NAS Capacity
Calculate usable network-attached storage capacity across drives and RAID configuration.
About NAS Capacity
NAS usable capacity starts from raw capacity (disk count × per-disk size), applies the RAID level's efficiency factor to account for parity or mirroring, then subtracts filesystem overhead for inode tables, journaling, and reserved blocks that most filesystems (ext4, XFS, ZFS) set aside. The result is a realistic estimate of the space actually available for files, typically 5-15% below the RAID-adjusted figure.
Formula
Usable capacity
usable = disks × disk_size × raid_efficiency% × (1 − filesystem_overhead%)
Worked example
With the default values below, NAS Capacity returns Usable Capacity of 22.80. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of Disksminimum 1
- 8
- Size per Diskminimum 0 TB
- 4 TB
- RAID Efficiencyaccepts 10–100
- 75
- Filesystem Overheadaccepts 0–20
- 5
Results
- Usable Capacity
- 22.80 TB
- Raw Capacity
- 32.00 TB
- Filesystem Overhead
- 1.20 TB
- Overall Efficiency (usable / raw)
- 71.3%
Step by step
- 1Values used
- 2Usable capacity
- 3Usable Capacity= 22.80 TB
- 4Raw Capacity= 32.00 TB
- 5Filesystem Overhead= 1.20 TB
- 6Overall Efficiency (usable / raw)= 71.3
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What RAID efficiency should I use for RAID 5 vs RAID 6?
RAID 5 efficiency is (n-1)/n where n is the disk count in the group — 87.5% for 8 disks. RAID 6 is (n-2)/n — 75% for 8 disks. RAID 10 is always 50% regardless of disk count since every disk is mirrored.
Why is filesystem overhead typically higher on NAS than block storage?
NAS exposes a filesystem directly (NFS/SMB shares) rather than raw blocks, so it always carries filesystem structure overhead — inode tables, journal, superblocks — on top of any RAID overhead, whereas some SAN/block deployments let the host filesystem's overhead be counted separately.
Does deduplication or compression change this calculation?
No — this calculates raw usable capacity before any data reduction. Deduplication and compression happen at the data layer on top of usable capacity and their effective ratio depends heavily on data type, so they're modeled separately rather than as a fixed overhead percentage.