RAID Usable Capacity Calculator
See the usable storage, fault tolerance and space efficiency of a RAID array by level, from the number and size of your drives.
How it works
RAID 0 = n·size · RAID 1 = size · RAID 5 = (n−1)·size · RAID 6 = (n−2)·size · RAID 10 = (n÷2)·size · efficiency = usable ÷ raw
Every redundant RAID level trades raw capacity for the ability to survive a failed drive, and the cost depends on the level: parity levels (5 and 6) sacrifice one or two drives’ worth of space, while mirroring (1 and 10) gives up half. RAID 0 keeps all the space but tolerates no failures — one dead drive loses everything. Efficiency is simply usable divided by raw; it rises as you add drives to a parity array, since the parity overhead is a fixed number of drives spread across more members. Note that drive vendors count a TB as 10¹² bytes, so the operating system will report a few percent less than these decimal figures.
Worked example
Six 4 TB drives (24 TB raw) in RAID 6 use two drives for parity, leaving (6 − 2) × 4 = 16 TB usable — 66.7% efficiency — and the array keeps running through the failure of any two drives at once.
Sources: Standard RAID levels — Wikipedia
Frequently asked questions
How is this different from a plain RAID capacity calculator?
It reports not just usable space but the space efficiency, exactly how much capacity redundancy costs, and how many drive failures each level survives — the trade-offs that actually drive the level choice.
Why is RAID 10 usable capacity only half?
RAID 10 mirrors every block onto a second drive, so half of all raw capacity holds copies. In return it offers fast rebuilds and can survive multiple failures as long as no mirrored pair loses both members.
Will my operating system show this exact size?
Slightly less. Manufacturers define a terabyte as 10¹² bytes, but operating systems often measure in tebibytes (2⁴⁰), so a 4 TB drive shows as about 3.64 TiB. Formatting overhead trims a little more.
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This calculator is for educational purposes. Double-check important results before acting on them.