GraphDiameter

GraphDiameter[g] gives the greatest distance between any pair of vertices in graph g.

Examples

Diameter of a path:

GraphDiameter[PathGraph[Range[10]]]
(* 9 *)

Diameter of a complete graph:

GraphDiameter[CompleteGraph[10]]
(* 1 - every vertex connects directly *)

Diameter of a grid:

GraphDiameter[GridGraph[{5, 5}]]
(* 8 *)

Please visit the official Wolfram Language Reference for more details.