Area is the amount of space within a shape. To calculate a rectangle's area, multiply its length by its width. When calculating the area of a vertical rectangle, such as a window, use height instead of length.

For example, a rectangle is 3 feet long, and 4 feet wide. To calculate the area, do the following:

length = 3 feet
width  = 4 feet
area   = 3 feet × 4 feet = 12 square feet

To understand why the length is multiplied by the width, think of a rectangle as a grid of smaller squares. In the example above, think of the rectangle as having 3 rows, with each row having 4 squares. The rectangle would have a total of 12 squares.