Perimeter is the distance around a shape. To calculate a rectangle's perimeter, add its length and width, and multiply the sum by 2, since a rectangle has 2 sides that are the length, and 2 sides that are the width. When calculating the perimeter 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 perimeter, do the following:

length    = 3 feet
width     = 4 feet
perimeter = (3 feet + 4 feet) × 2 = 14 feet