RAID Capacity and Retention
Usable capacity of a recorder or storage array by RAID level, how many drive failures it survives, and how many days of video it actually holds.
Planning estimates only. This calculator is not a substitute for engineered design or manufacturer specifications. Verify the final design against OEM documentation and the codes your authority having jurisdiction enforces.
The formula
Exactly what the calculator computes, in the order it computes it. Highlighted terms are your inputs.
// Drives that hold data, by level (n = drives in array, g = groups) RAID 0 n RAID 5 n − 1 RAID 10 n ÷ 2 RAID 1 1 RAID 6 n − 2 RAID 50 n − g RAID 60 n − 2g n = drivesInstalled − hotSpares array TB = dataDrives × driveSize usable TB = array TB × (1 − reserve) TiB = TB × 10¹² ÷ 1024⁴ // Retention, when a recording rate is given TB/day = aggregateMbps × 86,400 ÷ 8 ÷ 1,000,000 days = usable TB ÷ TB/day // Rebuild exposure for one failed drive hours = driveSize × 10¹² ÷ (rebuildRate × 10⁶) ÷ 3,600
What it assumes
Each of these is a way the number can be wrong for your job. Read them before you buy hardware.
- All drives are the same size. Arrays built from mixed drive sizes use the smallest drive's capacity for every member.
- Decimal terabytes in, both units out. Drives are sold in decimal TB (10¹² bytes). The operating system reports binary TiB, about 9% less. Both are shown.
- The reserve covers formatting and VMS headroom. Filesystems take a few percent, and most VMS platforms keep free space before overwriting the oldest video. Check your platform's setting.
- Retention assumes a constant write rate. Motion-based recording and variable bitrate will stretch retention; adding cameras or raising quality will shorten it. Size against the contract's retention requirement, then verify on site after a full retention period.
- Rebuild time is an estimate. Real rebuilds slow down while the recorder keeps writing video, and controllers throttle rebuild priority. Treat the figure as a minimum.
- Fault tolerance is not a backup. RAID protects against drive failure, not deletion, ransomware, controller failure, fire or theft of the recorder. Evidence that must be preserved needs to be exported.
- Hot spares are counted as unusable capacity. They are installed, powered and idle until a drive fails.
Questions people ask
- Should a video recorder use RAID 5 or RAID 6?
- With drives of 8 TB and larger, RAID 6. Rebuilding a large drive takes many hours, and RAID 5 loses the array if a second drive fails or an unreadable sector turns up during that rebuild.
More video surveillance tools
- Pixel Density Will this camera actually do the job at that distance? Pixels per foot and per metre, classified against the standard detect / observe / recognise / identify tiers.
- Lens / Field of View Solve for focal length, angle of view, scene width, or distance given the other three. The diagram updates as you type.
- VMS Storage How much storage this camera set actually needs for the retention you promised, with headroom.
- Network Bandwidth Aggregate stream load for a camera set, split into recording and live view, with uplink sizing for the access and distribution layers.
Need it designed, installed or maintained?
We design and maintain these systems for federal facilities. Bring us the drawings.