Measure the size of the trench footing with a steel tape measure. For example, a trench footing in a warm climate may be one foot wide, 30 inches deep and 40 feet long.
Multiply the height by the width by the length of the trench. In our example, we must multiply 1 X 2.5 X 40, which reveals the trench has a volume of 100 cubic feet.
Divide 100 by 27. A cubic yard, which is three feet wide, high and deep (3 X 3 X 3), contains 27 cubic feet. For this hypothetical footing you will need 3.7 cubic yards of concrete, or about half a truck load.