The formula to calculate the square footage of an area that is square or rectangular in shape is length multiplied by width equals the area's square footage. For example, if a rectangular shaped room measures 11 feet in length and 13 feet in width, 11 multiplied by 13 equals 143 square feet.
The formula to calculate square footage varies based on an area's shape. For a triangle, the width of the base is multiplied by the height of a triangle. This figure is then multiplied by a half. If the base measures 4, and the height is 5, 4 is multiplied by 5 and then multiplied by a half, equaling 10 square feet.
A parallelogram's square footage is determined by multiplying the base (a measure of width) times the height. A base of 10 and a height of 6 are multiplied together to yield 60 square feet.