Concrete footings

A footing is a buried strip or pad of concrete poured below ground that supports a wall or post. A slab is a flat layer of concrete poured on the ground, usually not buried.

Footing trenches are usually measured with a tape: length in feet, while width and depth are more convenient to read in inches. The calculator converts inches to feet before multiplying.

The calculator adds 10% to the calculated volume before the bag count is worked out, to cover uneven trench walls and spills.

Measure the trench in feet for length and in inches for width and depth. Convert the width and depth to feet by dividing by 12, then multiply: length in feet x (width in inches / 12) x (depth in inches / 12) = cubic feet of concrete. That volume, plus the 10% waste allowance shown on the calculator, is the amount to order.

For a 20 foot trench that is 16 inches wide and 8 inches deep, about 33 bags of 80 lb mix are needed once the 10% allowance is included. That works out to roughly 1.6 bags per linear foot of trench. For a different trench size, enter your own length, width, and depth into the calculator to get the bag count.

A 4 inch thickness alone does not answer whether a footing will support a house. The width and depth a building needs are set by the local building code and by the soil conditions, so have the building department or an engineer specify the footing size for the load and site. Once that size is known, the calculator will convert it to a concrete quantity.