The most common surprise in a new camera system is discovering that the recorder overwrites yesterday’s footage before you can review it. This guide shows how to size NVR hard drive storage correctly, using bitrate and retention days instead of guesswork. You will get a clear formula, a real sizing table, one worked example, common mistakes, and a checklist so your footage lasts as long as you actually need it.
What really drives storage, and it is not megapixels
Storage is consumed by bitrate, the amount of data each camera streams per second, measured in megabits per second (Mbps). Bitrate depends on resolution, frame rate, scene complexity, and the codec. Two 4MP cameras can use very different storage if one watches a still wall and the other watches a busy road. Megapixels influence bitrate, but they do not decide it alone.
The codec matters most. H.265 (HEVC) typically uses roughly half the bitrate of older H.264 for similar quality. Turning on H.265 is often the single biggest storage saving available, before you buy any larger drive.
The formula
Storage in gigabytes for one camera equals: bitrate (Mbps) divided by 8, times 3600, times recording hours per day, times retention days, divided by 1000. Divide by 8 converts bits to bytes; times 3600 converts seconds to an hour. Multiply per-camera totals to size the whole system. Continuous recording uses far more than motion-only recording, so decide your recording mode first.
Sizing table
These are practical bitrate ranges for H.265 at 15 to 20 frames per second, continuous recording, for a moderately busy scene. Use the higher end for busy outdoor views.
| Resolution | Typical H.265 bitrate | Storage per camera, 24h, 30 days |
| 2MP (1080p) | 2 to 4 Mbps | about 0.65 to 1.3 TB |
| 4MP | 4 to 6 Mbps | about 1.3 to 1.9 TB |
| 5MP | 6 to 8 Mbps | about 1.9 to 2.6 TB |
| 8MP (4K) | 8 to 12 Mbps | about 2.6 to 3.9 TB |
Treat these as planning estimates, not guarantees. Your real bitrate depends on movement, lighting, and settings. Always add headroom.
A worked example
A shop wants six 4MP cameras, continuous recording, 30 days of history, using H.265 at about 5 Mbps each. Per camera: 5 divided by 8 is 0.625 MB/s, times 3600 is 2250 MB per hour, times 24 is 54 GB per day, times 30 is about 1620 GB, roughly 1.6 TB. Six cameras need about 9.7 TB. After formatting overhead and headroom, a pair of 8 TB surveillance drives, or a single 16 TB, is the sensible choice. If the shop switched to motion-only recording during closed hours, real usage could drop by a third or more.
Common mistakes and how to fix them
- Sizing by camera count alone. Fix: size by bitrate and retention days, not “1 TB per camera” rules of thumb.
- Leaving H.264 on. Fix: switch to H.265 where cameras and NVR support it, then re-check bitrate.
- Forgetting continuous vs motion. Fix: decide the recording mode first; it can double or halve storage.
- Using desktop hard drives. Fix: use surveillance-rated drives built for constant writing from many streams; desktop drives fail early under 24/7 load.
- No headroom. Fix: add 20 to 30 percent so busy days and future cameras do not force early overwrites.
Storage sizing checklist
- List each camera’s resolution and target frame rate.
- Confirm the codec; enable H.265 if supported.
- Choose continuous or motion-only recording per camera.
- Set the retention days you legally or practically need.
- Apply the formula per camera, then total the system.
- Add 20 to 30 percent headroom.
- Buy surveillance-rated drives within your NVR’s supported capacity.
- After one busy week, check the recorder’s actual used space and adjust.
Conclusion
Storage sizing is arithmetic, not luck. Know your bitrate, pick a retention target, apply the formula, and add headroom. Your next step: open your NVR, note each camera’s live bitrate for a busy hour, and use those real numbers in the formula before you buy a drive.
Frequently asked questions
How many days of footage should I keep?
It depends on how quickly incidents are noticed and any local rules. Many small sites choose 14 to 30 days. Longer retention simply means more storage using the same formula.
Does motion-only recording save much?
Usually yes, especially for quiet indoor or overnight scenes, where it can cut storage by a third or more. Busy outdoor views with constant movement save far less.
Is H.265 always better than H.264?
For storage, generally yes, since it needs roughly half the bitrate for similar quality. Confirm both the cameras and the NVR support it, and that any viewing device can decode it.
Can I just add a bigger hard drive later?
Often, but only up to your NVR’s maximum supported capacity and number of bays. Check those limits before buying, so you are not forced to replace the recorder.
Why is my real storage use different from the estimate?
Bitrate changes with movement, lighting, and settings. Estimates assume an average scene. Always verify against the recorder’s actual used space after a week of normal activity.
References
ONVIF, the widely adopted industry standard for IP-based physical security devices, defines the streaming profiles behind these bitrate and codec settings. H.265 (HEVC) is a recognised international video compression standard, and its lower bitrate versus H.264 is the basis for the storage savings described here.