RAID Calculator Widget

Storage vendors, homelab blogs and IT teams can let readers size an array: choose the RAID level, the number of drives, their size and (for RAID 50/60) the span groups, and see usable capacity, efficiency, how many drive failures the array survives and what a 1,024-based OS will report.

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<div data-a2z-widget="raid-calculator" data-height="560"></div>
<p class="a2z-attribution" style="margin:6px 0 0;font:12px/1.4 system-ui,sans-serif;opacity:.8">RAID Calculator by <a href="https://a2z.tools/embed/raid-calculator" target="_blank" rel="nofollow noopener">A2Z Tools</a></p>
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Recommended. A small script (what it does) sizes the widget to fit its content, loads it lazily and keeps it isolated in its own frame. Works in WordPress (Custom HTML block), Squarespace, Webflow, Shopify, Blogger, React / Next.js and plain HTML - anywhere a <script> tag is allowed.

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How it works

RAID - redundant arrays of inexpensive disks, named in the 1987 Berkeley paper by Patterson, Gibson and Katz - trades capacity for redundancy. With n equal drives of size s, striping (RAID 0) uses all n x s and survives no failure; mirroring (RAID 1) keeps one drive's worth however many copies there are; single parity (RAID 5) gives (n - 1) x s and survives one failure; double parity (RAID 6) gives (n - 2) x s and survives two; RAID 10 mirrors pairs and stripes them, keeping half. RAID 50 and 60 stripe across g parity groups, losing one or two drives per group. The widget enforces minimums - 3 drives for RAID 5, 4 for RAID 6 and RAID 10, an even count for RAID 10, and equal groups of at least 3 or 4 for 50 and 60 - and states the rule when it refuses. Four 4 TB drives in RAID 5 give 12 TB (10.91 TiB) at 75% efficiency. It covers capacity only.

Calculation method

  • RAID 0: n x s (n >= 2), 0 failures; RAID 1: s (n >= 2), n - 1 failures
  • RAID 5: (n - 1) x s (n >= 3), 1 failure; RAID 6: (n - 2) x s (n >= 4), 2 failures
  • RAID 10: n / 2 x s (even n >= 4), 1 guaranteed, up to n / 2 if each failure is in a different mirror
  • RAID 50: (n - g) x s, g groups of >= 3; RAID 60: (n - 2g) x s, g groups of >= 4; n divisible by g
  • Efficiency = usable / raw; TiB = TB x 10^12 / 2^40

Worked examples

Small NAS

Inputs: RAID 5; 4 drives x 4 TB

Result: 12 TB usable (10.91 TiB); 75% efficiency; survives 1 failure

One drive's worth goes to parity.

Striped double parity

Inputs: RAID 60; 8 drives x 8 TB; 2 groups

Result: 32 TB usable (29.1 TiB); 50% efficiency; 2 failures guaranteed, up to 4

Each group of four loses two drives to parity.

Limitations

  • Capacity and fault tolerance only - no performance, rebuild time or failure-probability estimates.
  • Assumes identical drives and ignores hot spares, controller metadata and file-system overhead.
  • Vendor-specific layouts (RAID-Z, SHR, unRAID) differ and are not modelled.

Where publishers use it

  • A NAS manufacturer's buying guide comparing RAID 5 and RAID 6
  • A homelab blog planning a new storage server
  • A server reseller's configurator help page
  • An IT certification course on storage fundamentals
  • A video-production studio's page on safe shared storage

Questions

How much space do I get from four 4 TB drives in RAID 5?

(4 - 1) x 4 TB = 12 TB, a storage efficiency of 75%. An operating system counting in powers of 1,024 shows it as about 10.91 TiB, before file-system overhead.

RAID 6 or RAID 10?

With six 8 TB drives, RAID 6 gives 32 TB and survives any two failures; RAID 10 gives 24 TB and is guaranteed to survive only one (more if the failures hit different mirror pairs). RAID 6 stores more and tolerates any two failures; this widget does not compare speed.

Why does RAID 10 need an even number of drives?

RAID 10 is a stripe of mirrored pairs, so every drive needs a partner. With five drives one would be left over; most controllers either refuse or leave it as a hot spare.

What are span groups in RAID 50 and 60?

The drives are split into equal groups, each a RAID 5 (or RAID 6) set, and data is striped across the groups. Eight drives in two RAID 6 groups lose four drives to parity and keep 4 x size - but can survive up to four failures if they are spread across the groups.

Can I mix drive sizes?

Most hardware and software RAID treats every member as the size of the smallest drive, so mixing a 4 TB and an 8 TB drive wastes 4 TB. The widget assumes equal drives.

Is RAID a backup?

No. RAID keeps an array running through drive failures, but deletions, ransomware, file-system corruption and controller faults affect all copies at once. Keep separate backups.

Sources

  1. A Case for Redundant Arrays of Inexpensive Disks (RAID) - Patterson, Gibson and Katz, UCB/CSD-87-391 - University of California, Berkeley EECS . The technical report that defined RAID levels with mirroring and parity; levels 0, 6, 10, 50 and 60 are later industry conventions built on the same ideas. Checked 2026-10-01.

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools RAID Calculator
https://a2z.tools/embed/raid-calculator

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