The formula for finding perimeter of an object is based on the type of geometrical object. 1  Perimeter of a Square = 4L. If one side of a square measures 2 inches then the perimeter of the square is 2 in + 2 in + 2 in + 2 in or 8 inches.lol. 2  Perimeter of a Rectangle = 2W + 2L. If the width of a rectangle measures 4 inches and the length measures 2 inches then the perimeter of the rectangle would be 2 in + 2 in + 4 in + 4 in or 12 inches. Perimeter of a Triangle = A + B + C. Literally, this translates to side + side + side.