Circumference = 2 × π × r = 2 × 3.14286 × 13 = 81.7143 cm. Circumference is the distance around the circle, so the answer is a length unit, not a square unit. When radius is given, double it first because the diameter is 2r, then multiply by π. Circumference is not area; it is the boundary length around the circle, so the unit is not squared.