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.
- 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.
Frequently Asked Questions
How deep should footings be?
Footings must extend below the frost line, which varies by region (12 in in the south to 48 in or more in northern states). Check your local building code for the required depth.
What if I do not know my soil bearing capacity?
A geotechnical soil test is the most reliable method. Without a test, most codes allow a presumptive value of 1,500-2,000 psf for undisturbed soil, but this is conservative.
Learn More
Guide
Foundation Calculation Guide: Footings, Slabs, and Structural Loads
Learn how to calculate foundation requirements including footing sizes, concrete volumes, soil bearing capacity, and structural load distribution for residential buildings.
Ready to run the numbers?
Open Foundation Footing Calculator