RAID Capacity Calculator
Raw terabytes in, usable terabytes out — what the layout lets you keep and what the promise costs.
RAID Capacity Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Classic levels 0, 1, 5, 6, 1+0 only
- Decimal GB as storage is sold
In short: Eight 4 TB disks are 32.000000 GB of raw steel; in RAID 6 two of them work purely on parity, leaving 24.000000 GB usable — 75.000000%. RAID 5 on the same shelf returns 28.000000 GB (87.500000%), mirroring returns 16.000000 GB (50%). The layout decides the discount before a single file lands.
Formula
usable = raw − tax ··· raw = n × disk ··· tax: 0 · n/2 · 1 · 2 · n/2 disks
Capacity is the survival table with a price tag: parity layouts surrender one or two disks array-wide, mirrors surrender half of everything. Efficiency is usable over raw, and it falls as arrays get small — parity on four disks is a worse deal than parity on sixteen. Sizes here are decimal GB like every data page, the way drive makers print the box.
Worked Example
- Enter the whole disk count and one disk's size.
- Enter the level: 0, 1, 5, 6 or 10.
- Read usable capacity and the efficiency cut.
- Compare levels before the purchase order.
Defaults: 8 × 4 TB in RAID 6 → 24.000000 GB usable, 75.000000%. Drive the level to 5 → 28.000000 GB, 87.500000%; to 1+0 → 16.000000 GB, 50.000000%.
Strengths & Limits Of This Model
Where this engine is strong
- Tax disks named per level, not hidden in a percent
- Same table as the survival page, different question
Where it stops
- No filesystem or formatting overhead modeled
Practical Use Cases
Purchase orders
terabytes per rack unit
Level trades
the terabyte cost of tolerance
Growth plans
what one more disk buys per level
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
RAID Capacity Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How is usable RAID capacity calculated?
Multiply the disks by their size for raw, then subtract the layout's tax: RAID 0 keeps everything, RAID 5 loses one disk, RAID 6 loses two, mirroring keeps half. Eight 4 TB disks in RAID 6 are 32.000000 raw minus 8.000000 parity = 24.000000 GB usable.
Which RAID level gives the most usable space?
RAID 0 — all raw, no protection. Among protected layouts, RAID 5 is the most space-efficient at one disk of parity, which is exactly its appeal on big arrays. The efficiency read on this page puts the trade in percent so the discount is impossible to miss.
Why does mirroring lose half the capacity?
Every byte is written twice — the second copy is the recovery path, not storage. That is expensive per terabyte and cheap per recovery: a mirror rebuild is a straight copy, no parity math. 1+0 inherits the same fifty percent with striping on top.
Does efficiency improve with more disks?
For parity layouts, yes — one or two parity disks amortize across more data disks: RAID 5 on 4 disks keeps 75.000000%, on 16 keeps 93.750000%. Mirroring stays at exactly half regardless of width. The efficiency read moves only when the layout's tax is fixed and the array grows.
What does one more disk add?
Its full size divided by the layout: a parity disk adds one disk's worth of data space, a mirrored disk adds half. On the defaults, disk nine in RAID 6 adds 4.000000 GB of usable capacity — the page prices the marginal buy before you make it.
Are these the same terabytes the box prints?
Yes — decimal GB throughout, the way storage is sold. Operating systems that relabel the same bytes in binary units will show smaller-looking numbers; the bytes are identical. Every data page on this site states the convention so the arithmetic never argues.
How do I choose the level for a file server?
Capacity efficiency and two-disk tolerance push toward RAID 6 on wide arrays; small arrays and write-heavy databases pay for mirrors. The survival verdicts live on the RAID calculator page; this page prices what each verdict costs in terabytes.
What about hot spares?
A spare is a whole disk that holds no data until a failure — count it in the shelf, not in the capacity. Enter only the data disks here, then remember the spare when you price the rack.