Measure one side of a square and multiply it by four to get your perimeter. For example, a square that measures 4 inches would have a perimeter of 16 inches because all of the sides of a square are equal.
Calculate the perimeter of a rectangle by measuring the length and then the height. Multiply each of those measurements by two (to account for their matching sides) and add up your answers to get your perimeter. For example, a rectangle that is 6 inches long and 2 inches wide would have a perimeter of 16 inches.
Record the measurement of each side of a polygon that does not have any sides of the same length. Add those measurements together to obtain your perimeter. For example, if a triangle has sides of 2 inches, 4 inches and 7 inches, its perimeter is 13 inches.
Find the measurement of one side of any polygons that have equal sides. Multiply that measurement by the number of sides to discover the perimeter. For example, a pentagon with one side measuring 5 inches would have a perimeter of 25 inches. An octagon with one side measuring 5 inches would have a perimeter of 40 inches.