Skip to content

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.

Use this result well

Inputs that matter
Column Load, Allowable Bearing
Output to expect
Footing Size Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
Was this tool helpful?

Related Tools

Tools you might need next

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.

Concrete Masonry
Anchor Size Calculator

Calculate anchor size and rode length from boat length, weight, and conditions. Select ground tackle for safe anchoring in varied wind and bottom.

Sailing
Angular Size Calculator

Calculate angular diameter of the Moon, Sun, planets, and deep-sky objects. Free angular size calculator converts physical size and distance to arcminutes.

Astronomy

Reference & details

How it works

Required 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.