Footing Size Calculator
A footing is only as good as the soil beneath it. This calculator sizes spread footings by dividing your total load (including a safety factor) by the soil's bearing capacity to find the required area. It works for both square column footings and continuous wall footings, and includes concrete volume for ordering.
How Footing Sizing Works
The basic formula is: Footing Area = Total Load / Soil Bearing Capacity
For a square column footing, the width is the square root of that area. For a continuous wall footing, the width is the area divided by the wall length.
Typical Soil Bearing Capacities
| Soil Type | Bearing Capacity |
|---|---|
| Soft clay / Silt | 1,000 PSF |
| Firm clay | 1,500 PSF |
| Stiff clay / Sandy clay | 2,000 PSF |
| Sand / Gravel | 3,000 PSF |
| Dense sand / Gravel | 4,000 PSF |
| Hardpan / Bedrock | 6,000+ PSF |
PSF = pounds per square foot. These are general guidelines. A geotechnical report gives you the actual numbers for your specific site.
Depth Guidelines
The minimum footing depth depends on local frost lines and structural requirements:
- Residential (non-frost): 8-12 inches deep
- Frost-protected: Must extend below the local frost line (24-48 inches in northern states)
- Structural rule: Depth should be at least 1/3 of the footing width
Safety Factors
Residential footings typically use a safety factor of 1.5. Commercial structures may require 2.0-3.0. The safety factor accounts for uncertainty in soil conditions and load estimates.