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NAS / RAID Usable Capacity Calculator

Calculate usable capacity and fault tolerance for RAID 0, 1, 5, 6, and 10 arrays, including the formatting overhead and TB-vs-TiB unit gap that make your NAS show less than the sticker capacity.

Built and maintained by Paul Clark, Redmoon Software

When to use this

Use this when planning a new NAS or server build to see real usable space and fault tolerance before buying drives, and to compare RAID levels for the same drive count.

How it compares

Unlike a simple “sum of drive capacities” estimate, this calculator applies the actual usable-capacity formula for each RAID level, validates your drive count against that level’s minimum requirement, and separately shows the TB-vs-TiB unit gap that makes a NAS look smaller than expected.

Enter your values below. Calculations run locally as you type.

Array configuration

Usable capacity

Raw capacity
16.00 TB
4 × 4.00 TB
Usable (formatted)
10.80 TB
~9.82 TiB in OS
Fault tolerance
1 drive
can fail without data loss

4 × 4.00TB drives in RAID 5 yield about 10.80 TB of usable space after formatting overhead — your OS will likely report roughly 9.82 TiB.

How it works

Each RAID level trades usable capacity for fault tolerance differently: RAID 0 uses all raw capacity with none, RAID 1 mirrors drives in pairs, RAID 5 reserves one drive’s worth of capacity for parity, RAID 6 reserves two, and RAID 10 mirrors then stripes pairs.

Usable capacity is reduced further by a formatting overhead percentage, which accounts for filesystem metadata and the gap between manufacturers’ decimal-TB ratings and the binary-TiB units most operating systems display.

The calculator also checks your drive count against each RAID level’s minimum requirement — for example RAID 6 needs at least 4 drives, and RAID 10 needs an even number of at least 4.

FAQs

Why does my NAS show less space than the drives’ rated capacity?

Manufacturers rate drives in decimal terabytes (1 TB = 1,000,000,000,000 bytes), while operating systems typically report binary tebibytes. On top of that gap, RAID parity and filesystem metadata both consume real space.

RAID 5 vs. RAID 6 — which should I use?

RAID 5 tolerates one drive failure; RAID 6 tolerates two. As drives get larger, rebuilding a failed RAID 5 array can take a long time, during which a second failure would destroy the array — RAID 6’s extra parity protects against exactly that scenario.

Is RAID a backup?

No. RAID protects against a drive failure, but it does nothing to protect against accidental deletion, ransomware, fire, theft, or a controller failure that corrupts the whole array. You still need a separate backup, ideally offsite.

Worked example

Input

4 drives × 4TB in RAID 5, 10% formatting overhead.

Output

Raw: 16 TB. Usable (formatted): 10.8 TB (~9.82 TiB as reported by most OSes). Fault tolerance: 1 drive.

RAID 5 reserves one drive’s worth of capacity for parity, so usable capacity before overhead is (4-1)×4TB = 12TB. After 10% formatting overhead, that drops to 10.8TB, which will typically show as about 9.82 TiB in an operating system that reports binary units.

Common pitfalls

  • Assuming usable capacity equals raw drive capacity ignores both the RAID level’s parity/mirroring overhead and filesystem formatting overhead.
  • Mixing different drive capacities in an array without noticing that most RAID levels size the array to the smallest drive, wasting capacity on larger drives.
  • Treating RAID as a backup — it protects only against a drive failure, not against deletion, ransomware, theft, or fire.

What each level costs in capacity

RAID levels trade usable capacity for redundancy in different proportions. RAID 5 gives up one drive's worth to parity, RAID 6 two, RAID 10 half the total to mirroring, and RAID 0 nothing at all — while providing no redundancy whatsoever and failing entirely if any single drive dies.

The 10% overhead default covers filesystem metadata and the gap between the decimal terabytes drives are sold in and the binary units operating systems report, which together account for most of the "missing" capacity people notice on a new array.

Rebuild risk, and the thing RAID is not

RAID 5 on large modern drives carries a specific hazard worth understanding: rebuilding after a failure reads every sector of every surviving drive, which is both the heaviest possible load and precisely when a second drive is most likely to fail. On large arrays that window lasts many hours or days, and a second failure during it loses everything. RAID 6 exists to survive that case.

The more important point: RAID is availability, not backup. It protects against drive failure and against nothing else — not deletion, not corruption, not ransomware, not fire, not a controller writing garbage to every member at once. An array with no separate backup is a single copy that happens to tolerate one hardware fault.

Read more about this

· 8 min read

How to choose a RAID level: matching redundancy to how much you can lose

RAID 0, 1, 5, 6, and 10 make different trades between usable space, fault tolerance, rebuild risk, and performance. A practical decision guide for picking the level that fits your drive count, capacity, and tolerance for downtime.
· 7 min read

RAID capacity explained: why your NAS shows less space than the drives promise

A breakdown of the TB-versus-TiB unit gap, how RAID 0, 1, 5, 6, and 10 each trade usable capacity for fault tolerance, and why RAID protects against a dead drive but not against deleted files, ransomware, or a house fire.

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