Technology of metal die casting is characterized by production of casts complicated as to shape yet with positive mechanical properties and with high repeatability of production. However, casts are porous to a certain extent which eventually reduces their mechanical properties. One of the significant methods of porosity reduction of casts rests in correct design of a gating system. The submitted paper studies the influence of cross-section area of a runner on air entrapped in the cast volume. Seven alternatives of runners with the identical structural organization and variable cross-section area were compared. In case of a gating system design there was an assumption made that the runner with the largest cross section would deliver the lowest possible velocity to the melt before reaching the runner which would result in the lowest possible values of air entrapment. The air entrapment in the cast volume is evaluated behind the cores which were evaluated as critical points with regards to further processing. The results reached during examination of the melt flowing through runners proved the aforementioned assumption, yet the values of air entrapment in die casts volume did not show remarkable differences. In its final part, the paper clarifies the reached results and recommendations which should be taken into consideration when designing the gating system structure.