Arc Length Arc length s is a c 2 C C 10m Use 3 14 as an approximation for this is of the whole circle 4 For any arc length s of the circumference of a circle S SA S A circle has 360 This arc has a central angle of 90 That means that 8 360 2 r 31 4 A where 8 is the central angle and r is the radius of the circle O 90 5 S B