How to Calculate Concrete for a Footing

Footings carry the load of your walls and posts down to stable soil, so ordering the right amount of concrete matters. The good news: footing volume is just rectangular-prism math. This guide gives you the formula and a worked example for both continuous and pad footings.

The footing formula

A footing is a long or square block, so its volume is length times width times height. Keep every dimension in feet before you multiply, then convert to cubic yards because ready-mix is sold by the yard.

Remember there are 27 cubic feet in one cubic yard, and convert inches to feet by dividing by 12.

Step-by-step example: continuous footing

Imagine a continuous foundation footing that is 40 feet long, 16 inches wide, and 8 inches thick:

1. Convert width to feet: 16 / 12 = 1.333 ft
2. Convert thickness to feet: 8 / 12 = 0.667 ft
3. Volume in cubic feet: 40 x 1.333 x 0.667 = 35.6 cu ft
4. Cubic yards: 35.6 / 27 = 1.32 cu yd
5. Add 10% waste: 1.32 x 1.10 = 1.45 cubic yards

You would order about 1.5 cubic yards from the ready-mix plant.

Pad footings (isolated)

For posts and piers, footings are usually square pads. Multiply one pad's volume by the number of pads:

For example, four 2x2 foot pads at 12 inches thick: 2 x 2 x 1 = 4 cu ft each, x 4 pads = 16 cu ft, / 27 = 0.59 cu yd, +10% = 0.65 cu yd. That is small enough that bagged concrete may be simpler than a truck.

Reinforcement and code notes

• Rebar: Typical residential footings use #4 or #5 reinforcing bars running lengthwise, with ties at intervals. Exact sizes depend on the structure above.
• Thickness: A typical spread footing is 8–12 inches thick. Verify required dimensions with your local building department.
• Typical, not absolute: These are approximate guidelines. Always confirm requirements with local code, a structural engineer, or your contractor before pouring.

Use the calculator

Our Concrete Footing Calculator handles continuous and pad footings and outputs cubic yards plus bag counts. If your build also includes a block or brick wall above the footing, the Block Calculator and Brick Calculator will size those materials. See the full set on the Concrete & Masonry hub.

Measuring your footing on site

Before you trust any number, verify the dimensions from your plan and from the trench. Width and thickness are usually fixed by the structural design, while length is what you measure in the field. For a continuous footing, measure the full run of the wall; for pads, count every post location so none are missed.

Common footing mistakes

• Forgetting waste: Trenches are never perfectly shaped; add 10% so you are not short at the truck.
• Mixing units: Keep every dimension in feet before multiplying — the most common error.
• Undersizing: A footing that is too thin or narrow undermines the whole structure; confirm sizes with the plan or an engineer.
• Ignoring frost depth: In cold climates, footings must sit below the local frost line; this is a code requirement, not optional.