RAID Capacity Calculator
Before you buy drives for a NAS, know what you'll actually get. Pick your drive count, size and RAID level to see usable capacity, how many drives can fail without losing data, and exactly how much space the redundancy costs you. Covers RAID 0, 1, 5, 6, 10 and JBOD. Free, no sign-up.
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RAID questions, answered
Why is my usable capacity less than the drive total?
Two things shrink it. First, redundancy: parity and mirror levels reserve drives so the array survives a failure — that reserved space isn't usable. Second, the TB-versus-TiB gap: drives are sold in decimal terabytes but operating systems report binary tebibytes, so a "10 TB" drive shows as about 9.1 TiB. This calculator shows the raw usable figure before that formatting overhead.
Which RAID level should I choose?
For a typical multi-drive NAS, RAID 5 (one drive of parity) balances capacity and safety; RAID 6 (two drives of parity) is the safer choice once you're using large drives or many of them, because rebuilds take a long time and a second failure during a rebuild isn't rare. RAID 1 suits two-drive setups, RAID 10 favours performance, and RAID 0 or JBOD give maximum space with no protection — only for data you can afford to lose.
Does RAID replace backups?
No — and this is the most important point. RAID keeps your data online through a drive failure, but it does nothing against accidental deletion, file corruption, ransomware, theft or the whole NAS dying. Follow a 3-2-1 approach: keep your data in more than one place, on more than one type of media, with at least one copy off-site.
What happens during a rebuild?
When you replace a failed drive, the array rebuilds the missing data from parity or its mirror — which can take hours to days on large drives and stresses the remaining disks while it works. During that window a single-parity array (RAID 5) has no protection left, which is why RAID 6 is recommended for large arrays: it can survive a second failure mid-rebuild.