Recorder disk is the part of a CCTV quote most often guessed at, and the guess is usually wrong in the same direction — too small, discovered a month later when footage you needed has already been overwritten. The calculation is simple enough to do before you order.
The calculation
Four numbers multiply together:
Cameras × bitrate × recording hours per day × retention days
Bitrate is the one people do not know. It depends on resolution, frame rate and how much movement is in the scene — a static storeroom compresses far better than a busy shop floor at the same resolution. Camera datasheets quote a range; use the upper end, because a system sized on the lower end fails on the busiest day.
Decide retention first
This is the number that should drive the order, and it is a business decision rather than a technical one:
- 7 days — enough to review an incident someone reported promptly
- 15 days — covers a fortnight, catches slower-surfacing issues
- 30 days — the common commercial choice; covers a monthly cycle
- 90 days — where policy or insurance requires it
Ask what the longest realistic gap is between something happening and someone asking to see it. Stock discrepancies found at a monthly count need thirty days; a customer dispute reported the same afternoon needs two.
Continuous or motion-triggered
Continuous recording is predictable and the simplest to reason about. Motion-triggered recording extends retention substantially at the same disk, because most cameras watch nothing happening for most of the day.
The trade-off is the seconds before the trigger. Configure pre-record buffering, or you get footage that starts just after the moment you needed.
A mixed approach usually works best: continuous on entrances and tills, motion-triggered on corridors and storerooms.
Resolution multiplies through everything
Raising resolution raises bitrate, and bitrate appears in the multiplication — so resolution affects disk far more than it affects camera price. This is why we stock mostly 4MP: enough to identify a face at an entrance without forcing an oversized disk. See the range on our CCTV camera page.
Use higher resolution where identification matters and lower where you only need to detect movement. Applying one resolution to every camera is what makes disk requirements balloon.
Do not fill the disk
Leave headroom. A recorder running at capacity overwrites sooner than the calculation suggests, and adding cameras later reduces retention across every camera at once. Size for the cameras you expect within a year, not only those going in now.
Channels matter too
Choose channel count above camera count — a 16-channel recorder for twelve cameras leaves space for positions you identify after the first month of footage. Adding channels later usually means replacing the recorder, which costs more than buying the larger one at the start.
What sits around the recorder
The recorder needs a rack, a PDU and cable management, alongside the PoE switch feeding the cameras. Sixteen cameras at 4 Mbps is 64 Mbps of continuous traffic to the recorder, so the uplink should be gigabit. See our switches, racks and PDUs, and Cat6 cable.
Send us the two numbers
Tell us your camera count and your retention target, and whether recording is continuous or motion-triggered. We will work out the disk, the channel count and the switch capacity, and quote the whole system with GST invoicing.
Star Networking Services has supplied networking and security products in Chennai since 1998, across more than 300 projects.
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