How a continuous footing is measured
A strip footing is a simple rectangular prism: length × width × depth ÷ 12 gives cubic feet, ÷ 27 gives cubic yards. Standard residential footings pour 16–20 inches wide and 6–8 inches deep under load-bearing walls, widening to spread the wall's weight over more soil.
Before you dig
Call 811 before digging — utility marking is free and legally required in most states. The footing bottom must sit on undisturbed or compacted soil below the frost line; soft spots should be cut out and filled with extra concrete, which is exactly why ordering 10% more than the calculated volume is standard practice.