You are missing one element, the depth. Without the depth of the area, you cannot calculate yards. Square feet is a measurement of area, cubic yards (typically how concrete is sold) is a measurement of volume.
The formula:
Length X Width X Depth calculates volume. If measured in feet, divide the total by 27 to get cubic yards of concrete.
The formula:
Length X Width X Depth calculates volume. If measured in feet, divide the total by 27 to get cubic yards of concrete.
