Every camera on the plan writes to a disk somewhere, and the client's retention policy, thirty days say, fixes how much disk. The arithmetic is short. The assumptions inside it are where the estimates go wrong by a factor of three.

The multiplication

storage = cameras × bit rate × seconds per day × retention days ÷ 8

Twenty cameras at 4 Mbit/s for 30 days: 20 × 4,000,000 × 86,400 × 30 ÷ 8 = 25.9 TB. Round up for the file system and for a disk that will be replaced before it is full: 32 TB usable, which on a recorder with parity is four 12 TB drives.

What sets the bit rate

Resolution, frame rate, codec and scene complexity, in that order of leverage:

  • Resolution: a 4 MP stream is about twice a 2 MP stream at the same quality; 8 MP is twice that again.
  • Frame rate: bit rate scales a little less than linearly with frames. Fifteen frames a second is enough to see a hand reach a shelf; twelve is enough to see who walked in. Thirty is for reading a spinning number plate.
  • Codec: H.265 needs roughly 40 percent fewer bits than H.264 for the same picture. The vendor's smart codec, which spends bits on motion and starves the static background, cuts a quiet corridor by half again and does nothing for a busy shop floor.
  • Scene: rain, snow, trees in wind and a flickering fluorescent tube are all motion to an encoder. A car park in a storm can triple its bit rate for an hour.

Sensible planning figures for H.265 at 15 fps: 2 MP around 2 Mbit/s, 4 MP around 4, 8 MP around 8. Use the vendor's calculator for the model you specified and treat its output as a floor.

Motion recording

Recording only on motion cuts storage by 60 to 90 percent in a quiet building and by nothing at a busy entrance. It also means a still intruder is a gap in the timeline. Continuous recording at a low frame rate with a jump to full rate on motion is the usual compromise and the recorder decides it per camera.

Retention is a policy question

Thirty days is a common default, driven by how long it takes for an incident to be noticed and reported. Some sectors and some countries set a maximum as well as a minimum; a data-protection regime that says "no longer than necessary" wants a reason for every day past the one the business needs. Ask the client for the number, write it on the drawing, and size to it.

On the drawing

The schedule that lists each camera's resolution and frame rate is already the storage estimate; add a column for planned bit rate and the recorder specification writes itself. When a client asks for 8 MP everywhere, the same column shows what that costs in disks, which is usually the moment they ask which doors actually need it.