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Isolated Spread Footing Sizing Calculator

Size a square isolated column footing from the column load and the site's allowable soil bearing pressure.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Recommended footing side length

5.36 ft

Check your inputs

This is a preliminary sizing tool based on bearing pressure alone — final footing dimensions must also satisfy punching shear, one-way shear, and flexural (moment) checks per ACI 318, verified by a structural engineer.

Required footing area
28.7 ft²

For the dimensions entered, expect a recommended footing side length of 1.63 m (square footing side). Note that column load sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Standard preliminary footing sizing: required footing area = column load / allowable soil bearing pressure; assumed square, so side length = sqrt(area)

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why square, not rectangular?
A square footing is the most material-efficient shape for a purely axial (centered) column load — rectangular footings are used when site constraints or combined/eccentric loads require them.
Does this size account for the footing's own weight?
Not directly — for a more precise design, add the footing and backfill soil weight to the column load, or apply a small (5-10%) size increase as a simple approximation.
Is bearing pressure the only thing that governs footing size?
No — punching shear (the column literally punching through the footing) often governs footing thickness and can require a larger footing than bearing pressure alone, especially for heavily loaded columns on strong soil.