Foundation Footing Calculator Formula
Understand the math behind the foundation footing calculator. Each variable explained with a worked example.
Formulas Used
Minimum Footing Area
footing_area = min_areaSquare Footing Side
square_footing_side = min_area > 0 ? sqrt(min_area) : 0Round Footing Diameter
round_footing_dia = min_area > 0 ? 2 * sqrt(min_area / pi) : 0Variables
| Variable | Description | Default |
|---|---|---|
total_load | Total Load on Footing(lbs) | 20000 |
bearing_capacity | Soil Bearing Capacity(psf) | 2000 |
min_area | Derived value= bearing_capacity > 0 ? total_load / bearing_capacity : 0 | calculated |
How It Works
Footing Sizing
Footing Area = Total Load / Soil Bearing Capacity
The bearing capacity depends on soil type: soft clay 1,000 psf, medium clay/loose sand 2,000 psf, dense sand/gravel 3,000-4,000 psf, bedrock 8,000+ psf. Always get a soil test for structural footings.
Worked Example
20,000 lb column load on soil with 2,000 psf bearing.
total_load = 20000bearing_capacity = 2000
- 01Area = 20,000 / 2,000 = 10 sq ft
- 02Square footing = sqrt(10) = 3.16 ft per side
- 03Use a 3.5 x 3.5 ft footing (12.25 sq ft) for safety.
Ready to run the numbers?
Open Foundation Footing Calculator