Concrete footings
A wall or post rests on a footing, a strip or pad of concrete poured below ground. The bag count and estimated cost follow from the length, width and depth of the footing trench. Enter the length in feet and the width and depth in inches, and you get the number of 80 lb bags plus a 10% waste allowance for uneven trench walls.
Footings
Tap or type into any yellow number on the drawing to change it.
Avg US retail price, checked Aug 2026.
The calculator treats the footing as a long rectangular trench. It multiplies the length in feet by the width in inches converted to feet by the depth in inches converted to feet. This gives the volume in cubic feet. Each 80 lb bag of concrete yields about 0.6 cu ft, so the bag count is the volume divided by 0.6. The calculator then adds a 10% waste factor to cover uneven trench walls and spills. The price estimate uses $5.98 as the average cost per 80 lb bag, checked Aug 2026.
Depending on the unit system chosen, the length is measured in feet or meters, while the width and depth are measured in inches or centimeters.
How does a footing differ from a slab?
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.
Why does the calculator ask for width and depth in inches but length in feet?
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.
How much more concrete does the waste allowance require?
The calculator adds 10% to the calculated volume before the bag count is worked out, to cover uneven trench walls and spills.