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By Ali Zafar · Last updated November 2026

RAID Calculator

Enter how many equal disks you have and how big each one is. The calculator shows the usable capacity, the efficiency and how many disks can fail for RAID 0, 1, 5, 6 and 10, and marks the levels that are not possible with that many disks. It also shows the capacity as Windows reports it.

Disks

What it is: how many equal disks are in the recorder or array. How to choose: count only the disks in the array, not a spare. Example: 4.
What it is: the label capacity of one disk. How to choose: use the size on the disk label; all disks must be the same size. Example: 8.
What it is: the unit of the disk size (decimal: 1 TB = 1000 GB). How to choose: disk labels use decimal TB and GB. Example: TB.

RAID levels

—
raw capacity of all disks
File system overhead, hot spares and recorder reserves are not subtracted. The recorder or controller you use may support only some levels; check its manual.

How each level is calculated

For n equal disks of one size:

LevelUsable capacityDisks that can failMinimum disks
RAID 0n × disk02
RAID 11 × diskn − 12
RAID 5(n − 1) × disk13
RAID 6(n − 2) × disk24
RAID 10n / 2 × diskat least 1, up to 1 per mirrored pair4, even number

Efficiency is usable capacity divided by raw capacity. Windows shows capacity in binary units (TiB): decimal TB × 1012 / 240.

Worked example

Four 8 TB disks give 32 TB raw. RAID 0 is 32 TB, RAID 1 is 8 TB, RAID 5 is 24 TB (75% efficient, shown as 21.83 TiB in Windows), RAID 6 is 16 TB and RAID 10 is 16 TB.

For a CCTV recorder

RAID 0 gives the most space but one failed disk loses every recording. RAID 5 and 6 protect against disk failure at the cost of one or two disks of space. Remember that RAID is not a backup and that a rebuild with a large disk takes a long time.

Common mistakes

  • Mixing disk sizes: the array uses the smallest disk size for every disk.
  • Expecting RAID 10 with an odd number of disks. It needs an even number, at least four.
  • Planning storage from the Windows (TiB) number and then comparing with the decimal TB on the label.

Frequently asked questions

How much usable space does RAID 5 give?

RAID 5 gives (n minus 1) disks of space and survives one disk failure. With four 8 TB disks it gives 24 TB.

What is the difference between RAID 5 and RAID 6?

RAID 5 uses one disk of parity and survives one failure; RAID 6 uses two disks of parity and survives two failures, with one disk less of usable space.

How many disks do I need for RAID 10?

At least four, and an even number. Disks are paired as mirrors, and the pairs are striped together.

Why does Windows show less than the label size?

Disk labels use decimal units (1 TB = 10^12 bytes) while Windows reports binary TiB, which is about 9% smaller.

Is RAID a backup?

No. RAID keeps the system running when a disk fails, but it does not protect against deletion, fire, theft or a failed controller.

Related tools

Size the recording itself with the NVR Storage Calculator and the CCTV Storage Calculator. For the traffic into the recorder use the IP Camera Bandwidth Calculator. Read NVR Storage Calculation: 12 Mistakes before you buy disks.

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