Concrete Footing Calculator

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

Choose the footing you are pouring, enter the size, and the result updates instantly with the volume in cubic yards, the bags, the weight and the cost. A strip footing is a long prism, a square pad is side by side by depth, and a round pier is a cylinder. Each one then takes the same steps: add a waste allowance, divide by 27 for cubic yards, and round bags and ready-mix up.

Worked example

A 40 ft strip footing, 16 inches wide and 8 inches deep (Strip footing type), with a 10% waste allowance. Inputs: Footing length 40 ft, Width 16 in, Depth (thickness) 8 in, Number of footings 1, Waste allowance 10 %. Result: 1.45 yd³ (about 66 × 80 lb bags).

Concrete for common strip footings

Strip footing 16 inches wide and 8 inches deep with a 10% waste allowance. Tap a length to load it above.

Method & assumptions

This calculator handles the three footings you actually pour. A continuous strip footing is a long prism, so its volume is length times width times depth. A square pad footing under a post or column is side times side times depth. A round pier footing, the kind formed in a tube, is a cylinder: pi times the radius squared times the depth. Pick the type, and the calculator uses the right geometry, keeping the length or side in feet and the width, depth and diameter in inches.

Whichever footing you choose, the steps after the volume are the same. We add your waste allowance, because trench bottoms are rarely flat and you usually over-dig a little, then divide by 27 for cubic yards. Ready-mix is rounded up to the nearest quarter yard, which is how it is sold, and bags (40, 60 or 80 lb) are rounded up because you cannot buy part of a bag. Above about one cubic yard the result recommends ready-mix over hand-mixing, and the estimated weight, at about 150 pounds per cubic foot, helps you plan access and the subgrade.

Footing size is set by the load and the soil, not guessed: a common residential strip footing is 16 to 24 inches wide and at least 8 inches deep, with the bottom below the local frost line. Pad and pier footings under posts and columns are sized to spread the point load. Confirm the size against your plans and local code, and remember most footings carry rebar, which you can size with our rebar calculator. For round columns above the footing, use the sonotube calculator.

Pro tips and common mistakes

• Dig below the frost line. A footing that sits above frost depth will heave and crack the wall or post above it. Check your local frost depth and take the bottom of the footing below it.
• Keep the bottom flat and firm. Pour onto firm, level, undisturbed soil. Soft spots and a bumpy bottom waste concrete and let the footing settle unevenly.
• Size pads to the load. A pad or pier footing spreads a column load over the soil. Too small and it punches in; size it from the load and the soil bearing capacity, per your plans.
• Place the rebar on chairs. Footing steel does nothing lying in the dirt. Support it a few inches up so concrete surrounds it, and size it with our rebar calculator.
• Order a little extra. Over-dig and an uneven bottom eat concrete. The waste allowance covers most of it, but it is better to have a wheelbarrow left over than to come up short.

Frequently asked questions

References

• Foundation footing (Wikipedia)
• ACI 332: Residential concrete code
• QUIKRETE Concrete Mix yield (spec sheet)

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