RAID Capacity Calculator

Calculate the usable capacity of a RAID 0, 1, 5, 6 or 10 array from the number of disks and their sizes, with the fault tolerance of each level.

RAID level

How to enter the disks

Number of disks in the array (not counting hot spares)

disks

Capacity of one disk

Capacity unit (the same for all disks you enter)

Hot spares (0 to 10; empty = 0)

disks

Usable capacity —

Usable capacity (all units)
GB (powers of 1,000)	—
TB (powers of 1,000)	—
GiB (powers of 1,024)	—
TiB (powers of 1,024)	—

Details
Disks in the array	—
Hot spares (not counted in the array's disks or capacity)	—
Physical disks in total (including hot spares)	—
Total capacity of the disks in the array	—
Capacity not used because the disks differ in size	—
Share of capacity that is usable (usable ÷ total)	—
Fault tolerance	—

With disks of different sizes, the calculation uses every disk at the size of the smallest one (some implementations can use the leftover space).

A hot spare is a standby disk that takes the place of a failed disk. This calculator does not count it in the array's disks or capacity, only in the total of physical disks. It does not judge whether a hot spare's capacity is enough to replace a failed disk.

GB uses powers of 1,000 and GiB uses powers of 1,024. Manufacturers and operating systems may use different units, so the capacity shown by your operating system can look smaller.

RAID protects against disk failure. It is not a backup.

The value copied is the usable capacity in the unit you chose.

RAID levels and capacity formulas

Let n be the number of disks in the array, and S the capacity of one disk (if you mix disks of different sizes, the capacity of the smallest disk, S_min, is used).

Level	Minimum disks	Usable capacity	Fault tolerance
RAID 0	2	n × S	None (if one disk fails, all data is lost)
RAID 1	2 (this calculator uses exactly 2)	S	Up to 1 disk
RAID 5	3	(n − 1) × S	Up to 1 disk
RAID 6	4	(n − 2) × S	Up to 2 disks
RAID 10	4 (an even number)	(n ÷ 2) × S	Depends on which disks fail (see below)

The formulas for RAID 5 and 6 are derived from the definition that parity takes up the capacity of one disk and of two disks. For this English page, the Wikipedia article on RAID was opened, and the capacity formulas for these levels could not be confirmed in the text that was read (see "Sources" below), so treat them as the standard definitions rather than as quotations.

Fault tolerance of RAID 10

RAID 10 combines mirrors of two disks (RAID 1) in a stripe (RAID 0). It cannot be described by a single "number of disks that can fail."

If any one disk fails, no data is lost.
If two or more disks fail, it depends on which disks fail. If both disks of one mirror pair fail, data is lost. If each failure is in a different mirror pair, up to n ÷ 2 disks can fail, but this is not guaranteed.
This calculator does not decide which two disks form a mirror pair (this depends on the controller and the settings).

Hot spares

A hot spare is a standby disk in a RAID setup that takes the place of a disk that has failed or is failing. In this calculator, it is counted separately from the disks in the array: it is included in the total of physical disks, but not in the usable capacity (this is how this calculator treats it). It does not judge whether a hot spare is large enough to replace a failed disk.

Example

Four 4 TB disks (plus one hot spare):

RAID 5: (4 − 1) × 4 = 12 TB. There are 5 physical disks.
RAID 6: (4 − 2) × 4 = 8 TB
RAID 10: (4 ÷ 2) × 4 = 8 TB
RAID 0: 4 × 4 = 16 TB (no fault tolerance)

With disks of different sizes (4, 4, 4 and 6 TB) in RAID 10: using the smallest size, (4 ÷ 2) × 4 = 8 TB. Of the 18 TB in total, 2 TB is not used.

GB and GiB

GB uses powers of 1,000 (1 GB = 10⁹ bytes), and GiB uses powers of 1,024 (1 GiB = 2³⁰ bytes). Manufacturers and operating systems may use different units. That is why the usable capacity in your operating system can look smaller than the figure on the label. This calculator shows the result in all four units.

Notes

This is a theoretical capacity. Capacity lost to the file system and metadata, and vendor-specific schemes that use disks of different sizes efficiently (such as ZFS or Synology SHR), are not covered.
RAID performance (speed), rebuild time and failure probability are not covered.
RAID 01, RAID 50, RAID 60, JBOD and special Linux MD levels are not covered.
RAID is not a backup. It does not protect against mistakes, malware, fire and similar problems.

Sources

The minimum number of disks for RAID 5 (three) and RAID 6 (four) was confirmed in the text of the Wikipedia article "RAID", and the capacity of RAID 0 as the total of the drives (checked October 31, 2026). The article's text could only be read in part, and the capacity formulas for RAID 1, 5, 6 and 10 and a statement that RAID is not a backup were not found in the part that was read. The Japanese version of this calculator was written from the Wikipedia articles "Standard RAID levels", "Nested RAID levels" and "Hot spare". The text of the SNIA specification was not checked.

Frequently asked questions

How many disks does RAID 5 need?

At least 3. The usable capacity is (number of disks − 1) × the capacity of one disk.

Can RAID 1 use three or more disks?

Mirrors of three or more disks exist, but this calculator handles only the 2-disk configuration.

How many disks can fail in RAID 10?

It cannot be given as one number. If both disks of one mirror pair fail, data is lost. If the failures are in different pairs, several disks can fail, but this is not guaranteed.

Do hot spares count toward capacity?

No. They are standby disks, so they are not part of the usable capacity. They are counted in the total of physical disks.