Definitions
Raw capacity is all installed drive capacity. Usable capacity is what remains after the selected generic protection overhead. These formulas use decimal TB; convert to TiB only for a binary display.
| Layout | Usable formula | Minimum drives | Tolerance |
|---|---|---|---|
| Two-drive mirror | 1 × drive capacity | 2 | 1 drive |
| Single parity | (N − 1) × drive capacity | 3 | 1 drive |
| Dual parity | (N − 2) × drive capacity | 4 | 2 drives |
Unit conversion
TiB = TB × 10¹² ÷ 2⁴⁰. A 24 TB usable decimal target is about 21.8 TiB before filesystem or platform overhead.
Example
Four 12 TB drives in a generic single-parity layout provide 48 TB raw, 12 TB protection overhead, and 36 TB usable. In a generic dual-parity layout, usable capacity is 24 TB. The drives and raw capacity have not changed; the protection allocation has.
Common errors
- Comparing raw capacity with a usable requirement.
- Applying a parity formula to unsupported or mixed-size layouts.
- Rounding before calculating protection and headroom.
- Using RAID tolerance as a backup claim.
How tools use this reference
The RAID Capacity Calculator and NAS Bay, Drive Count & Capacity Planner use these generic models. Confirm exact vendor behavior and keep independently recoverable copies.
Limits
Mixed drive sizes, filesystem reserve, snapshots, proprietary hybrid arrays, expansion rules, and vendor implementation details can change the final usable result.