We introduce the notion of diagonal ranks of semigroups,which are numerical characteristics of semigroups. Some base propertiesof diagonal ranks are obtained. A new criterion for a monoidbeing a group is obtained using diagonal ranks.For some semigroup classes we investigate whether their diagonal acts are finitely generatedor not. For the semigroups of full transformations, partial transformations andbinary relations we find the general form of the generating pairs.