The industry standard of measurement for production and sale of concrete is the "cubic yard" (or, if using the metric system, "cubic meter").
Concrete is sold in increments of 1/2 cubic yard with a minimum order being 1 cubic yard. 1 cubic yard = 27 cubic ft.
Take the square footage (length x width) of the area to be poured and divide by:
162 for a 2 inch thick slab (capping, crawlspace dust cover etc.).
81 for a 4 inch thick slab (patios, side walk, general flatwork).
54 for a 6 inch thick slab (driveway aprons, industrial slab).
Columns, Piers, Round slabs:
To calculate the volume (cubic ft.) of a cylinder multiply (.785) x (raduis squared) x (height) then divide by (27) to convert to cubic yards.
To calculate the volume (cubic ft.) of a wall take the square footage (length x height) of the area to be poured and divide by:
162 for a 2 inch thick wall.
81 for a 4 inch thick wall.
54 for a 6 inch thick wall.