Despite many years of development in computer-generated holography, perfect phase-only holograms for most target images are still yet possible to compute. All computational phase retrieval algorithms end up with some error between the target image and the reconstruction of the computer-generated hologram (CGH), except for specific targets. This research focuses on the fundamental limits of phase-only CGH quantized to limited bit-depth levels, from the information theory point of view, revealing the information capacity of CGH and its effect on reconstruction quality, with an attempt to quantify how hard a target image is for phase-only hologram computation.