Perimeter is the total length (or the arc length) along the border or outer boundary of a closed two-dimensional plane or curve. For example, the perimeter of a circle is the total length around its boundary, while the perimeter of a polygon is the total length (or sum) of its sides. The word perimeter is derived from the Greek words "peri" (around) and "metron" (to measure).

The perimeter of a circle is commonly called the circle's circumference. With regards to the radius, r, or diameter, d, of a circle, the equation to solve for perimeter, p, is p = 2r = d, where pi, , is a constant number that is defined as the ratio of a circle's perimeter (or circumference) to its diameter, with its value approximately equal to 3.14159. As an example, if the radius of a circle is measured to be 20 centimeters and the approximate value of 3.14 is used...