When considering the shortest paths between two vertices (distance), the diameter of the graph is the number of edges in the longest of those distances. Therefore, to find the diameter of a graph, you need to star by finding the shortest path between each pair of vertices. The longest lenght of one of these paths os the diameter of the graph.
Example
The diameter of the graph below is 2. The longest distance between the vertice A (for example) and any of the other vertices in the graph is 2.
