🏗️ Footing Size Calculator
Calculate Foundation Footing Width Based on Soil & Load
📐 Footing Design Fundamentals
A footing distributes the weight of your building over a larger area to prevent settlement. The required size depends on the total load and the soil's ability to support that load without compressing.
Basic Formula
Required Area (sq ft) = Total Load (lbs) / Allowable Soil Bearing Capacity (psf)
Soil Bearing Capacities (Presumptive Values)
| Soil Type | Bearing Capacity (psf) |
|---|---|
| Soft Clay, Silt | 1,500 |
| Medium Stiffness Clay | 2,000 - 3,000 |
| Compact Sand & Gravel | 4,000 - 6,000 |
| Bedrock (solid) | 10,000+ |
Note: Always obtain a geotechnical report for critical projects. These are presumptive values from building codes.
"The biggest mistake is assuming your soil is 'good' without testing. I've seen contractors pour 16-inch wide footings on clay that can only support 1,200 psf—they needed 30 inches! If you're building on fill, assume ZERO bearing capacity until it's been compacted and tested. Also, frost heave is real: footings in cold climates MUST go below the frost line or you'll have cracked foundations within 2 years."
⚠️ Common Mistakes
1. Ignoring Frost Depth: Even if your soil is rock, if the footing is above the frost line, it will heave and crack your foundation. Check your local frost depth map.
2. Undersized Footings for Concentrated Loads: If you have a steel column or beam post, that's a HUGE concentrated load. A standard 16" wide wall footing won't work—you need a pad footing (e.g., 3ft x 3ft).
3. Forgetting the Safety Factor: This calculator uses the allowable soil bearing capacity, which already includes a ~2-3x safety factor. Don't divide by another safety factor or your footings will be giant.