay 2,000–3,000 psf, compact gravel 3,000–4,000 psf. A geotechnical report provides site-specific values — never guess on commercial or multi-story projects.
No. This calculator provides preliminary sizing for planning and material estimates. Final footings must meet local building code, frost depth, reinforcement, and depth requirements per a licensed engineer or approved prescriptive tables.
Use the load combination required by your jurisdiction. Many residential calculators use service (unfactored) loads with allowable bearing capacity. Engineered designs use factored loads with ultimate bearing checks.
Depth depends on frost line, soil type, and code — not this area calculator. In cold climates footings must extend below the frost depth (often 36–48″) regardless of area adequacy.
Strip footings under walls distribute load linearly. This spread footing calculator applies to isolated column and post pads. Wall footings require width based on wall load per linear foot.
About Structural Footing Size Calculator
Preliminary sizing for spread footings supporting columns or concentrated loads on soil. Enter the factored load and allowable soil bearing capacity to find minimum footing area and check whether your proposed width and length keep bearing pressure within limits.