Free tool
CCTV Storage Calculator
Size the hard drive for a camera system from the number of cameras, resolution, frame rate, codec and how many days of footage you need to keep.
How the figure is worked out
Each camera produces a bitrate that follows its resolution, frame rate and codec. That bitrate is multiplied by the recording hours, the number of days and the number of cameras, then adjusted for how much movement the scene contains and whether the recorder only writes on motion.
H.265 roughly halves the file size against H.264 at the same picture quality, and a smart codec saves more again on static scenes. Motion recording cuts storage dramatically, but it also means anything the detection misses was never recorded, so it is a trade rather than a free saving.
Buy more disk than the calculator says. Drives are sold in decimal terabytes and formatted in binary, a recorder keeps overhead, and the day you actually need footage is always the day the retention was one week too short.