How do I calculate concrete for a rectangular footing?

Multiply the footing length by its width and depth after converting all measurements to the same unit. Divide cubic feet by 27 for cubic yards, then add a practical waste allowance for excavation variation.

How much waste should I add to a footing estimate?

An 8% allowance is a practical starting point for a footing. Increase it when trench edges are loose, the bottom is uneven, or the excavation is larger than the planned dimensions.

Can I use a footing calculator for stepped footings?

Calculate each section with a consistent width and depth separately, then add the results. A single rectangular calculation does not include the additional volume from steps, pier enlargements, or bell shapes.

How Much Concrete Do I Need for a Footing?

Start with the actual footing shape

This guide applies to a straight footing with a consistent rectangular width and depth, including many wall footings, foundation trenches, and strip footings. Measure the part of the trench that will actually receive concrete. If the footing steps, widens for a pier, or changes depth, split it into separate sections.

Measure length, width, and depth

Use the total run length for the footing, then confirm the excavated width and depth. Plans may show dimensions in feet and inches, so convert every measurement to feet before calculating cubic feet. For example, 16 inches is 1.33 feet and 12 inches is 1 foot.

Use the footing volume formula

For one rectangular footing, multiply length by width by depth. The result is cubic feet. Divide by 27 to convert cubic feet into cubic yards for a ready-mix estimate.

Volume in cubic feet = length (ft) x width (ft) x depth (ft)
Volume in cubic yards = cubic feet / 27

Worked footing example

Suppose a footing is 20 feet long, 16 inches wide, and 12 inches deep. Convert 16 inches to 1.33 feet and 12 inches to 1 foot.

20 ft x 1.33 ft x 1 ft = 26.67 cubic feet
26.67 / 27 = 0.99 cubic yards

With an 8% allowance for trench variation, the planning estimate becomes about 1.07 cubic yards. This is a starting point, not a supplier order or an engineering specification.

Add a practical waste allowance

Footings usually need more allowance than a simple slab because excavated trenches are rarely exact. Loose soil, rounded corners, sloughed edges, uneven bottoms, and over-digging all increase the volume that receives concrete. Start with 8% for a reasonably clean trench, then increase the allowance when site conditions are less controlled.

Check these items before ordering

Confirm whether the footing includes steps, thickened sections, piers, or enlarged bases.
Check the actual trench dimensions after excavation, not only the dimensions on a drawing.
Confirm footing requirements, frost depth, reinforcement, and bearing conditions with the project documents and local requirements.
Ask the concrete supplier about delivery minimums, small-load fees, mix design, and timing.

Use the footing calculator

Enter the dimensions and your own local material prices in the Concrete Footing Calculator to estimate cubic yards, 60 lb bags, waste-adjusted volume, and a simple cost comparison.

Footing estimate FAQ

Can I calculate several matching footings together?

Yes. Calculate the volume for one footing and multiply by the number of matching runs, or enter the total length when the cross-section stays the same.

Does the estimate include rebar, gravel, or excavation?

No. Concrete volume is only one part of a footing project. Reinforcement, excavation, gravel, forms, labor, delivery, and local permit requirements must be planned separately.