Most video statements of work say something like recorded video shall be retained for a minimum of 30 days. Then the conversation turns to cameras: resolution, lenses, analytics. The retention line gets one row in the bill of materials, a recorder with “enough” drives.
Thirty days later, at the first quality check, someone pulls video from day 29 and it is not there.
Retention is decided by five numbers, and none of them is the camera’s megapixel count.
1. The bitrate the camera actually writes
Storage is bitrate multiplied by time. A 1080p camera at 15 frames per second, encoding H.265 on an ordinary lobby scene, averages somewhere around 1.5 Mbps. The same camera looking at a parking lot in the rain, or at trees moving in the wind, can write two or three times that.
Estimates are useful for planning. For a retention commitment, get the real number: set the stream up, let it run through a busy period, and read the average bitrate from the camera or the recording platform.
2. How much of the day it records
Continuous recording is simple arithmetic. Motion-triggered recording is a bet on how quiet the scene is, and people consistently bet low. Headlights, rain, insects drawn to infrared at night and HVAC-driven shadows all trigger recording.
If you plan on motion recording, measure the duty cycle on site for a full week before you depend on it.
3. Which terabyte you are counting
Drives are sold in decimal terabytes: 10¹² bytes. Operating systems and most recording platforms report binary tebibytes: 1024⁴ bytes, about 10% larger, so the same disks show a number about 9% smaller. An array sold as 100 TB shows up as roughly 91 TiB.
Nine percent is three days of a 30-day requirement. Size in one unit and say which one.
4. What the array gives up for protection
A recorder holding evidence should survive a drive failure, which means parity. Twelve 12 TB drives in RAID 6 hold ten drives’ worth of data, 120 TB of the 144 TB installed. Add a hot spare and it is 110 TB.
With large drives, RAID 6 rather than RAID 5 is the right default. A 12 TB drive takes well over a day to rebuild on a recorder that is still writing video, and RAID 5 loses everything if a second drive fails or an unreadable sector turns up during that day.
5. Headroom
Recording platforms keep free space before they start overwriting the oldest video, filesystems take a few percent, and somebody always adds cameras. Fifteen to twenty-five percent headroom over the calculated payload is normal practice.
Putting it together
Thirty-two 1080p cameras recording continuously, averaging 1.3 Mbps each after variable bitrate, need about 13.7 TB of raw video for 30 days. With 20% headroom, that is about 16.4 TB to buy, or roughly 15 TiB as the operating system will show it.
Run the other direction: a 12-drive RAID 6 array of 12 TB drives, with 5% reserved, holds about 114 TB. At an aggregate 160 Mbps of video, that is about 66 days.
Both numbers come straight out of two tools in our ESS Toolbox: the VMS storage calculator and the RAID capacity and retention calculator. Each shows its formula and every assumption it makes, so you can check the arithmetic before you sign for the hardware.
Then verify it
The only proof of 30-day retention is video from day 30. Put a retention check in the commissioning plan: on day 31, pull footage from the oldest expected date on every camera, and record that you did. It is the cheapest test in the whole project and the one most often skipped.