Exploring p. The distance around a circleis its. . circumference.. . Exploring p. The measurement of the inside of a circleis its. area.. . The Radius. The radius is a line segment that goes from the center point to any point on your circle.r = radius.. . . . . . The diameter. The diameter is a line segment that passes through the center point of your circle extending to two opposite points on the circle.d = diameter..