NAS Capacity Calculator
Calculate usable NAS capacity after RAID redundancy and filesystem overhead.
About NAS Capacity Calculator
NAS usable capacity accounts for two layers of overhead: RAID redundancy (parity or mirroring) and filesystem metadata. The RAID level determines how many disks' worth of capacity is consumed by redundancy, and the filesystem (ext4, XFS, Btrfs, ZFS) reserves additional space for metadata, journaling, and internal structures — typically 3-7% depending on configuration.
Formula
Usable NAS capacity
usable = raid_capacity × (1 - fs_overhead%)
- C_{raid}
- capacity after RAID overhead
- f
- filesystem overhead fraction
Worked example
With the default values below, NAS Capacity Calculator returns Usable Capacity of 15.20. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of Disksminimum 1
- 5
- Size per Diskminimum 0.1 TB
- 4 TB
- RAID Levelone of RAID 0, RAID 1, RAID 5, RAID 6
- RAID 5
- Filesystem Overheadaccepts 0–30 %
- 5 %
Results
- Usable Capacity
- 15.20 TB
- After RAID (before FS overhead)
- 16.00 TB
- Raw Capacity
- 20.00 TB
- Filesystem Overhead
- 0.80 TB
Step by step
- 1Raw capacity= 20 TB
- 2After RAID 5= 16 TB
- 3After filesystem overhead= 15.2 TB
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
Why do NAS vendors show less capacity than raw disk totals?
NAS capacity is reduced by RAID redundancy, filesystem overhead, and often additional reserves for snapshots or system partitions — vendors show usable capacity after all these deductions.
Does filesystem type affect overhead?
Yes. XFS and ext4 typically use 1-5% for metadata, while copy-on-write filesystems like ZFS and Btrfs may use 5-10% due to additional metadata for checksums, snapshots, and self-healing capabilities.