Quick answer: for four 4 MP cameras and thirty days of storage using motion-detection recording, estimate 4 to 8 TB. If that's all you wanted to know, pick 6 TB and you'll be fine. To understand where this number comes from and how to reduce it, read on.
The Formula
The calculation is based on three variables:
Volume (TB) = bitrate (Mbps) × number of cameras × hours per day × days ÷ 23,000
(The divisor 23,000 converts megabit-hours to terabytes.)
The bitrate mainly depends on resolution and compression:
| Resolution | Typical Bitrate (H.265) |
|---|---|
| 2 MP (Full HD) | 2 to 4 Mbps |
| 4 MP | 4 to 8 Mbps |
| 5 MP | 5 to 10 Mbps |
| 8 MP (4K) | 8 to 16 Mbps |
The range is wide because a fixed, calm scene (an empty hallway) generates much less data than a busy scene (a street with moving trees). Compression uses variable bitrate.
Sizing Table
For 30 days of archiving, with motion-detection recording:
| Cameras | 2 MP | 4 MP | 5 MP | 8 MP / 4K |
|---|---|---|---|---|
| 2 | 1 to 2 TB | 2 to 4 TB | 3 to 5 TB | 4 to 8 TB |
| 4 | 2 to 4 TB | 4 to 8 TB | 6 to 10 TB | 8 to 16 TB |
| 8 | 4 to 8 TB | 8 to 16 TB | 12 to 20 TB | 16 to 32 TB |
| 16 | 8 to 16 TB | 16 to 32 TB | 24 to 40 TB | 32 to 64 TB |
For continuous recording instead of motion-detection, double these values.
The Three Settings That Change Everything
1. H.265 instead of H.264
The H.265 (HEVC) codec reduces bitrate by approximately 40 to 50% for equivalent image quality. Recent Dahua models support it, and many add an H.265+ or Smart Codec variant that further optimizes static areas. Simply verify that your NVR and playback software also support it.
This is the most cost-effective setting: it almost halves your storage needs without any additional cost.
2. Motion-detection recording
Recording 24/7 when nothing is happening at night in a closed warehouse is pure waste. Triggered recording by detection typically reduces volume by 50 to 80% depending on scene activity.
One precaution: configure a pre-recording of a few seconds, otherwise you'll lose the entry into the field, which is often the most useful moment.
3. WizSense
This is where the real savings come into play for outdoor environments. A conventional motion detection camera records moving branches, rain, headlights, and shadows at sunset. WizSense cameras only trigger on human or vehicle presence.
For a wooded garden, the gain often exceeds 70% of the volume, and you also get notifications that become readable again.
Choosing the right drive, not just the right size
This is the component that's often overlooked, and the one that fails first. A surveillance drive writes continuously, 24/7, 365 days a year. An office drive is not designed for this workload: it will last a few months, then fail — usually without warning, and often on the day you need the footage.
A surveillance drive offers firmware optimized for permanent sequential writing, vibration tolerance when multiple drives coexist in a bay, error management that prioritizes video stream over re-reading, and a significantly higher annual workload rate.
If your recorder has multiple bays, two medium-sized drives are better than one very large one: the failure of one does not destroy the entire archive.
How many days do you really need to keep?
Seven days is sufficient for domestic use: a break-in is usually noticed within hours. Thirty days is the professional standard, and often the minimum expected by an insurer or in a specification. Beyond three months, the question becomes legal as much as technical: retaining identifiable personal images longer than necessary poses a proportionality problem under GDPR.
In Belgium, any camera installation must also be declared via the online portal provided for this purpose, and compliant signage must be displayed at the entrance to the monitored area. The declaration is free.
Frequently Asked Questions
What happens when the disk is full?
Recording loops and overwrites the oldest data. So the disk never truly fills up: its capacity determines your archive depth.
Is a microSD card in the camera enough?
It helps out in an isolated spot or acts as a buffer in case of a network outage. Its limitation is obvious: if the camera is stolen or destroyed, the images go with it.
Can I add a drive later?
Yes, if the recorder has a free bay. This is the main argument for not getting a single-bay model from the start.
Are the announced TBs real TBs?
No. A 4 TB drive shows approximately 3.6 TB usable. Factor in this 8 to 10% margin in your calculation.
Storage and memory cards · Dahua NVR recorders · 4 MP or 8 MP? · Which NVR to choose?