RAID Capacity Calculator
Enter your disk count and size to see usable capacity, redundancy and fault tolerance for RAID 0, 1, 5, 6 and 10.
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Single parity, one disk of overhead.
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What is the RAID Capacity Calculator?
This calculator shows usable capacity, storage efficiency and how many drives can fail for the common RAID levels.
How to use it
- 1Enter the number of disks and the size of each disk.
- 2Pick a RAID level.
- 3Read usable capacity, efficiency and fault tolerance.
How it works
RAID 0 uses all disks (no redundancy). RAID 1/10 mirror pairs (50% usable). RAID 5 loses one disk to parity, RAID 6 loses two. Usable = (disks − parity) × size.
Limitations & notes
- Filesystem overhead and vendor formatting reduce real usable space.
- RAID is not a backup — it protects against drive failure, not deletion or corruption.
Frequently asked questions
What is the difference between RAID 5 and RAID 6?
RAID 5 stores one disk's worth of parity and survives one drive failure; RAID 6 stores two and survives any two. With four 4 TB drives, RAID 5 gives 12 TB usable and RAID 6 gives 8 TB. RAID 6 is preferred for large drives because a second failure during a long rebuild, which can take a day or more on 16 TB disks, would destroy a RAID 5 array.
Is RAID 10 better than RAID 5?
RAID 10 trades capacity for speed and simpler rebuilds. It always uses 50% of raw space, so four 4 TB drives give 8 TB versus 12 TB in RAID 5. Writes are faster because there is no parity to calculate, and a rebuild only copies one mirror partner. It survives one failure for certain and more only if the failed drives are in different mirrored pairs.
Why does my operating system show less space than the calculator?
Drive makers use decimal terabytes, while Windows and many tools report binary units. A 12 TB array is about 10.9 TiB, which Windows labels as 10.9 TB. Filesystem metadata and reserved space take a little more. The calculator works in the units you enter, so convert with a storage unit converter if you want the figure your OS will display.
Can I mix different drive sizes in a RAID array?
In traditional RAID, every member is treated as the size of the smallest drive, so a 4 TB drive in an array of 8 TB drives wastes half of each larger disk. This calculator assumes identical drives and asks for one capacity per disk. Some systems, such as Synology SHR, Unraid or ZFS with matched vdevs, handle mixed sizes differently.
How many disks does each RAID level need?
RAID 0 and RAID 1 need at least 2 disks, RAID 5 needs 3, RAID 6 needs 4, and RAID 10 needs an even number of at least 4. The calculator flags invalid combinations, such as RAID 10 with 5 disks. Note that RAID 1 with more than two disks keeps extra full copies: three 4 TB disks still give only 4 TB usable but survive two failures.
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