The complexity of a crystal structure is a concept which does not have a clear definition. It is partially reflected by the amount of information related with all atoms in the unit cell: a crystal structure of more different atoms is regarded to be more complex. However, such an approach refers to the complexity of chemical units rather than the complexity of their configuration, especially for a molecular crystal. We present the analysis of informational entropy calculated on the basis of site symmetries in a space group, together with the analysis of minimal generating set cardinalities of the space groups. This approach ignores the chemical nature of the units in a crystal and compares crystal structures by the complexity of their 3D arrangement. The complexity of a crystal gradually increases from the lower to higher crystal systems; however, it is non-uniform within a given crystal system. Crystals of symmorphic space groups are generally more complex than those of non-symmorphic groups.