NVR vs DVR and Storage Sizing: Calculating Required HDD Capacity for Frame Rate, Resolution and Retention Period

Quick Answer: NVR (Network Video Recorder) records from IP cameras via ethernet; DVR (Digital Video Recorder) records from analogue cameras via coaxial cable. For new installations, always use NVR. Storage sizing formula: HDD (GB) = bitrate (Mbps) × number of cameras × 0.0075 × hours/day × days retention. A 4-camera system with 4MP cameras at 4Mbps per camera, recording 24/7 for 30 days, needs approximately 520GB. Allow 20–25% overhead for file system and OS. Most 4–8 camera domestic/small commercial systems use a 1–4TB HDD.

Summary

Choosing the right recording system and sizing the storage correctly are fundamental to a functional CCTV system. Too little storage and the system overwrites useful footage before an incident is reported; too much and you are wasting budget. The recording device also determines the system's capability for remote access, analytics, and expansion.

The shift from DVR to NVR reflects the broader transition from analogue to IP camera technology. DVRs accept coaxial BNC connections and record analogue or analogue-HD video. NVRs accept ethernet connections and record digital IP video. In a new installation, there is rarely a reason to choose DVR — IP cameras provide better resolution, more flexibility, and easier installation over greater distances. The only exception is retrofitting existing coaxial cable infrastructure, where an HD-analogue system (AHD, TVI, CVI) with a hybrid DVR may be more cost-effective than re-cabling.

Storage sizing is driven by four variables: bitrate (how much data each camera generates per second), number of cameras, recording hours per day, and retention period. Most modern NVRs and cameras display bitrate in the configuration menu — this is the most accurate input to the calculation. Alternatively, use manufacturer published bitrates for the camera model at the target resolution and frame rate.

Key Facts

Quick Reference Table

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Camera Resolution H.265 Bitrate (typical) H.264 Bitrate (typical) Storage per Camera per Day (H.265, 24hr)
2MP (1080p) 1.5–2.5 Mbps 3–5 Mbps 16–27 GB
4MP (2688×1520) 2.5–4.5 Mbps 5–9 Mbps 27–49 GB
8MP (4K UHD) 4–8 Mbps 8–16 Mbps 43–87 GB
12MP 8–16 Mbps 16–32 Mbps 87–173 GB

Bitrates vary significantly with scene complexity, frame rate, and camera quality. Verify actual bitrate in camera configuration.

Detailed Guidance

The Storage Calculation

The fundamental formula:

Storage (GB) = Bitrate (Mbps) × Cameras × 0.0075 × Hours/day × Days

Where 0.0075 converts Mbps to GB/hour (1 Mbps = 0.45 GB/hr; simplify to 0.0075 GB per Mbps per minute ≈ 0.45 GB/Mbps/hr).

More precisely:

Worked examples:

Example 1 — 4 domestic cameras, 30 days:

Example 2 — 8 commercial cameras, BS 8418, 31 days:

Example 3 — 16 camera commercial, motion-only recording (12 active hrs/day):

Frame Rate Selection

Frame rate (frames per second, fps) significantly affects both image quality and storage. Key considerations:

25fps (full rate):

15fps:

6fps:

Variable frame rate: Many modern NVRs support different frame rates per channel, and some cameras support reduced frame rate during periods of no motion (e.g. 1fps when static, 25fps on motion). This significantly reduces average storage without sacrificing critical event quality.

NVR Selection Criteria

Channel count: Never fill every channel at installation. Allow at least 20% spare capacity for future cameras. An 8-camera site needs a 10 or 16-channel NVR. Expanding the channel count later requires either purchasing a new NVR or adding a sub-NVR — expensive compared to specifying slightly more capacity upfront.

Maximum total bandwidth: Aggregate all cameras' bitrates and check against the NVR's maximum bandwidth:

CPU/processing power: For systems with multiple channels of high-resolution cameras plus AI analytics (person detection, vehicle detection, face recognition), processor performance matters. Entry-level NVRs struggle with simultaneous 4K decoding for display and recording. Specify NVRs from reputable manufacturers with sufficient processing power for the intended use.

Operating system: Most NVRs run embedded Linux. These are generally stable and resistant to malware. Avoid connecting the NVR to the internet with default passwords — CCTV NVRs are frequently targeted by botnets (Mirai and successors). Always:

Remote access:

HDD Selection and Maintenance

Surveillance-grade HDDs:

Standard desktop HDDs (WD Blue, Seagate Barracuda) are not rated for continuous write operation. They may work initially but typically fail within 1–2 years in CCTV service. The cost saving over surveillance HDDs is minimal; the cost of failure (replacement, lost footage) is significant.

HDD sizing tips:

RAID considerations:

When Footage Runs Out — Retention Checks

Set a reminder to verify actual footage retention after installation:

  1. After 30 days, check the NVR is still recording to the expected timeframe — if storage has run out faster than calculated, reduce bitrate or frame rate
  2. Verify that overwrite mode is enabled — some NVRs default to "stop recording when full" which means the system stops recording silently without alerting the customer
  3. Confirm the retention period meets the customer's requirements and any insurer specification

Frequently Asked Questions

Can I use a standard PC instead of an NVR?

Yes — PC-based software (Milestone XProtect, Blue Iris, iSpy) can record IP cameras. Advantages: more flexible, easier to expand, better analytics integration. Disadvantages: less reliable than embedded NVR (Windows updates, software crashes), requires more maintenance, higher power consumption, more expensive for the same reliability level. For professional installations, a purpose-built NVR is generally preferred. For home use with existing PC infrastructure, PC-based VMS software is a viable option.

How do I estimate storage when I don't know the camera bitrate?

Use these conservative estimates as a starting point:

Then add 50% buffer to account for scene complexity variation. After installation, check the actual bitrate in the camera menu and recalculate. If actual storage will run out before the retention period, reduce bitrate in camera settings (test quality before signing off).

My customer wants 90-day retention — how do I achieve this without spending a fortune?

Options to reduce storage requirements for long retention:

  1. Use H.265 encoding (if not already) — halves storage vs H.264
  2. Reduce frame rate for non-critical cameras to 6–8fps
  3. Enable motion-based quality enhancement: full quality on motion, low quality (1fps) on static background
  4. Add a second HDD (most NVRs support 2+ HDDs) rather than replacing the first
  5. Use an external HDD or NAS for secondary storage with footage archive

Regulations & Standards