The camera layout is agreed, the recorder is on the quote, and then somebody asks how many days of footage it will actually keep. It is the right question, because a recorder that overwrites itself after nine days is useless when an incident is discovered on day twelve. Storage is not guesswork; it is arithmetic with a few honest inputs. Here is the method, and a worked example with deliberately hypothetical numbers so you can substitute your own.
The inputs that decide everything
A network video recorder, or NVR, stores a continuous stream from each camera, and the size of that stream is its bitrate, expressed in megabits per second. Bitrate is the only figure that matters in the end; resolution, frame rate and compression are simply the things that push it up or down.
Resolution sets how many pixels each frame contains. Frame rate sets how many frames arrive every second. Compression, whether H.264 or the more efficient H.265, decides how much of that raw picture must actually be written to disk, and modern codecs only send what has changed between frames, so a quiet corridor produces far less data than a busy loading dock at the same settings. Scene complexity, lighting and image noise at night all nudge the bitrate as well, which is why the figure a manufacturer prints in a table is a starting point rather than a promise. Our commercial camera and NVR installations are sized from measured bitrates wherever a similar camera is already running.
The formula
Storage needed equals bitrate, times the number of seconds recorded per day, times the number of days you want to keep, times the number of cameras. Because bitrate is in megabits and disks are sold in gigabytes and terabytes, divide by eight to convert bits to bytes and then scale up through the units.
A worked example
Suppose a small warehouse has 12 cameras and the integrator has set each to a moderate stream that averages 4 megabits per second. Recording runs around the clock and the owner wants 30 days retained. Four megabits per second is half a megabyte per second. Over an hour that is 1,800 megabytes, and over a day roughly 43 gigabytes for one camera. Over 30 days one camera consumes about 1.3 terabytes, and 12 cameras together need roughly 15.6 terabytes of usable space before any margin is added.
Change any input and the answer moves in proportion. Halving the frame rate roughly halves the storage. Doubling the retention doubles it. Swapping H.264 for H.265 on the same scene typically brings the bitrate down noticeably, though how much depends on how much movement the camera sees. That proportionality is what makes the formula useful for what-if planning before any hardware is bought.
Recording strategy changes the number
Continuous recording is the simplest and the safest, since nothing is ever missed. Motion-triggered recording, or a low bitrate baseline with a high bitrate stream during motion, can cut storage sharply in spaces that are empty for most of the day, such as an office after hours or a stairwell. It relies on the camera detecting the event correctly, so it suits secondary views better than the front entrance. Many systems now record continuously at low quality and switch up when the camera’s analytics see a person or vehicle, which gives most of the savings without the risk of missing something.
Retention itself should be a business decision, not a leftover. A retail unit that reviews footage weekly needs less than a condominium corporation whose board meets monthly, and some regulated sectors have retention expectations of their own. Decide the number of days first, then size the recorder to it, rather than the other way round.
RAID, spare capacity and growth
The usable figure from the formula is not the number of terabytes to buy. Recorders and servers that use RAID, a way of spreading data across several disks so that one can fail without losing footage, give up part of the raw capacity to that protection. A mirrored pair loses half; larger parity arrays lose the equivalent of one or two disks. Surveillance-rated drives are designed for the constant writing a recorder demands and are worth specifying over desktop drives.
On top of that, leave headroom. Bitrates rise at night when image noise increases, snow and rain add motion to exterior scenes, and every building adds a camera or two within a couple of years. A recorder sized exactly to today’s arithmetic will fall short of its retention target the first winter. Where the camera count is large or spread over several buildings, a centralised video management system with expandable storage is easier to grow than a fixed recorder.
Common questions
How many days of footage should a business keep?
Long enough that an incident can be discovered and reviewed before it is overwritten. Many businesses settle on thirty days as a working default because complaints, chargebacks and insurance questions tend to surface within that window. Some sectors keep longer for their own regulatory reasons, and some, such as a busy retail counter, are content with less. Choose the number deliberately and size the recorder to it.
Does 4K recording need four times the storage of 1080p?
Roughly, at the same frame rate and compression, since a 4K frame contains about four times as many pixels. In practice the gap is often smaller, because higher resolution cameras are frequently run at lower frame rates and with more efficient compression. The honest approach is to take the bitrate the camera actually produces in your scene at your chosen settings and run the arithmetic on that.
Is H.265 always better than H.264?
For storage, yes: on the same scene it produces a smaller stream. The trade-offs are that decoding it needs more processing power on the recorder and on any workstation viewing many streams at once, and some older recorders and browsers cannot play it. When the recorder, cameras and viewing stations all support it, H.265 is the sensible default and stretches retention without buying more disks.
Can I just add a bigger hard drive later?
Sometimes. Many recorders have spare drive bays or accept an external storage unit, and a server-based video management system can grow almost without limit. Small recorders often cannot, and replacing the recorder later costs more than buying the right one now. Check the expansion path before purchase, and remember that adding a drive to a RAID array may mean rebuilding it, so growth is easier planned than improvised.
If you would like a recorder sized properly for your cameras, retention and growth, we will run the arithmetic against real bitrates rather than a brochure. See how we design commercial camera and NVR systems, or get in touch and tell us how many days of footage you need to keep.
