🧱 Footing Design

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

Related standards: ACI 318

Sizing a square spread footing so the pressure it puts on the soil stays within what the soil can safely carry.

A spread footing spreads a column's concentrated load over enough soil area that the bearing pressure underneath stays below the soil's allowable bearing capacity. The required area is the service (unfactored) load, plus a small allowance for the footing's own weight, divided by the allowable soil pressure; for a square footing, the side length is the square root of that area.

Depth is governed separately, mostly by shear (punching/beam shear around the column) and by frost depth requirements in cold climates — this calculator uses a simple rule-of-thumb ratio, not a full ACI 318 shear check.

Key formula

A_required = P × (1 + w) / q_allow
B = √A_required (square footing)
Rule-of-thumb depth ≈ B/4, never less than 12 in

Variables

P

column load (service, unfactored)

q_allow

allowable soil bearing pressure

w

allowance for the footing's own weight as a fraction of P (10% is a common preliminary figure)

How to use the Footing Design calculator

Use this to size the plan dimensions of a square spread footing under a single column or post: how wide it must be so that the pressure it puts on the soil stays at or below what the soil is allowed to carry. That is the first sizing step for a deck post, a porch column, a small building column or an equipment pad.

It works from the service load, the load the column actually delivers in use, and an allowable bearing pressure. It gives a plan size and a rule-of-thumb thickness; the real thickness and the reinforcing steel come from a shear and bending check that this calculator does not perform.

Column Load (service, unfactored) (kips)

The load the column delivers to the footing in service, dead plus live, without load factors. For a post, add up the area it supports times the design load per square foot, plus the post's own weight.

Allowable Soil Bearing Pressure (ksf)

The allowable soil bearing pressure: the pressure the soil may carry with a safety factor already applied. It comes from a soil report or, when there is none, a conservative presumptive value in your building code. It is not the ultimate capacity of the soil.

Footing Self-Weight Allowance (%)

An allowance for the footing's own weight, as a percentage of the column load. Ten percent is a common preliminary figure; raise it for a thick footing or a deep one with soil above it.

Worked example: a footing for a 24-kip column

A steel post in an addition delivers 24 kips of service load to the ground. The soil report allows 2 ksf (2,000 lb/ft²). What size of square footing is needed?

Worked by hand:

1. Add the footing's weight. The soil must carry the column plus the concrete under it: 24 kips × 1.10 = 26.4 kips.
2. Required area. Area = load ÷ allowable pressure = 26.4 ÷ 2 = 13.2 ft².
3. Side of the square. √13.2 = 3.63 ft, which is 43.6 in.
4. Round up to a buildable size. A 3 ft 9 in (3.75 ft) square footing has an area of 14.06 ft² and puts 1.88 ksf on the soil, comfortably under the 2 ksf allowed.
5. Thickness by rule of thumb. A quarter of the width is 43.6 in ÷ 4 = 10.9 in, which is under the 12 in floor, so the calculator returns 12 in.

Use a 3 ft 9 in square footing at least 12 in thick as the starting point. The bottom must also sit below the local frost depth, on undisturbed soil, and the final thickness and steel are set by a punching-shear, one-way-shear and bending check under the structural code.

Reading the result: how soil strength drives footing size

Footing area is inversely proportional to the allowable pressure, so the soil matters as much as the load. The same 24-kip column needs a footing about 4.2 ft across if the soil is only good for 1.5 ksf, and just 3.0 ft across at 3 ksf, which is why an unknown soil is the largest uncertainty in a small foundation.

When no soil report exists, many building codes allow a presumptive value, often somewhere between 1,500 and 2,000 lb/ft² for common clays and sands. Presumptive values are deliberately conservative and are not a substitute for a soil investigation on anything heavy.

• A wide, shallow footing is cheap to excavate but sensitive to frost; a narrow, deep one costs more digging. The bottom of any footing goes below the frost line, which ranges from a few inches in warm climates to four feet or more in cold ones.
• The calculator uses service loads because allowable soil pressures already include a safety factor. Factored loads belong in the structural design of the footing itself.
• Settlement is a separate matter. A footing can be within its bearing pressure and still settle too much on soft, compressible or fill soil.

Notes & limitations

• Allowable soil pressures are compared with SERVICE loads. Factored loads (such as 1.2D + 1.6L) belong in the structural design of the footing — its thickness and reinforcement — not in sizing its plan area against the allowable pressure.
• Real footing design also checks one-way (beam) shear and two-way (punching) shear around the column, and sizes reinforcing steel for the bending moment at the column face — none of that is covered by the simplified sizing shown here.
• q_allow itself comes from a geotechnical bearing capacity analysis (see the Soil Bearing Capacity calculator) with a safety factor already applied — don't use the ultimate bearing capacity directly.

Common mistakes

• Using the soil's ultimate bearing capacity as if it were the allowable pressure. The allowable value is the ultimate divided by a safety factor of about three; using the ultimate makes the footing about a third of the size it needs.
• Sizing with factored loads against an allowable pressure. That double-counts the safety margin; use the service load here.
• Forgetting the footing's own weight and the weight of soil sitting on it, which can be a tenth or more of the column load for a deep footing.
• Placing a footing on topsoil, fill, or soil that is frozen or saturated. The pressure limit applies to competent, undisturbed ground below those layers.
• Ignoring an off-center load or a moment. This calculator assumes a load through the center of the footing; a post at the edge, or a column resisting wind, produces uneven pressure that needs a larger or differently shaped footing.
• Treating the thickness as final. The one-quarter-of-width figure is a starting value, and a real footing must be checked for punching shear around the column and bending at its face.

Frequently asked questions

Papers worth reading

Some recent research on the bearing capacity of foundations Meyerhof, G. G. (1963), Canadian Geotechnical Journal. Where the soil side of a footing check comes from: how ultimate bearing capacity depends on footing shape and depth.