What is a Concrete Footing?
A concrete footing is the lowest part of a foundation system. It is poured directly into a trench cut into the earth and acts as the wide, stabilizing base that supports the foundation walls, columns, or structural posts above it.
The primary purpose of a footing is to distribute the immense weight of the building over a larger surface area of soil, preventing the structure from sinking, settling unevenly, or collapsing over time. Without a properly sized footing, a heavy structure can literally sink into the ground.
How to Calculate Concrete Volume for Footings
Footings are essentially long, rectangular prisms of concrete. Calculating the volume is similar to calculating a slab, but the dimensions are often handled differently because footings are usually narrow and deep (measured in inches), while their length runs the perimeter of the building (measured in feet).
The Formula
To find the exact volume in cubic yards:
- Convert the Width from inches to feet (divide by 12).
- Convert the Depth from inches to feet (divide by 12).
- Multiply the Length (ft) × Width (ft) × Depth (ft) to get the total Cubic Feet.
- Divide the total cubic feet by 27 to convert to Cubic Yards.
Volume (yd³) = (Length × (Width ÷ 12) × (Depth ÷ 12)) ÷ 27
Example Calculation
Let's say you are building a garage and need a continuous trench footing that is 100 feet long, 16 inches wide, and 12 inches deep.
- Width in feet:
16 ÷ 12 = 1.33 ft - Depth in feet:
12 ÷ 12 = 1.0 ft - Total Cubic Feet:
100 ft × 1.33 ft × 1.0 ft = 133.33 cubic feet - Total Cubic Yards:
133.33 ÷ 27 = 4.93 cubic yards
Trench Irregularities and Waste Factor
Trench footings are rarely perfectly smooth or geometrically flawless because they are dug into raw earth. Rocks fall out, trench walls cave in slightly, and the bottom is never perfectly level.
Because of these irregularities, the actual volume of the trench is almost always larger than your mathematical calculation. It is an industry standard to add a 10% waste factor when ordering concrete for trench footings. Running out of concrete halfway through a continuous foundation pour compromises the structural integrity of the entire building.
Frost Lines and Footing Depth
One of the most critical aspects of pouring a footing is ensuring the bottom of the footing rests below the frost line.
In cold climates, moisture in the soil freezes and expands during winter, creating "frost heave." If your footing is above the frost line, the expanding ice can literally lift the entire building, cracking the foundation. Always consult your local building code to determine the required frost depth in your municipality before digging your footing trenches.