What is my RAID capacity?
Pick a RAID level, the number of disks, and the disk size to see how much space you can use.
- Usable capacity
- 12 TB
4 disks of 4 TB in RAID 5 give 12 TB of usable space. Disk failures survived: 1.
- Usable capacity in TiB
- 10.91 TiB
- Total raw capacity
- 16 TB
- Used for protection
- 4 TB
- Storage efficiency
- 75%
- Disk failures survived
- 1
Usable capacity: 12 TB. 4 disks of 4 TB in RAID 5 give 12 TB of usable space. Disk failures survived: 1.
How to calculate
Computes the usable capacity, storage efficiency, and number of disk failures survived for a RAID 0, 1, 5, 6, or 10 array of identical disks.
Example with the default inputs (RAID level RAID 5, Number of disks 4, Size of each disk 4 TB): 4 disks of 4 TB in RAID 5 give 12 TB of usable space. Disk failures survived: 1.
Method: Usable = RAID 0: N × S; RAID 1: S; RAID 5: (N − 1) × S; RAID 6: (N − 2) × S; RAID 10: N ÷ 2 × S, for N disks of size S.
- All disks are the same size. In a mixed array, each disk counts as the smallest one.
- RAID 10 uses two-way mirror pairs and needs an even number of disks. RAID 1 mirrors the same data on every disk.
- Capacities are before the file system, hot spares, and the controller’s own metadata take their share.
- Disk sizes use decimal units (1 TB = 10¹² bytes), as drive makers label them.
Worked examples
Each example is checked against the calculator on every build.
- RAID level RAID 5, Number of disks 4, Size of each disk 4 TB gives Usable capacity 12 TB, Usable capacity in TiB 10.91 TiB, Total raw capacity 16 TB, Used for protection 4 TB, Storage efficiency 75%, Disk failures survived 1.Source: hand calculation in content.mdx: (4 − 1) × 4 TB = 12 TB = 10.91 TiB (raw values in bytes); SNIA dictionary, RAID 5 (n data + 1 parity)
- RAID level RAID 6, Number of disks 8, Size of each disk 10 TB gives Usable capacity 60 TB, Storage efficiency 75%, Disk failures survived 2.Source: hand calculation in content.mdx: (8 − 2) × 10 TB = 60 TB; SNIA dictionary, RAID 6 (survives any two failures)
- RAID level RAID 10, Number of disks 6, Size of each disk 2 TB gives Usable capacity 6 TB, Storage efficiency 50%, Disk failures survived 1, Most failures survived (RAID 10) 3.Source: hand calculation in content.mdx: 6 ÷ 2 × 2 TB = 6 TB; SNIA dictionary, RAID 10 (RAID 0 over RAID 1 pairs, one failure per pair)
- RAID level RAID 1, Number of disks 3, Size of each disk 2 TB gives Usable capacity 2 TB, Storage efficiency 33.333333%, Disk failures survived 2.Source: hand calculation in content.mdx: every disk holds the same data, so 2 TB usable and 3 − 1 = 2 failures
- RAID level RAID 0, Number of disks 3, Size of each disk 0.5 TB gives Usable capacity 1.5 TB, Storage efficiency 100%, Disk failures survived 0.Source: hand calculation in content.mdx: 3 × 500 GB = 1.5 TB; SNIA dictionary, RAID 0 (no redundancy)
How it works
For an array of N identical disks, each of size S:
| RAID level | How it stores data (SNIA) | Usable capacity | Failures survived | Fewest disks |
|---|---|---|---|---|
| RAID 0 | Striping: blocks spread over every disk, no redundancy | N × S | 0 | 2 |
| RAID 1 | Mirroring: every disk holds the same data | S | N − 1 | 2 |
| RAID 5 | Each stripe holds N − 1 data blocks and 1 parity block, spread over all disks | (N − 1) × S | 1 | 3 |
| RAID 6 | Each stripe holds N − 2 data blocks and 2 check blocks | (N − 2) × S | 2 | 4 |
| RAID 10 | RAID 0 striped over two-disk RAID 1 mirror pairs | N ÷ 2 × S | 1 (up to N ÷ 2, one per pair) | 4, even only |
From the usable capacity the calculator also shows:
- Total raw capacity: N × S.
