Cubic yard
The US unit concrete is sold in, equal to twenty-seven cubic feet. One yard covers about eighty-one square feet at four inches thick.
What does cubic yard mean?
Ordering is done in yards, and quantities are calculated from area and thickness with an allowance for subgrade irregularity, spillage, and form deflection. Under-ordering is far worse than over-ordering, because a short pour creates a cold joint that cannot be undone. Most suppliers charge a short-load fee below a minimum quantity.
In practice
The rule of thumb that one yard does roughly eighty square feet at four inches is the arithmetic most homeowners actually need.