How much storage does CCTV need? The arithmetic, with real numbers

Almost every system we are called out to has the same complaint: "it only keeps four days". That is not a fault. It is arithmetic that nobody did before buying the drive, and it takes two minutes.

The calculation

Storage per camera per day, in gigabytes, is roughly:

bitrate (Mbps) ÷ 8 × 3600 × 24 ÷ 1000

A 4MP camera recording H.265 continuously sits around 4 Mbps for a normal scene. So: 4 ÷ 8 × 3600 × 24 ÷ 1000 ≈ 43 GB per day.

Four of those cameras is about 173 GB a day. A 2 TB drive therefore holds a little under twelve days, and a 4 TB drive a little under twenty-four.

Treat that as an estimate, not a promise

H.265 is variable-bitrate: it spends bits on movement. A camera watching an empty corridor might average a third of the figure above; one watching a busy road at night, with the infrared on and rain in frame, can exceed it. Anybody who quotes you an exact number of days is guessing with more confidence than the format allows. Plan for the higher end and you will not be disappointed.

Motion recording changes the answer completely

Recording only on motion typically cuts storage by half to three-quarters for an indoor camera and rather less for a busy exterior. It is the single biggest lever you have, and the cost is the couple of seconds before the camera decides something is happening.

Our advice: continuous on the till and the entrance, motion on everything else. Those are the two you will actually scrub through.

Buy a surveillance drive, not a desktop one

A desktop hard drive is designed to be idle most of the time. A CCTV drive is written to twenty-four hours a day, forever, and desktop drives fail early doing it — usually silently, so you find out when you need the footage. Surveillance-rated drives are built for that duty cycle and cost very little more. See surveillance hard drives.

If you are using Wi-Fi cameras instead

The same arithmetic applies to a microSD card, and the numbers are unforgiving: a 256 GB card at 43 GB a day is under six days of continuous recording. This is why almost every Wi-Fi camera defaults to motion recording, and why cloud storage exists. If you need weeks of history, you want a recorder and a hard drive rather than a card.