Disk & Storage Calculator
Calculate usable capacity after RAID, formatting overhead, and growth — plus cloud storage cost.
Your details
Overhead & growth
Cost comparison
This calculator runs entirely on your device. Nothing you enter is uploaded, stored, or sold. How this works
Usable capacity
27.6 TiB
48.0 TB raw → 40.0 TB after RAID 5 → 27.6 TiB practical
Includes your inputs, the full breakdown, and every row of the table.
Raw capacity
48.0 TB
After RAID
40.0 TB
Tolerates 1 drive failing
Capacity efficiency
83%
Runway at current growth
4 years
12.0 TiB today growing 30%/yr
- 6 × 8.0 TB drives
- 48.0 TB raw
- RAID5 parity/mirror loss
- - 8.0 TB
- Decimal TB → binary TiB
- 36.4 TiB
- Filesystem overhead (5%)
- - 1.8 TiB
- Free space reserve (20%)
- - 6.9 TiB
- Practical usable
- 27.6 TiB
- Hardware cost
- $990
- Equivalent cloud cost per year
- $10,080
| Period | Data stored | Status |
|---|---|---|
| Year 1 | 15.6 TiB | Within capacity |
| Year 2 | 20.3 TiB | Within capacity |
| Year 3 | 26.4 TiB | Within capacity |
| Year 4 | 34.3 TiB | Over capacity |
| Year 5 | 44.6 TiB | Over capacity |
| Year 6 | 57.9 TiB | Over capacity |
| Year 7 | 75.3 TiB | Over capacity |
What this means
- Drive vendors sell decimal terabytes while operating systems report binary tebibytes, which is why an 8 TB drive shows as roughly 7.28 TiB. That is not lost space, just a different unit.
- Keep at least twenty percent free. Both ZFS and traditional filesystems slow considerably when they approach full, and copy-on-write filesystems degrade badly past ninety percent.
- RAID 5 with large drives is risky: rebuild times run into days and a second failure during rebuild destroys the array. RAID 6 or RAID-Z2 is the safer choice above about 6 TB per drive.
How the disk & storage calculation works
Usable storage is always less than the number printed on the box. Three separate reductions apply: RAID parity or mirroring, the decimal-to-binary unit conversion, and filesystem metadata overhead. A six-drive 8 TB RAID 5 array advertises 48 TB and delivers roughly 36 TiB of practical space.
Capacity planning also needs a growth assumption. Storage demand compounds, so a thirty percent annual growth rate doubles your footprint in under three years. Sizing only for today's data guarantees a migration much sooner than expected.
Frequently asked questions
Why does my 8 TB drive show as 7.27 TB?
Manufacturers count a terabyte as 10^12 bytes; operating systems count 2^40 bytes and often still label it 'TB'. The difference is about 9%, and no space is actually missing.
Is RAID 5 or RAID 6 better?
RAID 6 for any array with drives above roughly 6 TB. Rebuilds take so long on large drives that the chance of a second failure during the rebuild window becomes unacceptably high.
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Disclaimer. This calculator is provided for general information and educational purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Results are estimates based on the inputs you provide and the assumptions described above. Confirm any figure with a qualified professional before acting on it.