How much usable space will I actually get with my RAID disks? And how many disks can fail without data loss? Here’s how to calculate the usable capacity, fault tolerance, and efficiency of a RAID array, level by level — with a free calculator.
Usable Capacity by RAID Level
For n disks of identical capacity C:
- RAID 0: usable capacity = n × C (100%), but no security.
- RAID 1: usable capacity = C (half with two disks).
- RAID 5: usable capacity = (n − 1) × C.
- RAID 6: usable capacity = (n − 2) × C.
- RAID 10: usable capacity = (n / 2) × C.
Fault Tolerance by Level
- RAID 0: none — one disk lost means all data lost.
- RAID 1 and RAID 5: one disk.
- RAID 6: two disks.
- RAID 10: at least one disk, up to one per sub-mirror depending on distribution.
Concrete Example
With 6 disks of 4 TB each: in RAID 5, usable capacity = (6 − 1) × 4 = 20 TB, fault tolerance 1 disk. In RAID 6 = (6 − 2) × 4 = 16 TB, fault tolerance 2 disks. In RAID 10 = (6 / 2) × 4 = 12 TB, with better write performance.
Calculate Automatically
To get usable capacity, fault tolerance, and efficiency with one click, use our free RAID calculator: enter the number of disks, their size, and the RAID level.
Storage Efficiency
Efficiency = usable capacity / raw capacity. RAID 0 reaches 100%, RAID 1 and RAID 10 cap at 50%, while RAID 5 and RAID 6 improve with more disks (parity is distributed). However, beware: the larger a RAID 5 array, the riskier the rebuild after failure — hence the advantage of RAID 6 for large capacities.
FAQ
Which RAID offers the most usable capacity?
RAID 0 (100%), but with no security. Among secure levels, RAID 5 maximizes usable capacity.
RAID 5 or RAID 6?
RAID 6 is preferable as soon as the array has many disks or high-capacity disks, because it tolerates two failures.
Is RAID a backup?
No. It protects against disk failure, not deletion, ransomware, or disaster. A backup remains essential.
To choose well, read our RAID level guide and browse our hard drives & SSDs, our servers, and our storage bays.