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NAS & RAID Capacity Calculator

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RAID capacity calculator: find usable storage, parity overhead, and fault tolerance for RAID 0, 1, 5, 6, 10 and ZFS RAIDZ1/2/3. Enter drive count and size to compare every level side by side.

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All RAID levels compared for your drive count

RAID levelUsable spaceOverheadDrives lost before data lossMin drives

Usable-space figures show array capacity after RAID/RAIDZ parity or mirror overhead. Enter each drive's labeled (decimal) size; note your OS (Windows / TrueNAS / Synology) reports binary TiB, so it will display roughly 7–10% less than the decimal drive total (e.g. a 12 TB result shows as ~10.9 TiB). RAID 5/RAIDZ1 and RAID 6/RAIDZ2 share the same parity formula, so this tool reports one figure for both, and ZFS RAIDZ's variable-width parity avoids the RAID 5 write-hole. But real ZFS usable capacity is usually a little lower than the parity math: RAIDZ rounds every block up to a multiple of (parity + 1) sectors (“skip blocks”), and ZFS also reserves ~3.2% slop space it won’t let you fill. That padding is negligible with the default 128K recordsize on friendly geometries, but grows with small recordsize/volblocksize on ashift=12 (4K-sector) pools — small-block zvols can approach mirror-level overhead. For exact RAIDZ capacity that models skip blocks, see the OpenZFS capacity calculator. RAID does not replace backups. See the Proxmox Backup Server guide for a 3-2-1 strategy.

Worked examples

These are this calculator’s own outputs, computed with the same formula the tool runs in your browser — so you can see a real answer without touching a single input.

A 4-bay NAS (Synology DS423+ / QNAP / DIY) with four 4 TB drives in RAID 5 — this is the calculator’s own default state on page load

  • Inputs: Number of drives = 4, Capacity per drive = 4, Unit = TB, RAID level = RAID 5 / RAIDZ1
  • Result: Usable space 12.0 TB | Raw capacity 16.0 TB (across 4 × 4 TB) | Overhead (parity) 25%, 1 drive reserved | Drives lost before failure 1 (amber), 1 drive can fail safely

A 6-bay homelab NAS with six 8 TB drives in RAID 6 / ZFS RAIDZ2 — the double-parity setup most people move to once the array gets big

  • Inputs: Number of drives = 6, Capacity per drive = 8, Unit = TB, RAID level = RAID 6 / RAIDZ2
  • Result: Usable space 32.0 TB | Raw capacity 48.0 TB (across 6 × 8 TB) | Overhead (parity) 33%, 2 drives reserved | Drives lost before failure 2 (green), 2 drives can fail safely

A 4-bay box with four 8 TB drives in RAID 10 (two mirrored pairs) — the mirrored-stripe option people pick for faster rebuilds over RAID 5

  • Inputs: Number of drives = 4, Capacity per drive = 8, Unit = TB, RAID level = RAID 10
  • Result: Usable space 16.0 TB | Raw capacity 32.0 TB (across 4 × 8 TB) | Overhead (parity) 50%, 2 drives reserved | Drives lost before failure 1 (amber), 1 drive can fail safely

How to use this calculator

This RAID capacity calculator turns your drive count and drive size into usable storage for every level — RAID 0, 1, 5, 6, 10 and ZFS RAIDZ1/2/3. Select your RAID level, enter how many drives you have and their individual capacity, and it instantly shows usable space, redundancy level, and what happens when a drive fails.

Use the RAID comparison table at the bottom to see all levels side-by-side for your drive count.

The RAID capacity formula

Every RAID level starts from your raw capacity — drive count (N) times drive size — and reserves some of it for redundancy. The usable-space math is exactly what this calculator runs:

  • RAID 0 / JBOD — N x size. No redundancy, 100% usable.
  • RAID 1 (mirror) — size. One drive’s worth of usable space; every drive holds a full copy.
  • RAID 5 / RAIDZ1 — (N - 1) x size. One drive reserved for parity.
  • RAID 6 / RAIDZ2 — (N - 2) x size. Two drives reserved for parity.
  • RAIDZ3 — (N - 3) x size. Three drives reserved for parity.
  • RAID 10 — (N / 2) x size. Half the raw capacity, in mirrored pairs.

Worked example. Four 4 TB drives (16 TB raw): RAID 5 gives (4 - 1) x 4 = 12 TB usable (25% overhead); RAID 6 gives (4 - 2) x 4 = 8 TB usable (50% overhead); RAID 10 also gives 8 TB usable but rebuilds faster after a failure. Six 8 TB drives in RAIDZ2 give (6 - 2) x 8 = 32 TB usable.

Those are decimal TB, the way drives are labeled. Your OS reports binary TiB, so a 12 TB result shows as roughly 10.9 TiB — about 7-10% less — before any filesystem overhead.

