How to Find the Measure of Arcs

Arc Central Angle

• 1

  Draw the arc you want to measure along the circle's perimeter onto the paper with the pencil.

• 2

  Start at the tips of both arcs and draw lines to the center of the circle with the ruler to make two radii and form the arc's angle.

• 3

  Line the protractor's edge up with either of the radii, and measure the arc's central angle in degrees. Write the result down with pencil and paper for future reference.

Arc Length

• 4

  Convert the arc's central angle into radians by multiplying it by π/180 with a calculator. For example, if the central angle is 60 degrees:

  60 degrees x π/180= 60π/180=π/3

• 5

  Multiply the radius of the arc by its central angle in radians, with "R" representing the radius and "C" representing the central angle, with the calculator. For example, if the radius is 5, and rounding to the nearest hundredth, the result is:

  Arc length = RC = 5 x (π/3) = 5.23

• 6

  Write the final result down onto the paper with the pencil for future reference.