How to Calculate Concrete for Footings

A footing spreads a structure's load onto the soil so it doesn't sink or shift. The good news for estimating is that most footings are just rectangular prisms, so the concrete is straightforward: length × width × depth, divided by 27 for cubic yards. The harder part is sizing them correctly, which is a code and engineering question, not a math one.

Strip (continuous) footings

A strip footing runs continuously under a wall. To estimate it, measure the total length, the width (often 16–24 inches), and the depth (often 8–12 inches). For example, a 40-ft footing at 24 inches wide and 12 inches deep is 40 × 2 × 1 = 80 cubic feet, which is about 3 cubic yards. Multiply per run and add any returns and corners.

Spread footings (pads)

Spread footings are isolated square or rectangular pads under posts, columns or piers. Each is width × length × depth, multiplied by the number of pads. These frequently pair with something above — a column or a tube on a footing — and for the combined tube-plus-pad case the bell pier calculator adds both volumes together for you.

The frost-line rule

Sizing isn't just about the math. Footings must usually sit below the local frost line so they don't heave when the ground freezes and thaws — that depth varies widely by region. Footing width, depth, and reinforcement are governed by the load, the soil's bearing capacity, and your building code, so for anything structural, follow the code and an engineer's design. The estimating below is for ordering concrete, not for sizing the footing.

Once your dimensions are set, the footing calculator returns cubic yards, bag counts and truck loads, and most footings want rebar — estimate that with the rebar calculator.