How do I calculate RAID 5 capacity?

RAID 5 usable capacity equals (number of disks − 1) × the size of one disk, because one disk's worth of space is consumed by distributed parity. Four 4 TB disks in RAID 5 give 12 TB usable.

How much space does RAID 6 use for redundancy?

RAID 6 reserves two disks' worth of capacity for double parity, so usable space is (number of disks − 2) × disk size. In return it survives any two simultaneous disk failures.

What is the usable capacity of RAID 10?

RAID 10 gives you half of the raw capacity - (number of disks ÷ 2) × disk size - because every disk is mirrored. It requires an even number of at least four drives.

Why is my array smaller than the drives' advertised size?

Drives are sold in decimal terabytes while operating systems report binary tebibytes, and mixed-size arrays are limited by the smallest disk. This calculator uses the disk size you enter directly.

Does this RAID calculator send my data anywhere?

No. The calculation runs entirely in your browser with client-side JavaScript, so the disk counts and sizes you enter never leave your device.

RAID Calculator

Work out the usable capacity, redundancy and overhead of a RAID 0, 1, 5, 6 or 10 array from your disk count and size - instantly, in your browser.

RAID levelNumber of disksSize per diskUnit

RAID 5 result

Striping with single distributed parity.

Usable capacity: 12 TB
Raw capacity: 16 TB
Space efficiency: 75 %
Fault tolerance: 1 disk

Figures assume identical disks; mixed sizes are limited by the smallest member. RAID is not a backup - it only protects against disk failure, not deletion or corruption.

About RAID Calculator

Pick a RAID level, enter how many disks you have and the size of one disk in GB or TB, and the calculator returns four figures at once: usable capacity (the space you can actually store data on), raw capacity (the disks' combined total), space efficiency as a percentage, and fault tolerance (how many disks can fail before you lose data). It's a quick way to see the real trade-off a given array makes before you commit to buying drives.

Each level applies a different formula, and knowing them helps you interpret the result. RAID 0 stripes data across all disks for maximum capacity (n × size) but offers no redundancy, so a single failure destroys everything. RAID 1 mirrors, giving you one disk's worth of space while tolerating the loss of all but one member. RAID 5 uses single distributed parity, yielding (n − 1) × size and surviving one failure, and needs at least three disks. RAID 6 adds a second parity block for (n − 2) × size, survives two simultaneous failures, and needs at least four. RAID 10 stripes across mirrored pairs for (n ÷ 2) × size, requires an even number of at least four disks, and always survives at least one failure.

Space efficiency is simply usable divided by raw capacity, and it exposes the cost of redundancy clearly. RAID 0 is 100% efficient but unprotected; RAID 1 across two disks is only 50%; RAID 5's efficiency improves as you add disks because parity overhead stays fixed at one disk. In practice you are balancing three competing goals - capacity, redundancy and rebuild safety - and comparing a few configurations side by side is the fastest way to find the sweet spot for your workload.

A few caveats keep the numbers honest. The calculation assumes identical disks; a mixed array is limited by its smallest member, so the result reflects the disk size you enter. Drives are also sold in decimal terabytes while operating systems report binary tebibytes, so your formatted capacity will read a little lower than the raw figure here. Most importantly, RAID is not a backup: parity protects against physical disk failure, not accidental deletion, file corruption, ransomware or a controller fault. Because the tool runs entirely in your browser, the disk counts and sizes you enter never leave your device.

How to use it

1Choose the RAID level you are planning: 0, 1, 5, 6 or 10.
2Enter the number of disks and the capacity of a single disk, then select GB or TB.
3Read off the usable capacity, raw total, space efficiency and how many disk failures the array survives.
4Adjust the disk count or level to compare configurations and find the right balance of capacity and redundancy.

Common use cases

-Sizing a NAS or home server before buying drives.
-Comparing how much usable space RAID 5, RAID 6 and RAID 10 give for the same set of disks.
-Estimating the capacity cost of redundancy when planning a storage budget.
-Explaining to a client or teammate how many drive failures a proposed array can survive.
-Sanity-checking a vendor's quoted array capacity against the raw disk total.

Frequently asked questions

How do I calculate RAID 5 capacity?: RAID 5 usable capacity equals (number of disks − 1) × the size of one disk, because one disk's worth of space is consumed by distributed parity. Four 4 TB disks in RAID 5 give 12 TB usable.
How much space does RAID 6 use for redundancy?: RAID 6 reserves two disks' worth of capacity for double parity, so usable space is (number of disks − 2) × disk size. In return it survives any two simultaneous disk failures.
What is the usable capacity of RAID 10?: RAID 10 gives you half of the raw capacity - (number of disks ÷ 2) × disk size - because every disk is mirrored. It requires an even number of at least four drives.
Is RAID a substitute for backups?: No. RAID protects against physical disk failure but not against accidental deletion, file corruption, ransomware or a controller fault. You still need separate, independent backups.
Why is my array smaller than the drives' advertised size?: Drives are sold in decimal terabytes while operating systems report binary tebibytes, and mixed-size arrays are limited by the smallest disk. This calculator uses the disk size you enter directly.
Does this RAID calculator send my data anywhere?: No. The calculation runs entirely in your browser with client-side JavaScript, so the disk counts and sizes you enter never leave your device.