How Long Does a 4TB Hard Drive Last for Security Cameras?
Estimate 4TB NVR retention by camera count and bitrate for continuous recording, with worked 4-camera, 8-camera and 16-camera examples.
A 4TB surveillance drive can hold months of one low-bitrate camera or only a few days of a high-bitrate multi-camera system. Retention is determined by aggregate bitrate, not simply by resolution or channel count.
Continuous-recording examples
| System | Aggregate bitrate | Theoretical 4TB retention |
|---|---|---|
| 4 cameras × 2 Mbps | 8 Mbps | ~46 days |
| 4 cameras × 4 Mbps | 16 Mbps | ~23 days |
| 8 cameras × 4 Mbps | 32 Mbps | ~11.6 days |
| 8 cameras × 6 Mbps | 48 Mbps | ~7.7 days |
| 16 cameras × 4 Mbps | 64 Mbps | ~5.8 days |
The table uses simple decimal-capacity math before filesystem, formatting and recorder overhead, so practical retention will be lower. Variable bitrate also changes real use.
Why two 4K systems can have very different retention
One 4K camera may average 5 Mbps while another is configured at 12 Mbps. Codec, frame rate, image complexity and quality settings drive the final bitrate. That is why a bitrate-based calculator is more useful than a generic “4K equals X days” claim.
Motion recording can extend calendar retention
If the recorder writes only 25% of the day, the same disk can preserve roughly four times as many calendar days as 24/7 recording in a simplified model. Busy scenes, rain, insects and trees can push the actual recording duty cycle much higher.
Leave overwrite margin
Do not design a legal, insurance or business retention requirement to the exact theoretical day. If you truly need 30 days, size above 30 days on paper so normal variation does not erase evidence early.
Use the NVR Hard Drive Calculator for your exact bitrate and retention requirement. Compare with 8TB retention.