| Metric | Value | |
|---|---|---|
| Disks (active + spares) | {{ activeDisks.length }} + {{ spareCount }} | |
| RAID | {{ raidLabel }} | |
| Raw storage (all) ({{ unit }}) | {{ formatCap(rawAll) }} | |
| Spares capacity ({{ unit }}) | {{ formatCap(sparesRaw) }} | |
| Raw active ({{ unit }}) | {{ formatCap(rawActive) }} | |
| Equalized raw ({{ unit }}) | {{ formatCap(equalizedRaw) }} | |
| Overhead ({{ unit }}) | {{ formatCap(equalizedRaw - rawEffective) }} | |
| Raw post-overhead ({{ unit }}) | {{ formatCap(rawEffective) }} | |
| Target fill | {{ (targetFill*100).toFixed(0) }} % | |
| Protected efficiency | {{ (efficiency*100).toFixed(2) }} % | |
| Usable capacity ({{ unit }}) | {{ formatCap(usable) }} | |
| Redundancy overhead ({{ unit }}) | {{ formatCap(redundancyRaw) }} | |
| Reserved / free ({{ unit }}) | {{ formatCap(reservedRaw) }} |
Redundant Array of Independent Disks (RAID) combines multiple drives to improve availability and performance while trading some raw capacity for redundancy. Capacity planning must consider mirroring or parity overheads, unequal disk sizes, and practical limits like recommended occupancy and filesystem metadata. This tool focuses on usable capacity, helping you quantify how much space remains for data after protections and planning reserves.
You enter either a uniform disk size and count or a list of heterogeneous capacities, then choose a protection scheme and optional spares. The calculator equalizes mixed disks to the smallest active unit, deducts configurable filesystem overhead, and applies a target fill percentage, yielding an efficiency factor and a projected usable figure with a clear breakdown.
Use it when scoping new arrays, expanding shelves, or comparing layouts before procurement. Caution: estimates are planning aids that omit controller peculiarities, rebuild penalties, vendor-specific metadata, and workload-dependent effects.
Concept overview. Usable capacity reflects three stages: equalization, non-data overheads, and protection efficiency. Equalization constrains mixed arrays to the smallest active disk across the disks actually used by the chosen layout. Non-data overheads model filesystem and metadata. Efficiency captures the data fraction after mirroring or parity. Variables include active disk count, smallest active capacity, overhead rate, target fill, mirror width, and parity-per-group assumptions.
Overall sequence:
| RAID level | Minimum active disks | Fault tolerance | Efficiency (symbolic) | Notes |
|---|---|---|---|---|
| 0 | 1 | None | 1 | No redundancy; for testing or scratch workloads. |
| 1 | w | w−1 per set | 1/w | Mirroring; group width w ≥ 2. |
| 5 | 3 | 1 disk | (n−1)/n | Single parity across the stripe. |
| 6 | 4 | 2 disks | (n−2)/n | Dual parity across the stripe. |
| 10 | w×2 | w−1 per set | 1/w | Striped mirrors; w even. |
| 50 | ≥ 2 groups ×3 | 1 per group | (per−1)/per | Striped RAID5 groups. |
| 60 | ≥ 2 groups ×4 | 2 per group | (per−2)/per | Striped RAID6 groups. |
Fault tolerance is per mirror set or per parity group, not across the entire pool. Grouping may leave some disks unused when counts do not divide evenly.
| Parameter | Meaning | Unit/Datatype | Typical Range | Notes |
|---|---|---|---|---|
| Capacity per disk | Uniform capacity for all drives | MB/GB/TB/PB (SI) or MiB/GiB/TiB/PiB (IEC) | 0.5–40 TB | Ignored when a list of disks is provided. |
| Total disks | Drive count including actives and spares | Integer | 1–96 | Active set excludes hot spares. |
| Disk list | Comma/space separated capacities | Array of numbers | 2–96 entries | Mixed sizes are equalized to the smallest active. |
| Data protection | Chosen RAID level and geometry | Enum | 0/1/5/6/10/50/60 | Mirror width or group size may apply. |
| Mirror width | Members per mirror set | Integer | 2–4 | Used by RAID1 and RAID10. |
| Groups × per-group | Number of groups and members | Integers | 2–12 groups; 3–16 per group | Used by RAID50/60. |
| Hot spares | Global reserve drives | Integer | 0–N | Not part of active capacity. |
| Filesystem overhead | Non-data space for metadata | Proportion | 0–0.15 | Adjust per platform guidance. |
| Target fill | Planning occupancy threshold | Proportion | 0.50–0.99 | Lower for fragmentation-sensitive workloads. |
Worked example. Eight 10 TB disks, RAID6, no spares, 3% overhead, 85% target fill.
Equalization:
Overhead:
Target fill:
Protection efficiency (RAID6, n=8):
Usable capacity:
Concepts align with the RAID taxonomy (Patterson, Gibson, Katz), standard storage engineering texts, and the SNIA Dictionary’s definitions of mirroring, parity, and usable capacity.
Inputs describe device capacities rather than personal data; typical privacy regulations such as GDPR or HIPAA do not apply to these calculations.
Follow this sequence to estimate protected, practical storage for your layout.
Caution Equalization can significantly reduce capacity when disk sizes vary widely.
Yes. Mixed arrays are equalized to the smallest active disk times the number of disks used by the geometry, which can reduce capacity noticeably.
Hot spares are excluded from the active set and do not contribute to equalization or efficiency. They exist solely to accelerate recovery readiness.
Target fill models a practical occupancy ceiling to preserve performance headroom and maintenance space. Lower values are prudent when fragmentation is expected.
No. Calculations run locally in your browser using a reactive engine; inputs are not transmitted to a server.
Mirrors duplicate data across members, while parity levels reserve space for parity blocks. Efficiency expresses the data-to-raw ratio implied by each scheme.