A diagonal fan has an electric motor, a housing, and a diagonal impeller received in the housing. The diagonal fan is driven via the electric motor. The fan generates a diagonal flow during operation that is deflected in an axial flow direction by an inner wall of the housing. An outlet guide vane device is arranged adjacent to the diagonal impeller when viewed in the axial flow direction. The outlet guide vane device has a plurality of guide vanes that are distributed in a circumferential direction. The outlet guide vane device homogenizes an airflow generated by the diagonal impeller. The device has an air outlet with a specified outlet diameter B. The diagonal fan extends over a total axial length E. The ratio of the total axial length E to the outlet diameter B is configured such that 0.3≤E/B≤0.6.