Concrete Footing Calculator Formula
Understand the math behind the concrete footing calculator. Each variable explained with a worked example.
Formulas Used
Volume (Cubic Feet)
cubic_feet = length * width_ft * depth_ftVolume (Cubic Yards)
cubic_yards = length * width_ft * depth_ft / 27With 10% Waste
cubic_yards_waste = length * width_ft * depth_ft / 27 * 1.10Variables
| Variable | Description | Default |
|---|---|---|
length | Footing Length(feet) | 50 |
width | Footing Width(inches) | 20 |
depth | Footing Depth(inches) | 8 |
width_ft | Derived value= width / 12 | calculated |
depth_ft | Derived value= depth / 12 | calculated |
How It Works
How to Calculate Concrete for Footings
Formula
Volume = Length (ft) x Width (ft) x Depth (ft)
Convert width and depth from inches to feet by dividing by 12, then divide total cubic feet by 27 for cubic yards. Footings are typically 20 inches wide and 8 inches deep for residential construction, but always follow local building codes.
Worked Example
A continuous footing that is 50 ft long, 20 inches wide, and 8 inches deep.
- 01Width in feet: 20 / 12 = 1.667 ft
- 02Depth in feet: 8 / 12 = 0.667 ft
- 03Volume = 50 × 1.667 × 0.667 = 55.56 cu ft
- 04Cubic yards = 55.56 / 27 = 2.06 cu yd
- 05With 10% waste = 2.06 × 1.10 = 2.26 cu yd
Frequently Asked Questions
How deep should a footing be?
Footings must extend below the frost line in your area, typically 12-48 inches depending on climate. The minimum thickness is usually 6-8 inches for residential buildings.
How wide should a footing be?
A standard rule is that footings should be at least twice the width of the wall they support. For an 8-inch block wall, use at least a 16-inch wide footing.
What PSI concrete is used for footings?
Most residential footings use 2,500 to 3,000 PSI concrete. Commercial or structural footings may require 3,500 to 4,000 PSI or higher.
Learn More
Guide
How to Calculate Concrete Needed for Any Project
Learn how to calculate the exact amount of concrete needed for slabs, footings, columns, and other projects. Covers volume formulas, waste factors, and ordering tips.
Ready to run the numbers?
Open Concrete Footing Calculator