CCTV storage calculator

The multiplication

storage = cameras × bit rate × 86,400 s × days × share of the day recorded ÷ 8

Twenty 4 MP cameras at 4 Mbit/s for 30 days come to 25.9 TB. The calculator adds 20% on top for the file system and for a disk array that should never run full, and that is the usable capacity to specify.

Where the bit rate comes from

The planning figure is about 1 Mbit/s per megapixel in H.265 at 15 frames a second on an ordinary scene: 2 MP around 2 Mbit/s, 4 MP around 4, 8 MP around 8. H.264 needs about two thirds more for the same picture. Frame rate scales a little less than linearly. A smart codec, which spends bits on motion and starves the static background, halves a quiet corridor and does nothing for a busy shop floor, and a busy or windy scene can run half as high again as an ordinary one.

These are planning numbers, not a vendor's. Once the cameras are installed the recorder reports the real bit rate of each stream; check it against the estimate before the retention period runs out. Recording only on motion cuts storage in a quiet building and leaves gaps in the timeline, so set the share of the day from what the site actually does. Storage and retention goes through each assumption.