How Much Storage Does a CCTV System Need? Sizing a Recorder Properly
By Troy · September 11, 2026
Storage depends far more on resolution, frame rate and codec than on the number of cameras. Sixteen 4MP cameras at 15fps on H.265, recording continuously for 30 days, comes to about 11 TB; the same sixteen at 4K and 30fps on H.264 is about 83 TB. Work out the bitrate per camera, multiply by the retention period, then add headroom — and size drives against what the recorder reports, not what the box says.
Camera count is the least important input
The first question everyone asks is "how much storage for 16 cameras", and it is close to unanswerable. Sixteen 4MP cameras at 15fps on H.265, recording continuously for 30 days, comes to roughly 11 TB. The same sixteen cameras at 4K and 30fps on H.264 is about 83 TB. Seven times the disk, same camera count.
So the useful version of the question is: how many pixels per second are you recording, how hard are they being compressed, and for how long are you keeping them. Everything else is arithmetic.
The shortcut: bits per pixel
A codec spends roughly a fixed number of bits on each pixel it encodes. That means bitrate tracks pixels per second, and you can estimate any camera from one constant. For H.264 at 30fps on a typical scene, that constant is about 0.064 bits per pixel.
Check it against the figures manufacturers publish:
| Resolution | Published bitrate (H.264, 30fps) | Bits per pixel |
|---|---|---|
| 1080p (1920×1080) | ~4 Mbps | 0.0643 |
| 4MP (2560×1440) | ~7 Mbps | 0.0633 |
| 4K (3840×2160) | ~16 Mbps | 0.0643 |
Those agree to within 2% across a four-fold range of resolution, which is what makes a single number worth using rather than a fudge. Multiply width × height × 30 × 0.064 and divide by a million, and you have Mbps.
Frame rate does not scale the way you expect
Halving the frame rate does not halve the bitrate. With fewer frames there is more change between each one, so every remaining frame costs more to encode. In practice 15fps lands near 55% of the 30fps figure, not 50%, and 5fps is nearer 22% than 17%.
This matters because dropping frame rate is the first thing people do when a job is over budget on disk, and a calculator that scales straight down will under-size the result. On a retrofit where everything runs at 10 or 12fps, that error compounds across every camera.
Codec: where the real savings are
H.264 is the baseline and still what a lot of older recorders speak.
H.265 / HEVC is usually sold as "half the bitrate". Treat it as a bit less than half — around 55% of H.264 on surveillance footage is the safe planning figure. Check the recorder supports it before you spec it, because an NVR that has to transcode gives the saving straight back.
Smart codecs — H.265+, Zipstream, Smart Codec and the rest — go further by dropping the bitrate hard when nothing is moving. The savings are real and they are also entirely dependent on the scene staying still. A camera on a quiet stockroom will beat the claim; one watching a road in the rain will not get close. Plan at about 35% of H.264 rather than the headline number, or you will run out of disk in month two.
Motion recording saves less than people think
"We'll only record on motion" is treated as a free halving of the storage. It rarely is. A motion trigger on a car park, a street frontage or anywhere with foliage fires most of the day, and a site that records 20% of the time during a quiet survey week records far more over Christmas.
Guess high, and remember what motion recording does not change: bandwidth. The cameras stream to the recorder continuously whether it is writing to disk or not. Size the switch and the uplink off the full continuous figure.
The 9% that strands a job: TB versus TiB
Drives are sold in decimal terabytes. A "4 TB" drive holds 4,000,000,000,000 bytes. Your recorder counts in binary and reports that same drive as 3.64 TiB — about 9% less.
Size a job off the number on the box and a 30-day retention spec quietly becomes about 27 days. Nobody notices until somebody asks for footage from four weeks ago and it is not there, which is exactly the request a CCTV system exists to answer. Work in the recorder's units, not the manufacturer's.
Leave room on top of that for the operating system, for RAID parity if you are using it, and for the fact that every bitrate figure in this article is an estimate.
A worked example
Sixteen 4MP cameras at 15fps on H.265, continuous, kept 30 days.
- Pixels per second: 2560 × 1440 × 30 = 110.6 million
- At 0.064 bits per pixel: 7.08 Mbps for H.264 at 30fps
- At 15fps (the 0.85 power, not half): about 3.93 Mbps
- On H.265 (0.55): about 2.16 Mbps per camera
- Sixteen cameras: about 34.6 Mbps — that is your switch load
- Per day: 34.6 × 1,000,000 × 86,400 ÷ 8 = 374 GB
- Over 30 days: about 11.2 TB, which the recorder will call 10.2 TiB
That needs three 4 TB drives, not two: two of them report 7.28 TiB, which is short. The storage calculator does all of this, and takes mixed groups of cameras rather than assuming they are all the same.
Frequently asked questions
- How much storage do I need for 16 cameras?
- It depends far more on resolution, frame rate and codec than on the camera count. Sixteen 4MP cameras at 15fps on H.265, recording continuously for 30 days, is about 11 TB. The same sixteen at 4K and 30fps on H.264 is about 83 TB. Work out the bitrate per camera rather than using a rule of thumb.
- Why does my recorder show less space than the drives I bought?
- Drives are sold in decimal terabytes and reported in binary tebibytes. A 4 TB drive holds 4,000,000,000,000 bytes, which the recorder counts as 3.64 TiB — about 9% less. Size a job off the number on the box and a 30-day retention spec becomes about 27 days.
- Does H.265 really halve the storage?
- Close to it. Around 55% of the H.264 figure is the safe planning number for surveillance footage. Smart codecs like H.265+ or Zipstream go further by dropping the bitrate on static scenes, but that saving depends entirely on the scene staying static — plan at about 35% of H.264, not the headline claim.
- How long should CCTV footage be kept?
- Thirty days is what most specifications and insurers ask for. Seven days is the usual minimum, retail often runs 14, and licensed premises or council requirements can push it to 90. Check the specification before sizing, because retention multiplies everything else.
- Does recording on motion only halve the storage?
- Rarely. A motion trigger on a car park or a street fires most of the day, and a quiet survey week is not representative of the year. Guess high — and note that it does not reduce bandwidth at all, because the cameras stream continuously whether the recorder is writing or not.
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