RAID Capacity Sheet
Usable capacity by level, what the OS will show instead, and the cost of each terabyte that survives.
BFCBrilliance · bfcbrilliance.com/tools/raid-usable-capacity-calculator
The levels
- RAID 0
- n x size — survives NOTHING
- RAID 1
- one drive's size, however many you mirror
- RAID 5
- (n − 1) x size — survives 1 failure
- RAID 6
- (n − 2) x size — survives 2
- RAID 10
- floor(n ÷ 2) x size — survives 1 guaranteed
- Minimum drives
- RAID 0 and 1: 2 · 5: 3 · 6 and 10: 4
- OS reports 9.1% less
- sold in TB decimal, shown in TiB binary
- ⚠ RAID IS NOT A BACKUP
- it replicates a deletion perfectly
- ⚠ Compare cost per USABLE TB
- RAID 6 is only 25% over RAID 5
Layouts compared
| Level | Drives | Size | Usable TB | Can fail | Cost/TB |
|---|---|---|---|---|---|
This array
- Machine
- Date
- RAID level
- Drive count
- Drive size
- Price per drive
- RAW TB
- USABLE TB
- OS WILL SHOW (TiB)
- Drives that can fail
- COST PER USABLE TB
- Backup target
- Backup last verified
Before building an array
- A real BACKUP exists that is not this array — off-site or offline
- That backup has been RESTORED from at least once, not just written to
- Level chosen on cost per usable TB, not on raw capacity
- Rebuild time considered — it decides whether single parity is enough
- Minimum drive count met for the level
- RAID 10 drive count even, or the odd one assigned as a hot spare
- OS-reported capacity expected to be ~9.1% below the decimal figure
- Drive failure alerting actually configured and TESTED
RAID is not a backup
It protects against a DRIVE failing and nothing else. A deleted folder, ransomware, a corrupted file, a dead controller, theft or fire are all replicated perfectly across every disk in the array. Redundancy is uptime insurance, not data insurance — and an array with no separate backup is one mistake from total loss whatever level it runs.
Price the levels, do not eyeball them
Six 4 TB drives at 100 each: RAID 5 gives 20 TB usable at 30 per usable TB, RAID 6 gives 16 TB at 37.50. Twenty-five percent more for double the fault tolerance — which reads very differently from 'you lose another whole drive', and is usually worth it once rebuild windows are long.