NVR Storage Calculation: How Many TB Do Your Cameras Really Need?

Start with bitrate, not megapixels

The question we hear most is "how many TB does a 4MP camera need?" The honest answer: it depends on bitrate. Two 4MP cameras configured differently can use twice as much space as each other.

Storage (GB) per camera = Bitrate (Mbps) × 86,400 × days ÷ 8 ÷ 1000

86,400 is the number of seconds in a day. Divide by 8 to turn bits into bytes, and by 1000 to turn MB into GB.

A 4 Mbps camera recording around the clock: 4 × 86,400 ÷ 8 ÷ 1000 = 43.2 GB per day. Worth remembering for quick estimates on site.

Typical bitrates

H.265 needs roughly half the storage of H.264 at similar quality. Many brands add a smart codec (H.265+ and similar) that cuts further on static scenes. But night-time bitrate usually climbs because of sensor noise in low light, and so does any camera looking at moving trees or rain.

Smart codecs: useful, with two catches

Each brand has its own layer on top of H.264/H.265: H.265+ on Hikvision, Smart H.265+ on Dahua, WiseStream on Hanwha Vision, Zipstream on Axis. They lower quality on the static background, keep detail where things move, and stretch the gap between full keyframes. On a quiet corridor they can cut storage by a third or more. The catches: some third-party VMS and analytics engines don't handle them well, and long keyframe gaps make fast seeking during playback feel slow. Turn them on, then test playback and any analytics before you count on the savings.

Correct for night and rain

Your formula is only as good as the bitrate you put in. The bitrate a camera shows at 14:00 on a dry day is the lowest it will ever be. If you can, read the live bitrate on the NVR at 22:00 and during a rainy afternoon, and size from the higher figure. If you can't measure first, we add 20–30% to the daytime number for cameras with IR at night or a view of trees, roads or water.

Example: a factory with 16 cameras, 30 days

• 4MP cameras, H.265, 4 Mbps, recording 24 hours
• Per camera: 4 × 86,400 × 30 ÷ 8 ÷ 1000 = 1,296 GB ≈ 1.3 TB
• 16 cameras: 1,296 × 16 = 20,736 GB ≈ 20.7 TB
• Add 10–15% headroom → about 23–24 TB usable

Still on H.264 at 8 Mbps? You'd need about 41.5 TB. Nearly double, for roughly the same picture.

Example 2: a hotel with mixed cameras

Real sites rarely use one camera type. A 24-camera hotel, all H.265, recording continuously for 30 days:

30 days: 1.04 × 30 ≈ 31.1 TB. Add 15%: about 35.8 TB usable, which the NVR will show as roughly 32.6 TiB. Six 8 TB disks in RAID 6 give 32 TB, not enough. Eight 8 TB disks in RAID 6 give 48 TB (about 43.7 TiB), which fits with room for a few more cameras. That means an 8-bay NVR, or two smaller NVRs splitting the cameras.

Disks and RAID

An "8 TB" disk shows up in the NVR as about 7.27 TiB, because manufacturers count 1 TB = 1000 GB and the system counts in 1024s. Around 9% is gone before you record anything.

So for the 16 cameras above, RAID 5 on 4 × 8 TB is not enough once headroom and formatting are counted. We'd go for RAID 5 on 5 × 8 TB (32 TB), or RAID 6 on 6 × 8 TB if the site is remote and a replacement disk could take days to arrive. Big disks take a long time to rebuild, and if a second disk dies during a RAID 5 rebuild, the footage is gone.

Does motion recording really save space?

In some places. A warehouse at night with nobody around can save a lot. A door with traffic all day, a car park, or a camera that sees branches moving saves very little. We always size for continuous recording and treat whatever motion saves as extra headroom.

Don't forget bandwidth

• Into the NVR: 16 cameras × 4 Mbps = 64 Mbps. Easy for a gigabit uplink. Do check the NVR's incoming bandwidth limit, though: some small models only take 80–160 Mbps.
• Remote viewing: use the sub-stream (about 0.5–1 Mbps per camera). Four cameras at once needs roughly 2–4 Mbps of upload.
• When three or four managers open main streams on their phones at the same time, the office upload fills up and email slows down with it.

What breaks recording systems

1. Sizing from daytime bitrate, then finding you only keep 20 days instead of 30.
2. Desktop drives instead of surveillance drives built for 24/7 writes.
3. No RAID. One disk fails and the footage goes with it.
4. Never checking the NVR's maximum incoming bandwidth.
5. No UPS. The 17:00–19:00 brownouts when the grid is under peak load kill disks and corrupt video files.

How many days should you keep?

There is no single right number, but these are the ranges we see work in practice:

Remember that doubling retention doubles storage. If only four cameras need 90 days, put them on a separate NVR or a separate disk group instead of sizing the whole system for 90.

Checking an existing NVR in 15 minutes

1. Open playback and find the oldest recording. That is your real retention, whatever the design said.
2. Look at disk status (S.M.A.R.T.) and the RAID state. "Degraded" means one more failure loses footage.
3. Confirm overwrite is on, so a full disk doesn't stop recording.
4. Check the NVR clock and time sync. Video with the wrong time is close to useless as evidence.
5. Export a one-minute clip to a USB stick. If nobody on site can do that, train them now, not on the day of an incident.

Disks in Lao heat

Disks hate heat more than anything else in the NVR. A recorder in a guard house without air conditioning in April runs far hotter than its datasheet assumes, and disk failures follow. Give the NVR airflow, keep it off the floor where dust collects, and keep one spare surveillance disk of the same model on site. Getting a replacement through Thai distributors can take a week or more, and a RAID 5 array running degraded for a week is a risk you don't need.

What we recommend

Set retention by what you actually need: 30 days is common, while banks and cash-handling sites may need longer, so check your internal policy. Make H.265 the default. Use RAID 5 for small systems and RAID 6 once you have six or more disks or the site is in the provinces. And the NVR and PoE switches always go behind a UPS.

You can play with the numbers in our CCTV storage calculator: change the codec, days or camera count and watch the TB move. If you'd rather we work it out, just tell us two things: how many cameras, and how many days you need to keep.