RAID Capacity Calculator
Usable terabytes, efficiency, and fault tolerance from drives and level.
What it does
RAID trades raw terabytes for survival: striping is fast and full, mirroring halves capacity, parity levels keep one or two drives worth back. Four 4 TB disks give 16 raw but 12 usable under RAID 5.
The tool above takes drive count with size and level. It returns usable terabytes with efficiency and fault tolerance beside them.
The default of 4x4 TB at RAID 5 gives 12 TB at 75% surviving 1 drive. At RAID 10 the same disks give 8 TB at 50%, while 6x12 TB at RAID 6 gives 48 TB at 66.7% surviving 2.
The layouts follow storage industry RAID guidance with vendor documentation matching each formula. Both define RAID 5 as disks minus one, RAID 6 as minus two, RAID 10 as half.
How to use
- Enter the drive count planned.
- Enter matching drive size in TB.
- Pick the RAID level for the goal.
- Read usable first for the budget.
- Read tolerance before trusting data to it.
Formula
- N / S
- Drive count and matching size in TB.
- -1 / -2
- Parity drives held back at 5 and 6.
- /2
- Mirroring halves RAID 10 and RAID 1.
Matching sizes assumed; mixed arrays waste down to the smallest. Filesystems and reserves take their own slice after these numbers.
Worked example
Example: 4x4 TB at RAID 5
Defaults: 4 drives, 4 TB, RAID 5.
- Raw: 4 x 4 = 16 TB.
- Parity: one drive back, 3 x 4 = 12 TB usable.
- Efficiency: 12 / 16 = 75% of raw.
- Tolerance: any 1 drive fails safe.
- Tool shows 12 TB, 75% of raw, 1 drive.
Reference table
| Level | Usable | Survives |
|---|---|---|
| RAID 0 | 72 TB | None |
| RAID 5 | 60 TB | 1 drive |
| RAID 6 | 48 TB | 2 drives |
| RAID 10 | 36 TB | 1 per mirror pair |
Limits
- Matching drives assumed; mixed wastes to smallest.
- Filesystem and reserves take more after.
- Rebuilds stress survivors for hours or days.
- RAID never replaces offsite backups.
Common mistakes
- Filling arrays past 85% and choking performance.
- RAID 5 on huge disks with day long rebuilds.
- Odd counts on RAID 10.
- Counting parity as backup.
- Skipping hot spares on busy arrays.
Try it yourself
Quick checks. Answers sit tucked under each question.
4x4 TB RAID 10?
8 TB usable at 50%, 1 per mirror pair.
6x12 TB RAID 6?
48 TB at 66.7%, surviving 2 drives.
RAID 5 with 2 drives?
Rejected: RAID 5 needs at least 3 drives.
Questions
How much usable in RAID 5?
All but one drive: (N-1) times size. 4x4 TB gives 12 TB.
RAID 5 or 6?
6 survives two failures for one more drive of cost: the big disk default.
What is RAID 10?
Mirrored pairs striped: half usable with fast rebuilds.
Is RAID backup?
No. It survives drive death, not deletion, fire, or theft.
Mixed drive sizes?
Arrays waste down to the smallest: match drives.
How many drives for RAID 6?
At least 4, with two held as parity.