| Level | Usable | Efficiency | Can lose | Read | Write | Cost / usable TB |
|---|
RAID combines several drives into one array that's bigger, faster, more resilient — or some balance of the three. The catch is that resilience costs capacity: the drives that protect you against failure can't also store your data. This tool shows that trade-off directly. Set your drive count, size, and type, and every level's usable space, how many drives can fail, speed, and cost appear side by side.
Eight 4 TB drives give 32 TB raw. In RAID 5 you'd get 28 TB usable (one drive's worth goes to parity) and survive one failure. RAID 6 drops you to 24 TB but survives two — and on drives this large, that second parity matters more than it sounds.
The reason is the rebuild. After one failure, RAID 5 must read every remaining drive to rebuild — and on big consumer drives, the odds of hitting an unrecoverable read error partway through climb past 50%. That's why RAID 6 (or RAID-Z2) is the modern default for large arrays.
No — and this is the most important thing to know. RAID protects against drive failure, but not against deletion, ransomware, fire, theft, or a controller frying the whole array. You still need real backups, ideally off-site.
Redundancy has to live somewhere. RAID 5 spends one drive on parity, RAID 6 two, mirroring half. That "lost" capacity is exactly what lets the array survive a failure — it's the price of safety, not waste.
For a handful of small drives, RAID 5 is fine. For large drives (8 TB+) or many of them, prefer RAID 6 / RAID-Z2 — the rebuild-risk readout shows why single parity gets dangerous as arrays grow.
Most parity RAID treats every drive as if it were the smallest one, so the extra space on larger drives is stranded. This tool assumes matched drives; for mixed sizes, Unraid or ZFS handle it more gracefully. Matching your drives is almost always simpler.