When to use: Size the raw NVR/DVR disk capacity from per-camera bitrate, camera count, and retention days. Storage = bitrate × cameras × days × duty cycle. The recording duty cycle models motion-only or schedule-based recording, and a RAID/headroom margin accounts for parity, metadata, and filesystem overhead.
This calculator sizes the raw NVR or DVR disk capacity required for a video surveillance system based on per-camera bitrate, camera count, retention period, and recording duty cycle. It is the foundational calculation used in every CCTV storage specification.
The storage formula is: GB per camera per day = Bitrate (Mbps) × 86,400 seconds × Duty Cycle / 8 (bits to bytes) / 1,024 (MB to GB). Total storage is then: GB per camera per day × camera count × retention days. The result in GB is converted to TB (1 TB = 1,024 GB in binary units).
Duty cycle models the fraction of time cameras are actually recording. Continuous 24/7 recording uses a duty cycle of 1.0. Motion-triggered recording typically captures 30–50% of the time for a typical commercial installation, using a duty cycle of 0.4. Business-hours scheduling (8 AM to 8 PM, 5 days) corresponds to approximately 0.36.
A RAID or headroom margin is added on top of the calculated raw capacity to account for RAID parity drives (RAID 5 adds one drive, RAID 6 adds two), filesystem overhead, metadata, and future retention growth. A 20% margin is the standard planning assumption.
No universal standard mandates specific video retention periods — requirements are set by the AHJ, the owner's security policy, and any applicable regulations (HIPAA for healthcare facilities, FINRA for financial institutions, TSA for airports). Typical commercial specifications require 30–90 days retention for general areas and 90–365 days for high-security zones. Insurance requirements and legal discovery obligations can extend retention to two years or more for certain facilities.
Verify the actual duty cycle during commissioning — motion-triggered recording percentage depends heavily on the scene activity and sensitivity settings. A busy retail entrance may record 70–80% of the time even with motion triggering, while a stairwell may record 5–10%. Use the continuous recording calculation as the conservative bound for sizing, and confirm with a week of actual recording data during the commissioning period.
Use surveillance-rated hard drives (WD Purple, Seagate SkyHawk, Toshiba S300) in NVRs and DVRs. Desktop or laptop drives are not rated for continuous 24/7 write duty cycles and will fail prematurely. For RAID arrays, do not count parity drives in the usable capacity calculation — RAID 5 with 5 × 8 TB drives provides 4 × 8 = 32 TB usable; RAID 6 with 6 × 8 TB drives provides 4 × 8 = 32 TB usable.
Enter the per-camera bitrate in Mbps (obtain from the camera datasheet or the CCTV Bandwidth Calculator), camera count, retention period in days, recording duty cycle, and RAID/headroom margin percentage. The tool shows per-camera per-day storage, per-camera total retention storage, raw total in GB and TB, and total with the RAID/headroom margin applied. The recommended drive purchase rounds up to the next whole terabyte.
The bitrate is available in the camera datasheet (under "bit rate" or "bandwidth" specifications) or from the camera web interface after installation. For planning purposes, use the CCTV Bandwidth Calculator to estimate bitrate based on resolution, frame rate, and codec. H.265 cameras at 1080p and 15 fps typically use 1.5–3 Mbps.
At 2 Mbps per camera (1080p H.265, 15 fps, medium scene), continuous recording: 2 Mbps × 86400 × 1.0 / 8 / 1024 = 21.1 GB/cam/day × 16 cameras × 30 days = 10,125 GB = ~9.9 TB raw. With 20% margin: ~11.9 TB. A 12 TB or 16 TB NVR would be appropriate.
This calculator uses binary units: 1 GB = 1,024 MB, 1 TB = 1,024 GB. Hard drive manufacturers use decimal units: 1 TB = 1,000 GB. A 4 TB drive labeled by the manufacturer provides approximately 3.64 TB in binary units (3,726 GB binary). When sizing storage, use the actual usable capacity in binary units.
H.265 (HEVC) reduces bitrate — and therefore storage — by approximately 50% compared to H.264 at equivalent visual quality. H.265+ smart codecs from major camera manufacturers can achieve 60–70% reduction on largely static scenes. Use the Video Storage Sizing Tool to compare codecs side by side with your specific inputs.
Yes, for low-to-medium activity scenes. A stairwell recording 10% of the time uses 90% less storage than continuous. A busy entrance recording 70% of the time saves only 30%. For scenes where storage is critical, use VBR (variable bit rate) encoding with a low minimum bitrate to automatically reduce storage during periods of inactivity.
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