Selection prerequisites: One of the following:
•One or more arcs
•A circle and two points on the circle
•A circle and three points on the circle
This Measure menu command measures the length of each selected arc using the distance units chosen on the Units panel of the Preferences dialog box.
If one or more arcs are selected, this command measures the length of each selected arc. 

If a circle and two points are selected, this command measures the length of the minor arc on the circle defined by the two selected endpoints. . 

If a circle and three points are selected, this command measures the length of the minor or major arc on the circle that starts at the first selected point, passes through the second point, and ends at the third point. 
See also: