Storage is the most frequently miscalculated item in a surveillance system. The usual outcome: a system is installed, and two weeks later someone discovers it holds five days of footage instead of thirty — by which point the disk has already been bought.
Calculate your capacity
Capacity required
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With 15% headroom
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Suggested disk size
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Figures are estimates for H.265. A high-motion scene will use more; motion recording may use less. Treat the result as a planning indicator, not a technical commitment.
The arithmetic behind it
All the calculator does is multiply a data rate by time. Bitrate is measured in megabits per second and storage in gigabytes, so we divide by 8 to convert bits to bytes:
Capacity (GB) = bitrate (Mbps) × 3600 × recording hours per day × days × cameras ÷ 8 ÷ 1024
The only variable requiring judgement is bitrate, which depends on resolution, frame rate, codec and how much the scene moves. The figures below are practical estimates for H.265 at 15 fps — a sound starting point, though a busy scene can exceed them.
| Resolution | Estimated bitrate | Typical use |
|---|---|---|
| 2 MP (1080p) | ~2 Mbps | Corridors, entrances, offices |
| 4 MP | ~4 Mbps | The best-balanced choice for most premises |
| 5 MP | ~5 Mbps | Wide areas needing detail |
| 8 MP (4K) | ~8 Mbps | Plate reading, till points, large areas |
Three things that move the number a lot
- Motion recording instead of continuous cuts capacity substantially on sites that are quiet overnight — but it depends on sensitivity being tuned correctly, or you lose footage.
- H.265 saves roughly half the space of H.264 at comparable quality. Confirm both camera and recorder genuinely support it.
- Frame rate: dropping from 25 to 15 fps shrinks the file noticeably, and in most surveillance applications nobody sees the difference.
Leave headroom
Calculate, then add at least 15%. Disks should not be filled to the last byte, bitrates spike in busy scenes, and you may add a camera or two later. Remember too that a 4 TB disk gives you roughly 3.6 TB once formatted.
Finally: use surveillance-rated drives. Desktop disks are not designed for continuous 24-hour writing, and a drive failure in a CCTV system means losing the footage at the moment you need it.