What a footing does
A footing spreads the weight of a wall over enough soil that the ground does not compress or fail beneath it. Soil can carry only a certain pressure, its allowable bearing pressure. If the load per foot of wall divided by the footing width exceeds that pressure, the footing will settle, and uneven settlement cracks walls and floors.
The formula
For a continuous strip footing under a wall:
Width = load per linear foot ÷ (allowable soil pressure − footing self-weight)
The footing’s own weight is 150 lb per cubic foot of concrete times its thickness. An 8-inch footing weighs 100 psf, which uses up part of the soil’s capacity.
Worked example
A wall puts 2,000 lb per linear foot on its footing, the soil is clay rated at 1,500 psf, and the footing is 8 inches thick:
- Footing self-weight: 150 × 8/12 = 100 psf.
- Net capacity: 1,500 − 100 = 1,400 psf.
- Minimum width: 2,000 ÷ 1,400 = 1.43 ft = 17.1 inches.
- Rounded up to 18 inches, the soil pressure is 2,000 ÷ 1.5 + 100 = 1,433 psf, within the limit.
- For 120 ft of footing: 1.5 × 0.667 × 120 ÷ 27 = 4.44 cubic yards, or about 4.9 yd³ with a 10% allowance.
Soil bearing values
Without a soils report, the International Residential Code allows these presumptive values:
| Soil | Allowable pressure |
|---|---|
| Clay, sandy clay, silty clay, silt | 1,500 psf |
| Sand, silty sand, clayey sand, silty or clayey gravel | 2,000 psf |
| Sandy gravel, gravel | 3,000 psf |
| Sedimentary and foliated rock | 4,000 psf |
| Crystalline bedrock | 12,000 psf |
Your building department may require a soils test where expansive clay, fill or high groundwater is suspected.
Estimating the load
The load comes from the roof, floors and walls the footing supports, including snow and occupants. For a typical house, the IRC tables of minimum footing widths by number of stories and soil type already include these loads. Use this calculator to check a footing under a heavier wall, a masonry chimney or an addition, and use the load from your plans or engineer.
Code minimums and good practice
- Footings must be at least 12 inches wide and 6 inches thick under the IRC, and wider where the tables require.
- The bottom of the footing must sit below the frost line, which ranges from a few inches in the South to 4 feet or more in the northern states.
- Place footings on undisturbed soil, never on loose fill, and keep the trench free of water.
- Many builders place two continuous #4 bars near the bottom of the footing, and seismic areas require reinforcement; check local rules.
Point loads from posts and columns need square pad footings, sized as load ÷ net pressure for the area. For poor soils, slopes or heavy structures, hire a structural or geotechnical engineer.