NAS Capacity Calculator
Calculate usable NAS capacity after RAID redundancy and filesystem overhead.
Inputs
Space reserved by filesystem metadata, journaling, etc.
Usable Capacity
15.20TB
After RAID (before FS overhead)
16.00TB
Raw Capacity
20.00TB
Filesystem Overhead
0.80TB
Step by step
Raw capacity
5 × 4 TB
= 20 TB
After RAID 5
(5-1) × 4
= 16 TB
After filesystem overhead
16 × (1 - 0.05)
= 15.2 TB
How it works
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
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.