RAID Capacity Calculator
Calculate usable capacity, fault tolerance, and efficiency for RAID 0, 1, 5, 6, 10, and JBOD from your drive count and size. Free NAS planning tool.
Capacity math only — real formatted space is a few percent lower, and drive makers count TB as 10¹² bytes (so a "4 TB" drive shows ~3.64 TiB in your OS). RAID is not a backup: it protects against drive failure, not deletion, ransomware, or controller failure. Keep a separate 3-2-1 backup.
Choosing a level
The trade is always capacity vs. how many drives can die before you lose data. RAID 5 (one parity drive) is the classic balance for small arrays. RAID 6 (two parity) is the honest choice once drives get large, because rebuilding a failed 12+ TB drive takes so long that a second failure during the rebuild is a real risk. RAID 10 trades half your capacity for speed and fast, low-stress rebuilds. Full breakdown: RAID levels explained for small-business NAS.
RAID is not a backup
This is the mistake that costs people their data. RAID keeps you running when a drive fails — it does nothing against accidental deletion, ransomware, a failed controller, fire, or theft. You still need a separate backup following the 3-2-1 rule (three copies, two media, one off-site).