How it works
The formula, without the black box.
working TB = RAID-usable TB × (1 − free-space reserve)
RAID 5 subtracts one drive-equivalent, RAID 6 subtracts two, RAID 1 uses one mirrored drive’s capacity, and RAID 10 uses half of an even-numbered set. Proprietary layouts may differ.
Inputs that deserve a second look
- Drive count
- Equal-sized drives assumed. Mixed arrays commonly use the smallest drive as the effective size.
- Drive size
- Decimal TB printed by the drive manufacturer.
- Layout
- Capacity math only; rebuild behavior, failure tolerance, and performance need separate review.
- Reserve
- Working space intentionally excluded from the video target.
Common planning mistakes
- Calling RAID a backup.
- Ignoring a hot spare or vendor-reserved capacity.
- Using a RAID level unsupported by the recorder.
- Selecting only by usable capacity while ignoring rebuild and failure risk.
Questions installers ask
How much usable capacity is four 12 TB drives in RAID 5?
The simple equal-drive calculation is 36 TB before reserve. With a 15% free-space reserve, working capacity is 30.6 TB.
Does RAID guarantee retention?
No. Retention also depends on actual bitrate and recorder behavior. RAID reduces certain drive-failure risks; it is not a retention guarantee or independent backup.
Why decimal TB?
Drive labels use decimal units. Some operating systems display binary tebibytes, making the same bytes appear as a smaller number.
Sources and boundaries
These links support the method or terminology. They do not turn a planning estimate into a site guarantee.
SNIA RAID overview ↗
Storage-industry definition and terminology.