How to Calculate Concrete for Footings
Continuous footings run along the entire perimeter of a foundation wall. Their volume equals length times width times depth. For example, a footing that is 20 inches wide and 10 inches deep running 120 linear feet around a house foundation requires a straightforward rectangular volume calculation converted to cubic yards.
Spread footings are square or rectangular pads beneath columns or posts. Each footing's volume is calculated individually and then multiplied by the number of identical footings. A typical deck post footing is 24 inches square and 12 inches deep. Corner and mid-span footings may have different dimensions to handle different load conditions.
Pier footings are cylindrical, calculated using pi times the radius squared times depth. Common diameters are 8, 10, and 12 inches. Cylindrical forms (sonotubes) define the shape. The concrete volume for a round pier is roughly 21% less than a square footing of the same width and depth, which saves material on multi-pier projects.
Example Calculation
A house foundation requiring 150 linear feet of continuous footing, 20 inches wide and 8 inches deep.
- Convert to feet: 20 inches = 1.67 feet wide, 8 inches = 0.67 feet deep
- Volume = 150 x 1.67 x 0.67 = 167.8 cubic feet
- Convert to cubic yards: 167.8 / 27 = 6.21 cubic yards
- Add 10% waste: 6.21 x 1.10 = 6.84 cubic yards — order 7 cubic yards
Tips for Accurate Results
- Footings must extend below the frost line in your area. Local building codes specify the minimum depth, typically 36-48 inches in northern climates and 12-18 inches in southern regions.
- Over-excavated footing trenches should be filled with concrete, not backfill. Disturbed soil beneath footings can compact unevenly and cause differential settlement.
- Place two horizontal runs of #4 rebar in continuous footings to add tensile strength. Position rebar 3 inches from the bottom and 3 inches from the top of the footing.
- Pour footings and let them cure for at least 2-3 days before placing foundation walls on top. This allows the footing to gain enough strength to support the wall weight.
Frequently Asked Questions
How wide should a concrete footing be?
Footing width depends on the load it carries and the soil bearing capacity. A general rule is that footings should be twice the width of the wall they support. For a standard 8-inch foundation wall, footings are typically 16-20 inches wide. Weak soils with low bearing capacity require wider footings to spread the load over a larger area.
How deep do footings need to be?
Footing depth must extend below the local frost line to prevent heaving. In the northern United States, this ranges from 36 to 48 inches. In the south, 12 to 18 inches is common. The minimum footing thickness is typically 6 to 8 inches for residential construction. Always verify requirements with your local building department before excavating.
What is the difference between a footing and a foundation?
A footing is the widened concrete base that sits at the bottom of an excavation and distributes building loads to the soil. A foundation wall sits on top of the footing and extends up to or above ground level. Together they form the foundation system. The footing is always wider than the wall above it to spread weight across more soil area.
Can I pour footings and walls at the same time?
Monolithic pours combining footings and walls are possible for some construction methods, particularly insulated concrete forms. However, traditional construction pours footings first, allows partial curing, then builds foundation walls on top. Monolithic pours require more complex formwork and are best left to experienced contractors with crews large enough to handle the volume.
Do footings need rebar reinforcement?
Yes, most building codes require at least two continuous horizontal bars of #4 rebar in residential footings. Rebar provides tensile strength that concrete alone lacks, preventing cracks from differential settlement or soil movement. In areas with expansive clay soils or seismic activity, more extensive reinforcement including vertical dowels may be required by code.