Footing Size Calculator
Enter the column load in kips and the soil's allowable bearing in ksf to get the required footing area, snapped to practical dimensions, with the actual bearing pressure shown.
Last updated: 2026-09-22
How the calculation works
- Required area divides the load by the allowable pressure, with a 10% allowance for the footing's own weight and the soil above it.
- Square and circular footings invert the area formula directly; rectangular uses a 1.5:1 aspect for long columns or equipment pads.
- The practical size snaps up to the nearest half foot — forms are laid out in whole inches, and half-foot increments match lumber and excavation practice.
- The actual bearing pressure with the snapped size shows how much margin the practical size leaves.
Formula
A_required = 1.1 × Load / q_allow square side = √A circular dia = √(4A/π) p_actual = 1.1 × Load / A_provided
| Symbol | Meaning | Unit |
|---|---|---|
A | Required bearing area | ft² |
q | Allowable soil bearing pressure | ksf |
1.1 | Self-weight allowance (footing + soil above) | — |
Worked example
Interpreting the result
This is a bearing-only preliminary size — real footing design also checks punching shear (column punching through the footing), one-way shear and the flexural reinforcement in the bottom mat, all of which set the thickness and rebar. The utilization figure tells you whether the snapped size is comfortable (85–95%) or whether you should step up a half foot for construction tolerance. Soil values without a geotech report are guesses: the 'typical' ranges in the notes are for preliminary thinking only, and building a footing on guessed bearing capacity is how foundations settle.
Assumptions
- Service loads (unfactored) against allowable bearing — the standard ASD preliminary method.
- Centered column load, no moment transfer at the base.
- Footing bearing at depth with no adjacent surcharge or slope effects.
Limitations
- No thickness, shear or flexural steel design — the output is the plan dimension only.
- Uplift, moment frames and eccentric loads need full structural design.
- Frost depth, expansive soils and water table effects are site-specific and not modeled.
Soil bearing numbers without a report
The 'typical' values floating around (sand 1500 psf, clay 2000 psf, gravel 3000 psf) come from old code prescriptive tables and describe soil classes, not your soil. Within a single building lot, bearing capacity can vary 2–3× between a clay pocket and a sand lens. The honest hierarchy: a geotechnical report with actual borings is authoritative; a hand penetrometer or DCP field test gives a defensible estimate; a code table gives a number to start a conversation with an engineer. Residential decks and small additions often proceed on the prescriptive tables in local code — anything with a roof, masonry or more than two stories does not.
Frequently asked questions
How big should a footing be for a deck post?
A typical residential deck post carries 8–15 kips. On medium soil (2 ksf), that's a 2.5–3 ft square footing at 10–12 in thick with #4 bars each way. Check your local code — many jurisdictions publish prescriptive deck footing tables by post spacing and soil class.
What is allowable soil bearing pressure?
The maximum pressure the soil can carry with an adequate safety factor against shear failure and excessive settlement — typically 1500–6000 psf for common soils, established by a geotechnical report from actual testing rather than the textbook ranges.
Why include footing self-weight?
The footing and the soil column above it are dead weight the soil carries in addition to the column load — typically 8–12% extra. Ignoring it oversizes the risk of under-sizing the footing by one increment; including a flat 10% is the standard preliminary allowance.