Footing Size Calculator
Size a square footing from the column load and soil bearing capacity: enter the load in pounds and the soil's psf rating to get the required footing area and side length.
How this calculator works
A footing takes the load pressing down through a column or post and spreads it across the soil so the ground is not overloaded. The stronger the soil, the smaller the footing can be; the heavier the load, the bigger it has to be.
To find the size, you divide the load by the soil’s bearing capacity, which gives the area of soil you need under the footing. For a square footing, taking the square root of that area gives the length of each side.
Formula: Required area (sq ft) = load (lb) ÷ soil bearing capacity (psf); side of a square footing = √area.
Worked example
A 9,600 lb load on 2,000 psf soil:
- Required area: 9,600 ÷ 2,000 = 4.8 sq ft
- Side of square footing: √4.8 ≈ 2.19 ft
- That is about 26.29 in per side
Notes
Typical residential soils are assumed at 1,500 to 2,000 psf unless a soils report says otherwise. Weaker soil or a heavier load both push the footing wider, so when you are unsure, lean toward the lower bearing value to stay conservative.
This calculator sizes the footprint only. It does not set footing thickness, reinforcement, or depth below the frost line, all of which matter for a footing that performs as intended. Treat the result as a starting point for a conversation with your engineer and building department, not as a finished design.
How to use
Enter the total load the footing carries in pounds and the soil’s bearing capacity in psf, then read off the required area and the side length of a square footing. Use a conservative soil value when you do not have a soils report, and confirm the final size with a professional.
This is a planning estimate, not an engineered design. Structural sizing is safety-critical and code-governed — confirm all members, connections, and footings with a licensed structural engineer and your local building code.
Frequently asked questions
What is soil bearing capacity in psf?
Soil bearing capacity is how much weight a square foot of soil can safely support, measured in pounds per square foot, or psf. Stronger soils carry more per square foot, so a footing on strong soil can be smaller. The value comes from soil type and, for anything important, a soils report from a qualified professional.
Why does a bigger load need a bigger footing?
A footing spreads a concentrated load out over the soil beneath it. The heavier the load, the more soil area you need so that each square foot stays within its safe limit. Dividing the load by the soil's capacity gives the required area, so as the load grows the area grows right along with it.
Why take the square root of the area?
A square footing has equal sides, and the area of a square is its side multiplied by itself. To work backward from an area to the side length, you take the square root. If the required area is nine square feet, the side is the square root of nine, which is three feet. This gives the footing width to build.
Does this replace an engineer's footing design?
No. This is a planning estimate that ignores footing thickness, reinforcement, frost depth, eccentric loads, and local code. Real footing design is safety-critical and depends on a soils report and code requirements. Use this to get a rough sense of size, then have a licensed structural engineer confirm the actual footing before you build.
Estimates only. Verify quantities with your supplier before purchasing.