RAID Calculator
AvailableCalculate usable capacity, storage efficiency, theoretical performance and fault tolerance for RAID 0, 1, 1E, 5, 5E, 5EE, 6, 10, 50 and 60 — with a disk layout diagram, capacity breakdown and a failure simulator.
Configure the array
Capacity
Disk layout
Logical, illustrative layoutEach bay shows one disk’s share of data, parity, mirror copies, spare and unused space. Controllers spread parity and mirror blocks across disks; the diagram shows the amounts, not physical block positions. Click disks to simulate failures.
Performance & fault tolerance
How it works
Each RAID level spends part of the raw capacity on redundancy: mirrored levels store copies, parity levels store one (RAID 5) or two (RAID 6) disks’ worth of parity, and 5E/5EE keep spare capacity for rebuilds. Every disk in an array (or in each group of a nested array) is used only up to the size of its smallest member. The calculator applies these rules exactly, then subtracts an optional reservation for the filesystem or system.
RAID levels at a glance
N = number of disks, S = size of the smallest disk, G = number of groups, g = disks per group.
| Level | Min. disks | Usable capacity | Failures survived (guaranteed) |
|---|---|---|---|
| RAID 0 | 2 | N × S | 0 |
| RAID 1 | 2 | S | N − 1 |
| RAID 1E | 3 | N × S ÷ 2 | 1 (more if not neighbours) |
| RAID 5 | 3 | (N − 1) × S | 1 |
| RAID 5E | 4 | (N − 2) × S with one spare | 1, then 1 more after rebuild |
| RAID 5EE | 4 | (N − 2) × S with one spare | 1, then 1 more after rebuild |
| RAID 6 | 4 | (N − 2) × S | 2 |
| RAID 10 | 4 | (N ÷ 2) × S with pairs | 1 (up to one per pair) |
| RAID 50 | 6 | G × (g − 1) × S | 1 (up to one per group) |
| RAID 60 | 8 | G × (g − 2) × S | 2 (up to two per group) |
RAID 1E, 5E and 5EE are vendor-specific; the calculator states which layout it models. RAID protects against some disk failures only — it is not a backup.
Frequently asked questions
Is RAID a backup?
No. RAID keeps a system running through some disk failures, but it copies deletions, corruption and ransomware encryption instantly, and a controller fault, fire or theft can take the whole array. Keep separate backups, ideally including one offline or off-site copy.
Why is the available space smaller than the calculated capacity?
Disk makers use decimal units (1 TB = 10¹² bytes) while many operating systems show binary units (TiB) labelled “TB”; filesystems, metadata, snapshots and system partitions take further space. Use the reservation option for a rough allowance.
RAID 5 or RAID 6?
RAID 6 survives two simultaneous disk failures, RAID 5 only one. With large hard disks a rebuild can take a day or more, and the array has no redundancy left during a RAID 5 rebuild, so RAID 6 (or RAID 10) is usually preferred for large HDD arrays.
Can I mix disks of different sizes?
In classic RAID each disk contributes only as much as the smallest disk in its array or group; the rest stays unused. Some NAS systems (e.g. hybrid RAID schemes) use the extra space differently — this calculator shows the conservative classic result.
How many disks can fail in RAID 10?
At least one is always survivable. More can fail as long as no mirror pair loses both disks — with 8 disks up to 4, if each failure hits a different pair. Use the failure simulator to test a scenario.