UK The formula
Every NVR storage estimate starts from the same sum. A camera sends its video at a given bitrate, measured in megabits per second, and the recorder writes that stream to disk for as long as it records. Multiply the bitrate by the seconds recorded each day and by the days kept, then divide by eight, because a byte has eight bits.
Storage (MB) = bitrate (Mbps) × seconds recorded per day × days of retention ÷ 8
A day has 86,400 seconds, so one camera at 4 Mbps recording around the clock writes 4 × 86,400 ÷ 8 = 43,200 MB, about 43 GB a day. Kept for 30 days, that is about 1.3 TB for one camera. Divide megabytes by 1,000 for gigabytes and by 1,000,000 for terabytes, the same decimal measures drive makers use. The sum also works backwards: divide the space available by the daily figure to find how many days a recorder keeps.
Typical bitrates and daily storage
| Camera | Codec | Typical bitrate | Per day (24 h) | 30 days |
|---|---|---|---|---|
| 1080p (2 MP) | H.264 | 4 Mbps | 43 GB | 1.3 TB |
| 1080p (2 MP) | H.265 | 2 Mbps | 22 GB | 0.65 TB |
| 4 MP | H.264 | 6 Mbps | 65 GB | 1.9 TB |
| 4 MP | H.265 | 3 Mbps | 32 GB | 0.97 TB |
| 4K (8 MP) | H.264 | 16 Mbps | 173 GB | 5.2 TB |
| 4K (8 MP) | H.265 | 8 Mbps | 86 GB | 2.6 TB |
These are estimates for continuous recording of a typical scene at 25 to 30 frames per second. Real bitrates swing widely: a busy road, rain, trees moving in the wind or low-light noise can double them, while a quiet corridor with a low-noise image may need half. Our comparison of H.264 vs H.265 explains why the newer codec needs roughly half the bits for similar quality.
What changes the answer
The formula is fixed, but each input moves with the way the system is set up.
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Recording mode. Motion or event recording writes only when something happens. A quiet perimeter camera might record 10 to 30% of the day, while a busy entrance may record almost continuously, so size from the busiest cameras rather than the average.
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Frame rate. Dropping from 25 to 12.5 frames per second often cuts the bitrate by a third to a half. It rarely halves it, because the key frames that refresh the whole picture are still sent.
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Bitrate control. Constant bitrate makes storage predictable. Variable bitrate saves space on quiet scenes but rises on busy ones, so size from its maximum or from an average measured over a busy day.
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Second streams. Some recorders also store each camera’s low-resolution sub-stream for fast search and remote viewing. Add its bitrate if so.
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Hours in service. A bus in service for 12 hours records 43,200 seconds a day, not 86,400. Vehicles and shift-based sites need far less space than the 24-hour figures suggest.
From raw figure to drives
The formula gives the space the video needs. The drives need to be bigger, for three reasons.
First, headroom. Keep 15 to 20% of the space free so the file system works efficiently and the recorder can overwrite the oldest footage cleanly. Second, protection. RAID 1 mirrors two drives, so usable space is half the total, and RAID 5 gives up one drive’s capacity to parity. Third, the way capacity is counted. A drive sold as 16 TB shows about 14.5 TB in most operating systems, which count in binary multiples.
Plan for growth as well. Cameras added later, or a move from 1080p to 4 MP, raise the daily figure for every day of retention, so leave space or spare drive bays for them.
A worked example
| Step | Assumption | Result |
|---|---|---|
| Per camera per day | 4 MP camera, H.265 at 3 Mbps, 3 × 86,400 ÷ 8 | 32.4 GB |
| All cameras per day | 16 cameras | 518 GB |
| Retention | 30 days | 15.6 TB |
| Headroom | Add 20% | 18.7 TB |
| Drives | RAID 5 with four 8 TB drives | 24 TB usable |
For a vehicle the same method gives much smaller numbers. Eight 1080p H.265 cameras at 2 Mbps on a bus running 12 hours a day write 2 × 43,200 ÷ 8 = 10.8 GB each, 86 GB a day for all eight and about 605 GB a week. A 1 TB SSD therefore keeps around nine days with 20% headroom before it starts overwriting.
Choosing drives for continuous recording
Surveillance writes around the clock, so endurance matters as much as capacity. Hard drives rated for surveillance workloads suit fixed sites with long retention. In vehicles and on outdoor masts, SSDs cope better with vibration and temperature, but check their write endurance (TBW): 16 cameras at 3 Mbps write about 0.5 TB a day, or roughly 190 TB a year. Recorders that write across several drives at once spread that load, which helps both speed and endurance.
Our industrial NVRs take SSDs in front-access trays with RAID 1 or 5, so a failed drive can be swapped while recording carries on. For a wider view of recorder design, cameras and power, see our guide to PoE in surveillance and NVR systems.
Quick answers on NVR storage
NVR storage: common questions
Does H.265 always halve storage?
Not always. Savings of 30 to 50% over H.264 are typical, depending on the scene, the camera and its encoder settings.
How long should footage be kept?
As long as your policy requires. Many sites keep 14 to 31 days. The right period comes from your own risk assessment and the data protection rules where the cameras operate.
Is there a quick figure per camera?
About 22 GB a day. That is a 1080p H.265 camera at around 2 Mbps recording continuously, or about 0.65 TB a month.
What happens when the drives are full?
The oldest footage is overwritten. Most recorders loop, so capacity sets the retention period unless footage is locked or exported first.
Does recording audio add much?
Very little. A typical compressed audio stream is a few tens of kilobits per second, well under 1 GB a day, which is small beside the video.
We fit the drives and set up RAID as standard, so the recorder arrives sized for the retention you need.