SettingsSettings for this calculationUS
The total axial load the column transfers to the footing.
Use the unfactored (service-level) load for bearing capacity sizing, per standard geotechnical practice.
The soil's allowable bearing capacity, from a geotechnical report.
Always use a site-specific geotechnical value for real construction.
Recommended footing side length
5.36 ft
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 sq ft
They open the calculator with your figures already in it
Isolated Spread Footing Sizing Calculator: 5.36 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard preliminary footing sizing: required footing area = column load / allowable soil bearing pressure; assumed square, so side length = sqrt(area)
Inputs used
- Column Load
- 89.92 kip
- Allowable Soil Bearing Pressure
- 3132.82 psf
Intermediate steps
- Required footing area
- 28.7 sq ft
Confidence note: 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.
What this calculation does not cover
- SIZES THE PLAN AREA, NOTHING ELSE. This returns a side length from bearing pressure alone and says nothing about thickness, reinforcement or founding depth. Punching shear, one-way shear and flexure set the concrete and the steel, and on a heavily loaded column over strong ground those checks routinely demand more footing than bearing pressure does. It is a preliminary size, not a foundation design.
- Bearing capacity is not settlement. A footing sized exactly to the allowable pressure can still settle more than the structure will tolerate, and on a compressible clay the settlement check is usually the one that decides the size.
- Assumes the load is axial and lands in the middle of the pad. Any moment, eccentricity, uplift or lateral thrust skews the pressure under the footing — once the resultant leaves the middle third one edge lifts and the peak pressure runs far above the average used here — and those cases generally want a rectangular footing rather than the square this assumes.
- The footing's own weight, the backfill sitting on top of it and any plinth or slab above are not added to the load you enter, so the pressure actually delivered to the soil is higher than the pressure this sizes for.
- The allowable pressure you enter carries the whole geotechnical burden. It must already include its factor of safety — enter an ultimate capacity instead and you over-credit the ground by roughly a factor of three — and it must belong to the elevation you are actually founding at, not the shallow detail. Frost depth, groundwater, adjacent footings whose stress bulbs overlap, expansive or collapsible ground, and any code minimum footing size sit entirely outside this calculation, and none of it substitutes for a site geotechnical report.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Standard preliminary footing sizing: required footing area = column load / allowable soil bearing pressure; assumed square, so side length = sqrt(area)
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