Plan your RAID array before purchase. See exactly how much usable storage you get, how many drives can fail, and storage efficiency for every RAID level.
WHICH RAID LEVEL?
RAID 5 is the most common for NAS/server use — good balance of storage efficiency (~75% with 4 drives) and fault tolerance. RAID 6 survives 2 drive failures. RAID 10 has the best rebuild performance but uses 50% of capacity. Never use RAID 0 for important data.
RAID Calculator — Usable Storage, Fault Tolerance and Efficiency for All RAID Levels
Our free RAID calculator calculates usable storage capacity, fault tolerance (number of drives that can fail), and storage efficiency for RAID 0, RAID 1, RAID 5, RAID 6 and RAID 10. Plan your NAS, SAN or server storage array before buying hardware — and avoid the costly mistake of buying drives only to find you have less storage than expected.
RAID Level Comparison — Which RAID Should You Use?
- RAID 0 (Striping) — All capacity is usable. Zero fault tolerance — one drive fails and all data is lost. Only suitable for non-critical temporary storage or caching.
- RAID 1 (Mirroring) — Data is written identically to two drives. 50% storage efficiency. Survives one drive failure. Best for critical small datasets like OS drives.
- RAID 5 (Striping + Parity) — Requires minimum 3 drives. One drive worth of capacity used for parity. Survives one drive failure. Best balance of performance, capacity and redundancy for most NAS and server deployments.
- RAID 6 (Striping + Double Parity) — Requires minimum 4 drives. Two drives used for parity. Survives two simultaneous drive failures. Recommended when using large drives (4TB+) where rebuild times increase the risk of a second failure.
- RAID 10 (Mirror + Stripe) — Requires minimum 4 drives (must be even number). 50% efficiency. Fastest rebuild time and best performance. Recommended for high-traffic databases and virtualisation hosts.
How RAID Storage Capacity is Calculated
- RAID 0: Usable = Number of Drives × Drive Size
- RAID 1: Usable = Drive Size (only one copy counts)
- RAID 5: Usable = (Number of Drives − 1) × Drive Size
- RAID 6: Usable = (Number of Drives − 2) × Drive Size
- RAID 10: Usable = (Number of Drives ÷ 2) × Drive Size
Important RAID Warnings
RAID is not a backup. RAID protects against drive failure but not against accidental deletion, ransomware, fire, or controller failure. Always follow the 3-2-1 backup rule: 3 copies, 2 different media types, 1 offsite. Also note that with large drives (8TB+), RAID 5 rebuild times can exceed 24 hours — increasing the risk of a second failure during rebuild. Consider RAID 6 or ZFS for large arrays.