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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.

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.

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.

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