This work shows the quantification of the flexibility in power distribution systems in the scenario in which non-conventional renewable sources are connected to it. From a set of metrics available in the literature, one is selected based on its applicability to operational and distribution system planning scenarios. The theoretical foundation and detail of its computational implementation is shown. On the basis of this, its calculation is addressed for a distribution system in which non-conventional renewable sources and storage systems are present. From the results it is possible to identify quantifiable characteristics of flexibility to the variation in the operation of this type of systems.