How much usable space does RAID 5 give?

RAID 5 keeps one drive's worth of parity, so usable capacity is (number of drives - 1) times the drive size. Four 4 TB drives give (4 - 1) x 4 = 12 TB usable from 16 TB raw, a space efficiency of 75%, and the array survives any single drive failure.

Which RAID level survives the most drive failures?

RAID 6 survives any two simultaneous drive failures. RAID 1 survives everything except the loss of all drives, because each drive holds a full copy. RAID 10 always survives one failure and can survive one per mirrored pair if luck holds, while RAID 5 tolerates exactly one and RAID 0 tolerates none.

Is RAID a substitute for backup?

No. RAID protects against drive hardware failure only. Accidental deletion, ransomware, file corruption, controller failure, fire and theft all take out every copy in the array at once, so you still need a separate backup, ideally with one copy offsite or in the cloud.

RAID Calculator

This RAID calculator works out how much usable storage you get from a set of drives at each common RAID level, along with the space efficiency and how many drive failures the array can survive. RAID trades raw capacity for speed or redundancy: RAID 0 stripes data across every drive for maximum space and speed but dies with any single drive, RAID 1 mirrors everything so capacity stays at one drive's worth, RAID 5 spreads one drive of parity across the array, RAID 6 spreads two, and RAID 10 stripes across mirrored pairs. Choose your RAID level, enter how many drives you have and the size of each, and the calculator returns the usable capacity, the raw total, the efficiency percentage and the fault tolerance, flagging combinations that are not valid, such as RAID 5 with fewer than three drives. Home NAS builders sizing a Synology or QNAP, gamers striping SSDs, and anyone planning a small office server can compare levels in seconds before buying drives. Capacities are in the decimal terabytes drive makers advertise; your operating system will report a smaller number in binary units, and the array is always limited by its smallest drive, so mixing sizes wastes the difference.

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RAID level

Number of drives

drives

Capacity of each drive

12 TB

usable capacity of your RAID 5 array

Raw capacity16 TB

Space efficiency75%

Fault tolerance1 drive

RAID 5 keeps one drive's worth of parity, so your 4 x 4 TB array gives 12 TB of usable space and keeps working if any single drive fails.

Assumes identical drives; with mixed sizes every drive is treated as the smallest. Sizes are decimal TB as marketed, so your OS will show less. RAID protects against drive failure only: it is not a backup.

How it works

Raw capacity is simply the drive count multiplied by the drive size. Usable capacity then depends on the level. RAID 0 uses every byte: n x size, with no redundancy at all. RAID 1 stores a full copy on every drive, so usable capacity equals one drive regardless of how many you mirror, and the array survives until the last drive dies. RAID 5 dedicates the equivalent of one drive to parity spread across the set: (n - 1) x size, surviving any one failure, with a minimum of three drives. RAID 6 dedicates two drives' worth: (n - 2) x size, surviving any two failures, minimum four drives. RAID 10 pairs the drives into mirrors and stripes across the pairs: (n / 2) x size, needing an even number of at least four drives; it always survives one failure, and up to one per pair if the failures land kindly. Space efficiency is usable divided by raw.

Worked example

You are filling a four-bay NAS with 4 TB drives, so raw capacity is 4 x 4 = 16 TB. In RAID 5 the usable space is (4 - 1) x 4 = 12 TB, an efficiency of 12 / 16 = 75%, and the array keeps running through any single drive failure while you swap in a replacement. The alternatives from the same four drives: RAID 6 gives (4 - 2) x 4 = 8 TB but survives two simultaneous failures, RAID 10 also gives 8 TB with faster rebuilds, RAID 1 gives just 4 TB copied four times, and RAID 0 gives the full 16 TB with no protection at all. For most home NAS setups this makes RAID 5 the capacity sweet spot, with RAID 6 worth the sacrifice once drives get large and rebuild times stretch.

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