ArcLength

ArcLength[reg] gives the length of a one-dimensional region (curve).

Examples

Length of a line:

ArcLength[Line[{{0, 0}, {3, 4}}]]
(* 5 *)

Circumference of a circle:

ArcLength[Circle[{0, 0}, r]]
(* 2 Pi r *)

Please visit the official Wolfram Language Reference for more details.