How to Calculate Concrete for Footings

A simple strip footing

A continuous strip footing of uniform width and depth is calculated exactly like a slab: length × width × depth, in consistent units, converted to cubic yards or cubic metres at the end.

Measure the length along the centreline of the footing. For a closed rectangle, centreline length × width × depth gives the footing volume with no corner adjustment. If you measure along the outside faces instead, each corner is counted twice, so subtract one width × width square per corner. The Concrete Footing Calculator multiplies the length you enter, so enter the centreline length.

Footing and stem wall: two separate volumes

In a T-shaped foundation, the footing is the wide base and the stem wall is the narrower foundation wall that sits on it. Calculate each as its own rectangle and keep the two figures apart on your take-off, so each pour can be ordered on its own if they are cast separately.

A stepped footing on a sloping site is split the same way: work out each level run separately and add them.

Pad footings

A pad footing is a single block under a post or column: length × width × thickness, times the number of pads. Four pads 24 × 24 in and 12 in thick are 2 × 2 × 1 = 4 ft³ each, or 16 ft³ (0.59 yd³) in total.

Round footings and piers

A round pier or tube form is a cylinder: π × radius² × depth. A 12 in diameter pier 4 ft deep holds π × 0.5² × 4 = 3.14 ft³ (0.12 yd³); six of them hold 18.85 ft³ (0.70 yd³). The Sonotube Concrete Calculator and Concrete Pier Calculator do this arithmetic, and post holes are covered in the fence-post guide.

Minimum footing size

Footing sizes come from the drawings or the local code. In jurisdictions that adopt the International Residential Code, footings are sized from Tables R403.1(1) to R403.1(3) but must be at least 12 in wide and 6 in thick (Section R403.1.1), and must project at least 2 in past the wall but no more than the footing's thickness. That wording was checked as reproduced in the 2025 California Residential Code; the model IRC text wasn't checked, and earlier or locally amended editions may differ.

Frost depth and engineered designs govern where they apply: an engineered foundation follows its drawings.

Reinforcement is a separate calculation

If the drawings call for rebar, count it separately: rebar quantity depends on the bar spacing and layout, not on the concrete volume. The rebar guide walks through the count.