Use the design load per linear foot and the allowable bearing pressure on compatible force/length and force/area bases. Bearing strength, load combinations, eccentricity, frost depth and footing thickness require the actual design basis; this arithmetic alone cannot establish them.
This is a rough width check and not a structural design. Soil varies across a site, and wet or fill soil can be much weaker. Codes prescribe minimum footing sizes for light framed houses, and larger structures need an engineer.
Frost depth varies widely by region and controls the depth.
Required footing width (inches) from load per linear foot
| Load per linear foot | 2,000 |
|---|---|
| Allowable soil bearing pressure | 1,500 |
| Required footing width (inches) | 16 |
| Required footing width (feet) | 1.3 |
Common error: Guessing the soil. A footing sized for firm soil on soft clay can settle unevenly and crack the wall above it.
Footing Size: formula
Footing Size: changing Load per linear foot
| Load per linear foot | Required footing width (inches) | Required footing width (feet) |
|---|---|---|
| 1,600 | 12.8 | 1.1 |
| 1,800 | 14.4 | 1.2 |
| 2,000 (example) | 16 | 1.3 |
| 2,200 | 17.6 | 1.5 |
| 2,400 | 19.2 | 1.6 |
Footing Size: compare input ranges
| Input | Tested from | To | Required footing width (inches): low input | Required footing width (inches): high input | Swing |
|---|---|---|---|---|---|
| Load per linear foot | 1,800 | 2,200 | 14.4 | 17.6 | 3.2 (20%) |
| Allowable soil bearing pressure | 1,350 | 1,650 | 17.8 | 14.5 | 3.2 (20%) |
Footing Size: individual input comparisons
| Input | Value | Required footing width (inches) | Required footing width (feet) |
|---|---|---|---|
| Load per linear foot | 1,600 | 12.8 | 1.1 |
| 1,800 | 14.4 | 1.2 | |
| 2,000 | 16 | 1.3 | |
| 2,200 | 17.6 | 1.5 | |
| 2,400 | 19.2 | 1.6 | |
| Allowable soil bearing pressure | 1,200 | 20 | 1.7 |
| 1,350 | 17.8 | 1.5 | |
| 1,500 | 16 | 1.3 | |
| 1,650 | 14.5 | 1.2 | |
| 1,800 | 13.3 | 1.1 |
Required footing width (inches): Load per linear foot against Allowable soil bearing pressure
| Load per linear foot / Allowable soil bearing pressure | 1,200 | 1,500 | 1,800 |
|---|---|---|---|
| 1,600 | 16 | 12.8 | 10.7 |
| 1,800 | 18 | 14.4 | 12 |
| 2,000 | 20 | 16 | 13.3 |
| 2,200 | 22 | 17.6 | 14.7 |
| 2,400 | 24 | 19.2 | 16 |
Footing Size: values and definitions
| Quantity and definition | Value used |
|---|---|
| Load per linear foot — Wall or point load carried per linear foot of footing, in pounds. | 2,000 |
| Allowable soil bearing pressure — Allowable soil bearing pressure, in pounds per square foot (1,500 psf is a common presumptive value for clay/sandy-clay soils). | 1,500 |
| Required footing width (inches) | 16 |
| Required footing width (feet) | 1.3 |
Load per linear foot: half and double
| Load per linear foot | Required footing width (inches) |
|---|---|
| 1,000 | 8 |
| 2,000 | 16 |
| 4,000 | 32 |
A footing spreads the weight of a wall over enough soil that the ground can carry it. A load of 2,000 pounds per linear foot on soil that can bear 1,500 pounds per square foot needs a width of about 1.33 feet, or 16 inches. Weaker soil needs a wider footing, and the width grows directly as the bearing value falls.
The bearing value comes from a soil report or a code table, and the depth is set by frost and the design.
Use the soil bearing value from a geotechnical report or the code's presumptive table for the soil type, and let an engineer confirm the footing for anything structural.
Estimate the concrete on the concrete volume calculator, reinforcement on the rebar page and block or wall materials on the CMU block calculator. The rectangle area and cubic yard pages give supporting numbers, and the pressure converter changes bearing pressure units.
What is soil bearing capacity?
The pressure the ground can support without excessive settlement or failure, measured in pounds per square foot.
How deep should a footing be?
Below the local frost line to prevent heaving, which ranges from a few inches in warm climates to four feet or more, and on undisturbed soil.
Can I size my own footing?
For small non-structural projects a code table may suffice, but houses and heavy loads need an engineer or a code-based design.