RAID calculator

Choose the drives and the RAID level. See how much space you can use, how many drives can fail without losing data, and how fast the array reads and writes compared with one drive.

RAID level

Striping with one drive’s worth of parity.

How the drives are used

4 drives: 3 for data, 1 for parity, 0 for copies

Usable capacity 12 TB 12 TB of 16 TB raw space, 75% usable.
Raw capacity
16 TB
As the operating system shows it
10.91 TiB
Space you can use
75%
Drives that can fail
1 at any time
Read speed
3× one drive
Write speed
3× one drive for long writes; small random writes are slower

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How it is calculated

Each RAID level trades space for safety. Striping spreads data across drives for speed, mirroring keeps full copies, and parity stores just enough extra information to rebuild one or two lost drives. The usable space follows from how many drives’ worth of capacity goes to copies or parity:

RAID 0  = n × size
RAID 1  = size
RAID 5  = (n − 1) × size
RAID 6  = (n − 2) × size
RAID 10 = n ÷ 2 × size

Speeds are the textbook multiples of one drive for large sequential reads and writes. Real arrays land below them, and parity levels lose more on small random writes, because every write also reads and rewrites parity.

Drives are sold in decimal terabytes (1 TB = 1,000,000,000,000 bytes), while most operating systems count in binary tebibytes (1 TiB = 1,099,511,627,776 bytes). That is why a 4 TB drive shows up as about 3.64 TB, and why the calculator shows both.

Example

Four 4 TB drives in RAID 5: (4 − 1) × 4 TB = 12 TB usable, 75% of the 16 TB raw. Any one drive can fail. In RAID 10 the same drives give 8 TB and survive two failures if they hit different pairs.

RAID levels at a glance

LevelDrives, at leastUsableCan fail
RAID 02n0
RAID 121n − 1
RAID 53n − 11
RAID 64n − 22
RAID 104n ÷ 21+

Questions

RAID 5 or RAID 10: which should I choose?

RAID 5 gives more space: with four drives, three hold data. RAID 10 gives half, but writes faster, rebuilds faster and survives two failures in different pairs. For databases and busy virtual machines RAID 10 is the usual choice; for large files that are mostly read, RAID 5 or RAID 6 makes better use of the drives.

How much usable space does RAID 5 give?

All drives but one: (n − 1) × the size of the smallest drive. Three 2 TB drives give 4 TB, eight 8 TB drives give 56 TB. With large drives consider RAID 6, because a second drive can fail during the long rebuild.

Is RAID a backup?

No. RAID keeps a server running when a drive dies, but a deleted file, ransomware, a broken update or a failed controller hits every drive at once. Keep backups on another machine, ideally with another provider or in another location.

Can I mix drives of different sizes?

You can, but classic RAID uses only as much of each drive as the smallest one has. Four drives of 4, 4, 4 and 8 TB in RAID 5 give 12 TB, not 16 TB, so the calculator assumes drives of one size.