QUICK ANSWER
Do not assume one retention period applies to every premises. Establish the period required for the activity and classification, then size storage from camera count, bitrates and recording duration. Advertised disk capacity may differ from usable capacity after formatting or resilience measures.
Establish retention before choosing disks
Record the applicable retention requirement in the design. “A month of storage” is not a meaningful promise without channel count, recording settings and usable capacity. Sector-specific requirements must not be generalised.
Bitrate, encoding, resolution, frame rate and scene complexity affect consumption. Do not reduce image quality or switch to motion-only recording just to claim a longer retention period when that conflicts with the site's requirements.
A worked continuous-recording example
For decimal units, estimated TB = cameras × Mbps per camera × days × 0.0108. This assumes continuous 24-hour recording at a constant bitrate. It is a calculation example, not a prescribed legal retention period or final purchasing recommendation.
For 16 cameras at 4 Mbps over an illustrative 31 days, base capacity is approximately 21.43 TB. An illustrative 20% design allowance brings this to 25.71 TB usable capacity. That allowance is not a universal legal requirement and does not automatically account for RAID overhead.
Verify retention after commissioning
Check the oldest retrievable recording on each channel, disk health, schedules and alerts. Test actual playback. RAID is not an independent backup, and redundancy does not necessarily extend retention.
Recalculate when adding cameras or changing recording settings, then monitor actual consumption. Record the commissioned settings so later retention problems can be investigated.
Estimate storage capacity
Continuous recording at constant bitrate. Days are an illustrative input, not a prescribed retention period.
RAID overhead, formatting and variable bitrates are not included. Validate usable capacity before selecting equipment.Frequently asked questions
Is an 8 TB disk enough for every system?
No. It depends on bitrate, camera count, retention, recording mode and usable capacity.
Does the 31-day example prescribe retention for all sites?
No. It is an illustrative calculation input; the applicable retention period must be established separately.
Sources and review information
Content reviewed 4 November 2026, with the technical requirements document, edition 2.0 dated 17 November 2024. Confirm the activity-specific requirements and current instructions at implementation.
FROM GUIDANCE TO IMPLEMENTATION
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