Storage capacity utility
RAID Usable Capacity Calculator
Calculate nominal usable capacity for equal-size RAID 0, 1, 5, 6, and 10 arrays. The result exposes the formula and unit conversion instead of pretending controller, filesystem, performance, or reliability behavior can be derived from drive count alone.
Interactive calculation
Calculate nominal array capacity
This version assumes equal-capacity drives. Enter the number of array members and the advertised decimal capacity of each drive.
N = number of drives. C = equal capacity per drive. 1 TB here means 1,000,000,000,000 bytes.
Nominal usable capacity
12 TB
≈ 10.91 TiB before filesystem, metadata, reserved-space, and implementation overhead
- Raw drive capacity
- 16 TB
- Capacity efficiency
- 75%
- Formula
- (N − 1) × C
One drive-equivalent of capacity is consumed by distributed parity.
Scope: equal-capacity members only. Mixed-size arrays, hot spares, vendor-specific layouts, RAID-Z, SHR, dRAID, erasure coding, and controller metadata are outside this formula.
Interpretation: capacity arithmetic does not predict speed, rebuild time, reliability, fault behavior, recoverability, or whether a given controller supports the layout. RAID is not a substitute for a separate backup.
Capacity model
What each formula reserves
For N equal-capacity drives of C TB each, RAID 0 uses N × C. RAID 5 reserves one drive-equivalent for distributed parity, so nominal usable capacity is (N − 1) × C. RAID 6 reserves two drive-equivalents, giving (N − 2) × C.
RAID 10 is modeled as striped mirrored pairs, so it requires an even drive count and yields (N ÷ 2) × C. RAID 1 is modeled as one data copy mirrored across all entered members, so its usable capacity is C rather than N × C.
Boundaries
Nominal RAID capacity is not formatted free space
Drive labels use decimal capacity. Operating systems may display the same byte count using binary TiB or label binary values differently. The calculator shows both decimal TB and the corresponding TiB before additional overhead.
Real arrays can reserve capacity for controller metadata, filesystems, snapshots, hot spares, alignment, vendor-specific layouts, or other features. Those details are deliberately excluded because they are not universal consequences of a RAID level.
References
RAID layout and capacity sources
Primary platform documentation supports the level definitions, parity model, and minimum-member constraints. Specific controllers and software stacks can impose stricter requirements, so this tool remains a generic equal-drive arithmetic model.