NAS RAID Capacity Calculator

Convert the raw disks in a NAS into the usable capacity each RAID level actually leaves you after redundancy.

About NAS RAID Capacity Calculator

Each RAID level reserves a different amount of capacity for redundancy: RAID 5 gives up one disk, RAID 6 two, and RAID 10 half. Filesystem metadata and reserved blocks then take a further slice off the formatted total. Buying drives against raw capacity leaves arrays short once parity and formatting are accounted for, and expanding a populated array later is far more disruptive.

Formula

NAS RAID Capacity

Usable = data disks × disk size × (1 − filesystem overhead)

data disks: Disks carrying data after redundancy is deducted
disk size: Capacity of a single drive

Worked example

With the default values below, NAS RAID Capacity Calculator returns Usable Capacity of 11.400. Change any input above and every figure here is recalculated in your browser.

Inputs used

Number of disksaccepts 1–64: 4
Capacity of each diskminimum 0.1 TB: 4 TB
RAID levelone of RAID 0 — striping, no redundancy, RAID 1 — mirror, RAID 5 — single parity, RAID 6 — dual parity, RAID 10 — striped mirrors: RAID 5 — single parity
Filesystem and reserve overheadaccepts 0–100 %: 5 %

Results

Usable Capacity: 11.400 TB
Raw Capacity: 16.00 TB
Capacity Lost to Redundancy: 4.00 TB
Storage Efficiency: 75.0%
Fault Tolerance: Survives one disk failure

Step by step

1Values used
2NAS RAID Capacity
3Usable Capacity= 11.400 TB
4Raw Capacity= 16.00 TB
5Capacity Lost to Redundancy= 4.00 TB
6Storage Efficiency= 75.0
7Fault Tolerance= Survives one disk failure

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 3 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

How is NAS RAID Capacity calculated?

Usable = data disks × disk size × (1 − filesystem overhead) Each RAID level reserves a different amount of capacity for redundancy: RAID 5 gives up one disk, RAID 6 two, and RAID 10 half. Filesystem metadata and reserved blocks then take a further slice off the formatted total.

Why does NAS RAID Capacity matter?

Buying drives against raw capacity leaves arrays short once parity and formatting are accounted for, and expanding a populated array later is far more disruptive.

What values do I need to enter?

This calculator takes 4 inputs: Number of disks, Capacity of each disk, RAID level, Filesystem and reserve overhead. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.