Plan an mdadm or hardware RAID array before you buy disks. Enter a count and size, or mixed custom sizes, and this RAID calculator shows usable capacity, efficiency, and fault tolerance for RAID 0, 1, 5, 6, and 10. Copy an mdadm --create command when you are ready. For copy-on-write filesystems with different math, use the Btrfs RAID calculator or the ZFS capacity calculator. This tool is part of the Linux tools on TechOpt.io.
How this RAID calculator estimates usable capacity
Classic RAID treats every disk as the size of the smallest member. Extra space on larger disks is unused. Hot spares are subtracted before the formula runs — they are not in the array until a rebuild. The formulas below use n active disks and d as the smallest disk, matching mdadm(8):
| Level | Usable | Min disks | Fault tolerance |
|---|---|---|---|
| RAID 0 | n × d | 2 | None — any failure loses the array |
| RAID 1 | d | 2 | n − 1 disks |
| RAID 5 | (n − 1) × d | 3 | 1 disk |
| RAID 6 | (n − 2) × d | 4 | 2 disks |
| RAID 10 | (n / 2) × d | 4 (even) | 1 disk per mirror pair |
Four 4 TB disks in RAID 5 therefore give 12 TB usable (75%). The same disks in RAID 10 give 8 TB; in RAID 6 they also give 8 TB with a second parity disk. Recommended fill is 80% of usable space so the array has room to grow and rebuild.
TB vs TiB
Vendors advertise decimal terabytes (1 TB = 1012 bytes). Linux usually reports tebibytes (1 TiB = 240 bytes). An 8 TB drive is about 7.28 TiB. Switch units in the calculator; the hero number always shows the other unit underneath.
FAQ
How much usable space does RAID 5 have?
RAID 5 usable capacity is the smallest disk multiplied by one fewer than the number of active disks. Four 4 TB disks in RAID 5 give 12 TB usable (75% efficiency) and survive one disk failure.
RAID 5 vs RAID 6 vs RAID 10?
RAID 5 keeps more usable space and survives one failure. RAID 6 spends a second disk on parity so two disks can fail, which matters on large or slow-to-rebuild arrays. RAID 10 uses half the raw capacity, rebuilds by copying a mirror, and is usually the better pick for write-heavy workloads. Compare all three for your disks in the table above.
Why is my 8 TB drive only 7.28 TiB?
Drive makers advertise decimal terabytes (1 TB = 10^12 bytes). Operating systems usually report tebibytes (1 TiB = 2^40 bytes). 8 TB is about 7.28 TiB. Toggle TB and TiB in this calculator to see both.
Does RAID 10 need an even number of disks?
Yes. Linux mdadm RAID 10 stripes across mirrored pairs, so you need at least four disks and an even count. Odd counts are invalid here; they are not silently rounded down.
Does this RAID calculator upload my disk layout?
No. It runs entirely in your browser. Disk sizes never leave your machine.
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