There is a very simple relation to calculate the length of an arc, but to understand the relation we have to first know where it comes from.
Arc is a portion of circumference or the curved path with a constant radius stretching from point A to point B. Arc length is the measure of the length of this arc. Arc have a central point around which it is curved, it is the center of the circle that would be formed if the arc is given a full 360o rotation. The other characteristic it has is the angle between the started and ending points if a line was stretched between the end points of arc and the center point, then the angle between these two lines will be the central angle.
All of these terminologies may be getting overwhelming but just take a breath, and look at the figure illustrated below, this will clear all the doubts and concepts about arcs.
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