A ZFS caveat (RAIDZ padding). RAIDZ uses the parity formula above, but real usable capacity on ZFS is usually a little lower than the raw math shows. RAIDZ rounds every block up to a multiple of (parity + 1) sectors — the “skip blocks” that catch people out — and ZFS also reserves about 3.2% slop space it won’t let you fill. The padding is negligible with the default 128K recordsize on friendly drive counts, but grows with small recordsize/volblocksize on ashift=12 (4K-sector) pools, where small-block zvols (VM disks) can approach mirror-level overhead. So treat the RAIDZ figures here as the parity ceiling; for exact capacity that models skip blocks, use the OpenZFS capacity calculator.

Choosing a RAID level

Zero tolerance for data loss at any point — use RAID 6 or ZFS RAIDZ2. Both survive two simultaneous drive failures and are safe during the rebuild window after the first failure.

Best usable space with one-drive redundancy — RAID 5 or ZFS RAIDZ1. Same parity formula, though ZFS’s real usable capacity runs a little lower once RAIDZ padding is counted (see the ZFS note above); the ZFS version is also safer (no write hole).

Simple mirroring for small arrays — RAID 1 (2 drives) or RAID 10 (4+ drives in mirrored pairs). Lower capacity efficiency but simpler failure recovery and better random read performance.

Maximum space, no redundancy — RAID 0 or JBOD. One drive failure loses everything. Only appropriate for scratch storage or content you have elsewhere.

RAID does not replace backups

RAID protects against drive hardware failure. It does not protect against:

  • Accidental file deletion (the deletion propagates across all drives instantly)
  • Ransomware (encrypts data on all drives simultaneously)
  • Controller failure (the RAID controller itself can corrupt data on failure)
  • Fire, theft, or physical disaster

For a complete data protection strategy: RAID for uptime during a drive failure, plus a 3-2-1 backup strategy with at least one off-site copy. The two layers address different failure modes.

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Frequently asked questions

Which RAID level gives the most usable space?
RAID 0 and JBOD give 100% usable space with no redundancy — if any drive fails, all data is lost. RAID 5 uses N-1 drives for storage (one drive’s worth of parity). RAID 6 and RAIDZ2 use N-2 (two parity drives). RAID 1 uses half the raw capacity (mirroring). For maximum usable space with at least one drive failure tolerance, RAID 5 and RAIDZ1 win.
What's the difference between RAID 5 and ZFS RAIDZ1?
Both tolerate one drive failure and use N-1 drives for usable storage. The difference is in how parity is calculated and written. RAID 5 uses traditional stripe-based fixed-width parity, which creates a write hole vulnerability (partial stripe writes after a crash can corrupt data). ZFS RAIDZ1 uses variable-width parity stripes and is copy-on-write — there’s no write hole. For homelab NAS use, RAIDZ1 is generally preferred over software RAID 5.
Can I use drives of different sizes in a RAID array?
In traditional RAID (hardware or md/mdadm software RAID), mismatched drive sizes waste capacity — the array uses the smallest drive’s size as the per-drive capacity. In ZFS, you can mix sizes with some caveats: RAIDZ uses the smallest drive size within a vdev. ZFS mirrors can use mismatched sizes but the usable space per mirror is limited by the smaller drive.
How does RAID 6 protect against two simultaneous drive failures?
RAID 6 stores two independent parity blocks per stripe using different algorithms (P+Q parity, typically Reed-Solomon). If any two drives fail simultaneously, the data can be reconstructed from the remaining drives and both parity blocks. This makes RAID 6 safe during a drive rebuild — if a second drive fails while the array is rebuilding after the first failure, data survives. RAID 5 does not protect against a second failure during rebuild.
How do you calculate RAID capacity?
Usable RAID capacity is your raw capacity (drive count x drive size) minus whatever the RAID level reserves for redundancy. RAID 0 and JBOD keep 100% (N x size). RAID 5 and RAIDZ1 reserve one drive: (N - 1) x size. RAID 6 and RAIDZ2 reserve two: (N - 2) x size. RAID 1 mirroring leaves one drive’s worth usable, and RAID 10 leaves half. For example, four 4 TB drives in RAID 5 give (4 - 1) x 4 = 12 TB usable. The calculator above runs this math for every level at once.
How much usable space does RAID 6 give?
RAID 6 (and ZFS RAIDZ2) reserve two drives for parity, so usable space is (N - 2) x drive size. Six 8 TB drives give (6 - 2) x 8 = 32 TB usable — 33% overhead — while surviving any two simultaneous drive failures. That two-drive tolerance is why RAID 6 is the safe choice for large arrays: it stays protected even during the rebuild after a first drive fails.
Does RAID replace backups?
No. RAID protects against drive failure only — it does not protect against accidental deletion, ransomware, controller failure, fire, or theft. RAID and backups serve different purposes. A 3-2-1 backup strategy requires a separate copy off the RAID array entirely. The calculator shows your storage redundancy; use the Proxmox Backup Server guide for the backup layer.