This is the question we are asked more than any other, usually halfway through a quote, and the honest answer is that it is arithmetic rather than a trade secret.
You can work yours out in the calculator and skip the rest of this page. If you would rather understand the number before you trust it, read on.
The formula
Storage is four things multiplied together:
- How many cameras are recording
- The bitrate each one produces, in megabits per second
- How many hours a day each camera records
- How many days of footage you have to keep
That gives you a figure in megabits. Divide by eight for megabytes, then by a thousand twice to reach terabytes.
Why the units matter more than they should
Here is where most published calculators go wrong, and it is worth knowing because it decides which drive you buy.
A hard drive labelled 4 TB holds four trillion bytes. That is the decimal convention, and it is what is printed on the box. Plenty of calculators divide by 1024 instead, which is the binary convention, and then print the answer with a "GB" label anyway.
Mixing the two lands you roughly ten percent adrift. On a small system that is an inconvenience. On a system where the answer was already close to a drive size boundary, it is the difference between the recorder holding thirty days and holding twenty-six.
Our calculator uses decimal throughout, because the whole output is "which drive do I buy" and drives are sold decimal.
Bitrate is the number people guess at
Resolution gets the attention, but bitrate is what fills the disk. A 4MP camera on a low bitrate can consume less than a 2MP camera set generously.
Typical figures are a starting point, not gospel. What actually drives the number is:
- Motion in the scene. A car park at night compresses to almost nothing. A busy street with trees moving does not.
- Light. Low light introduces sensor noise, and noise does not compress. Cameras frequently use more storage overnight than during the day.
- The codec. H.265 typically produces meaningfully smaller files than H.264 for the same picture.
If you have a system already installed, read the real bitrate off the recorder rather than using a table. Your own figure beats anybody's published one.
Add margin, then round up to a real drive
Two adjustments turn a calculation into a purchase.
First, add about ten percent. Scenes get busier, someone raises a bitrate, and a drive that is completely full is a drive that has stopped recording the thing you needed.
Second, round up to a size that is actually sold. There is no 7 TB drive. Working out that you need 6.8 TB means buying 8 TB, and it is better to know that before you quote.
A worked example
Eight cameras, 4MP, H.265, recording continuously for thirty days at 4 Mbps each:
- 8 cameras at 4 Mbps is 32 Mbps
- 32 Mbps is 4 megabytes per second
- Over a day that is 345,600 MB, so about 345 GB
- Over thirty days, about 10,368 GB, so about 10.4 TB
- Add ten percent margin: about 11.4 TB
- Buy 12 TB, or two 6 TB drives if the recorder takes them
Then check the recorder can take it
The last step people miss. A recorder has a fixed number of drive bays and a maximum size per bay. Working out that you need 24 TB is only useful if the box in front of you will hold it.
If you are specifying a system rather than expanding one, tell us the retention you need and we will size the recorder and the drives together. The survey is free.
