The Low Frequency Array (LOFAR) is a synthesis radio telescope covering the 10–240 MHz range. LOFAR is the first operational pathfinder to the Square Kilometre Array (SKA), a future radio telescope envisaged to be at least an order of magnitude more sensitive than current instruments. LOFAR exploits the same hierarchical beamforming structure as envisaged for the SKA phased array systems. In this paper, we describe the system requirements imposed by calibratability, i.e., the ability to perform a proper self-calibration of the instrument, for the low frequency regime. We derive requirements on station size, aperture efficiency and side lobe level. We also discuss the impact of the polarimetric response of the stations. We discuss the LOFAR design choices made to satisfy these requirements and indicate their implications for SKA phased array systems. This demonstrates that calibratability imposes requirements complementary to those based on imaging requirements and that self-calibratability has a significant impact on configuration design considerations.