- Used for protection: raw capacity − usable capacity.
- Storage efficiency: usable ÷ raw × 100%, which is 100% for RAID 0, 100% ÷ N for RAID 1, (N − 1) ÷ N for RAID 5, (N − 2) ÷ N for RAID 6, and 50% for RAID 10.
- Usable capacity in TiB: the usable bytes divided by 2⁴⁰.
- Most failures survived (RAID 10 only): N ÷ 2, when each failed disk is in a different mirror pair.
Assumptions
- All disks are the same size. Disk sizes are in decimal units, as drive makers print them: 1 GB = 10⁹ bytes and 1 TB = 10¹² bytes. GiB and TiB are 2³⁰ and 2⁴⁰ bytes.
- RAID 10 uses two-way mirror pairs, so it needs an even number of disks, 4 or more. A level with fewer disks than its minimum, or RAID 10 with an odd number, has no answer.
- "Failures survived" counts disks that can fail at the same time, in any combination, without losing data.
- The results are before the file system, hot spares, and the RAID controller's metadata, which all use some space.
Worked examples by hand
Four 4 TB disks in RAID 5 (the default). Usable = (4 − 1) × 4 TB = 12 TB. Raw = 4 × 4 TB = 16 TB, so 4 TB holds parity. Efficiency = 12 ÷ 16 = 75%. The array survives 1 failed disk. In binary units, 12 × 10¹² ÷ 2⁴⁰ = 10.91 TiB.
Eight 10 TB disks in RAID 6. Usable = (8 − 2) × 10 TB = 60 TB of 80 TB raw, an efficiency of 60 ÷ 80 = 75%. Any 2 disks can fail.
Six 2 TB disks in RAID 10. Three mirror pairs, so usable = 6 ÷ 2 × 2 TB = 6 TB, an efficiency of 50%. It always survives 1 failure, and up to 3 if each failure is in a different pair.
Three 2 TB disks in RAID 1. Every disk holds the same data, so usable = 2 TB, an efficiency of 1 ÷ 3 = 33.33%, and 2 disks can fail.
Three 500 GB disks in RAID 0. Usable = 3 × 500 GB = 1.5 TB, an efficiency of 100%, and 0 failures survived: losing any disk loses the array.
Other questions people ask
How do I calculate RAID 5 capacity?
Multiply the size of one disk by the number of disks minus 1. RAID 5 spreads one disk’s worth of parity across the array. Four 4 TB disks give (4 − 1) × 4 TB = 12 TB of usable space, and the array survives one disk failure.
What is the difference between RAID 5 and RAID 6?
RAID 5 keeps one disk’s worth of parity and survives one failed disk. RAID 6 keeps two and survives any two failed disks at the same time. RAID 6 costs one more disk of capacity, which matters most with large disks that take a long time to rebuild.
How many disks can fail in RAID 10?
At least one. RAID 10 stripes data over mirror pairs, and each pair survives losing one of its two disks. If every failed disk is in a different pair, up to half the disks can fail, but two failures in the same pair lose data.
Why does my operating system show less space than the calculator?
Drive makers count 1 TB as 1,000,000,000,000 bytes, but Windows and many other systems divide by 2⁴⁰ and still label the result TB. 12 TB is 10.91 TiB, which Windows shows as 10.9 TB. The file system and the RAID controller’s own metadata take a little more.
Is RAID a backup?
No. RAID keeps the data available when a disk fails, but it copies deletions, corruption, and ransomware to every disk at once, and a fire or theft takes the whole array. Keep separate backups as well.
What happens if my disks are different sizes?
Standard RAID 1, 5, 6, and 10 use the same amount of space on every disk, so each disk counts as the smallest one and the extra space on larger disks is unused. Enter the size of the smallest disk to see the usable capacity.
Which RAID level should I choose?
RAID 0 gives all the space and speed but no protection. RAID 1 mirrors two disks for simple protection. RAID 5 gives the most space with protection against one failure, RAID 6 protects against two, and RAID 10 protects well and rebuilds fast at the cost of half the space.