Footing Size Calculator
A footing size calculator divides column load by soil bearing capacity: 9,600 lb on 2,000 psf soil needs 4.80 sq ft, or a 26.29 in square footing.
Footing size calculator: the formula
A footing size calculator takes the load that lands on a post or column and divides it by what the soil can safely carry. The result is the bearing area in square feet. Take the square root and you have the side of a square footing, which this tool also shows in inches.
Footing area (sq ft) = load (lb) ÷ soil bearing (psf). Side (ft) = square root of the area. Side (in) = side in feet × 12.
The math in plain words
Picture a post standing on a patch of ground. Every square foot of that ground can hold up only so much weight before it starts to sink. If your soil is rated at 2,000 psf, each square foot holds 2,000 lb.
A 9,600 lb load needs 9,600 ÷ 2,000 = 4.80 sq ft of soil under it. A square with 4.80 sq ft has a side of 2.191 ft, and 2.191 × 12 = 26.29 in. In the field you round up to a whole number, so you would form a 27 in square.
The tool does not add the weight of the concrete. A 27 × 27 in footing that is 12 in thick holds 2.25 × 2.25 × 1 = 5.06 cubic feet. At 150 lb per cubic foot that is about 760 lb, so add it to the column load before you divide.
Worked examples
Example 1, the default values. Load 9,600 lb, soil 2,000 psf.
- Area: 9,600 ÷ 2,000 = 4.80 sq ft
- Side: 2.191 ft × 12 = 26.29 in
Example 2, a heavier load on weaker ground. Load 15,000 lb, soil 1,500 psf.
- Area: 15,000 ÷ 1,500 = 10.00 sq ft
- Side: 3.162 ft × 12 = 37.95 in
Put the same 15,000 lb on 3,000 psf gravel and the area drops to 5.00 sq ft, a side of 26.83 in. The weaker soil costs you about 11 extra inches on every side.
Typical presumptive bearing values
Model building codes publish conservative presumptive values for common soil types. The table below follows that commonly published list, and the right-hand columns show what each one does to a 9,600 lb load. Your local code and any soil report override these numbers.
| Soil type | Bearing (psf) | Area for 9,600 lb | Square side |
|---|---|---|---|
| Clay, sandy clay, silty clay | 1,500 | 6.40 sq ft | 30.36 in |
| Sand, silty sand, clayey sand | 2,000 | 4.80 sq ft | 26.29 in |
| Sandy gravel and gravel | 3,000 | 3.20 sq ft | 21.47 in |
| Sedimentary and foliated rock | 4,000 | 2.40 sq ft | 18.59 in |
Most codes also set a minimum footing width regardless of the math, so a tiny result does not mean a tiny footing is allowed.
How to find the load
Start with the area the post supports, called the tributary area. For a deck post, that is roughly half the distance to the next post in each direction. Multiply that area by the combined dead and live load in pounds per square foot that your local code requires. A post holding up 120 sq ft of floor at 80 psf carries 120 × 80 = 9,600 lb.
Snow, a roof above, or a hot tub changes the number a lot. The deck load calculator is a good place to build the load figure, and local code or your building department sets the snow figure where roofs carry snow.
How to find the soil value
Do not guess the soil. Ask the building department which presumptive value it accepts for your area. For a house or any large structure, a geotechnical report gives a tested number. Fill, soft organic soil, and ground that stays wet can all be far weaker than the table suggests.
Common mistakes
- Forgetting the footing’s own weight, which can add several hundred pounds.
- Using a bearing value for gravel when the hole is in clay.
- Treating this result as the full design. Thickness, reinforcing steel, and depth below the frost line are separate decisions.
- Placing footings so close together that their pressure zones overlap.
- Rounding the side down. Always round up.
Round tubes and bulk concrete
If you pour into round forms, size the tube so its cross-section is at least the area you found. The Sonotube calculator converts a tube diameter and height into bags, and the concrete footing calculator estimates the volume for a rectangular footing. The concrete weight calculator helps with the self-weight step above.
Structural work like this carries real consequences if it is wrong. Have a licensed engineer or your local building department confirm any footing that supports a house, a deck over a few feet high, or a roof.
Frequently asked questions
What is soil bearing capacity in psf?
It is the pressure the ground can carry before it settles too much, in pounds per square foot. Soft clay may be rated near 1,500 psf, while dense gravel is often rated at 3,000 psf or more. The number belongs to your site, so it comes from the local building department's presumptive table or a soil report.
Why does a bigger load need a bigger footing?
The soil has a fixed strength per square foot. Double the load and you need double the area to keep the same pressure on the ground. A 19,200 lb load on 2,000 psf soil needs 9.60 sq ft, which is twice the 4.80 sq ft that 9,600 lb needs.
Why take the square root of the area?
A square footing has equal sides, so side times side equals the area. The side is the square root of the area. With 4.80 sq ft required, the side is 2.19 ft, which is 26.29 in.
Does the load I enter need to include the footing's own weight?
Yes, add it yourself. Concrete weighs about 150 lb per cubic foot, so a 27 × 27 in footing that is 12 in thick adds roughly 760 lb. The calculator uses the number you type and adds nothing on its own.
Does this replace an engineer's footing design?
No. This gives the bearing area only. Footing thickness, steel, depth below the frost line, and combined loads are separate checks. For a house, an addition, or anything heavy, have a licensed engineer or your local building department confirm the design.
Estimates only. Verify quantities with your supplier before purchasing.