[header] How to calculate the area of a rectangle [title] Understand the rectangle. [step] The rectangle is a quadrilateral, which means it has four sides. Its opposite sides are equal in length, so the sides along its length are equal, and the sides along its width are equal as well. 
If one side of the rectangle is 10, for example, then the opposite side's length will also be 10. [substeps] Also, every square is a rectangle, but not all rectangles are squares.