RAID Capacity Calculator
Buying N Drives Doesn't Mean N Drives of Usable Space
Every RAID level beyond simple striping spends some of the raw disk capacity on redundancy, parity, or mirroring, in exchange for the array surviving one or more disk failures without losing data. How much capacity gets sacrificed — and how much fault tolerance you get in return — depends entirely on which RAID level you choose, which is why the same eight drives can yield very different usable totals depending on the array configuration.
The Formulas
RAID 1: Usable = Disk Size (mirrored pair)
RAID 5: Usable = (Number of Disks − 1) × Disk Size
RAID 6: Usable = (Number of Disks − 2) × Disk Size
RAID 10: Usable = (Number of Disks ÷ 2) × Disk Size
RAID 5 sacrifices one disk's worth of capacity to parity data spread across the array; RAID 6 sacrifices two disks' worth for double parity, surviving two simultaneous failures; RAID 10 mirrors pairs of disks and then stripes across those mirrored pairs, giving up exactly half the raw capacity.
Choosing a RAID Level Is a Capacity-vs-Risk Decision
- RAID 0 — maximizes usable capacity and read/write speed but offers zero fault tolerance; a single disk failure loses everything, so it's suited only to data that's disposable or backed up elsewhere.
- RAID 5 — a common middle ground for file servers, giving good usable capacity with tolerance for one disk failure, though rebuild times on large disks can be lengthy.
- RAID 6 — the extra parity disk matters more as drives get larger, since rebuild windows (during which a second failure would be catastrophic on RAID 5) stretch out with capacity.
- RAID 10 — favored for performance-sensitive workloads like databases, since it avoids the parity-calculation overhead of RAID 5/6 while still tolerating drive failures.
Eight 2 TB Disks, Every RAID Level Compared
| RAID Level | Usable Capacity | Fault Tolerance |
|---|---|---|
| RAID 0 | 16,000 GB | None |
| RAID 1 | 2,000 GB | Survives failure of all but one disk in each mirrored pair |
| RAID 5 | 14,000 GB | 1 disk failure |
| RAID 6 | 12,000 GB | 2 disk failures |
| RAID 10 | 8,000 GB | Multiple failures, if not both disks in the same mirrored pair |
RAID 1 in this calculator reflects the capacity of a single mirrored pair (one disk's worth), since RAID 1 arrays are built from disks mirrored in pairs rather than striped across the whole set.
How to Use This Calculator
- Select the RAID Level: 0, 1, 5, 6, or 10.
- Enter the Number of Disks in the array.
- Enter the Disk Size (GB) for each drive (assumes identical drives, as RAID arrays require).
- Select Calculate to see the usable capacity, raw total capacity, and the fault tolerance for that configuration.
Related Calculations
Estimating how long that storage will last under a given write load? See the SSD Lifespan Calculator, or convert between storage units with the Storage Calculator.