Simple Calculator Tools

RAID Storage Capacity Calculator

Enter your number of drives, drive size and RAID level to see exactly how much usable storage you'll actually get.

RAID array settings

Enter your drive count, drive size and RAID level.

Usable capacity—Enter your drive details above.
Raw (total) capacity—
Overhead capacity—
Fault tolerance—
Quick insight

RAID Storage Capacity Calculator

Enter your drive count, size and RAID level to see usable capacity.

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What Is a RAID Storage Capacity Calculator?

RAID (Redundant Array of Independent Disks) combines multiple physical drives into one logical volume, trading some raw capacity for speed, redundancy, or both. Different RAID levels use different amounts of your drives' total space for parity or mirroring, so the usable capacity you end up with is always less than the sum of your drives. This calculator applies the standard capacity formula for each RAID level so you know exactly what to expect before you build an array.

This is the same math used by NAS setup wizards and RAID controller utilities when they show you "usable" versus "raw" capacity during array creation.

How to Read Your Results

Usable Capacity

The actual storage space available for your files after the RAID level's overhead is accounted for.

Raw (Total) Capacity

The simple sum of every drive's capacity, before any RAID overhead is subtracted.

Overhead Capacity

The space "spent" on redundancy — mirrored copies or parity data — rather than usable storage.

Fault Tolerance

How many drive failures the array can survive without losing data, which is the main reason to choose a redundant RAID level over plain striping.

Real-World Example

You're building a small file server with 4 drives at 4TB each and want good performance with real redundancy, so you choose RAID 5.

FieldValue
Drives4 × 4TB
Raw capacity16 TB
Usable capacity (RAID 5)12 TB
Fault toleranceSurvives 1 drive failure

RAID 5 uses the equivalent of one drive (4TB) for parity, leaving 12TB usable out of 16TB raw — and if any single drive fails, the array can rebuild from the remaining three.

Tips for Choosing a RAID Level

  • RAID 0 maximizes capacity and speed but has zero fault tolerance — only use it for disposable or already-backed-up data.
  • RAID 1 and RAID 10 rebuild fastest after a drive failure since they just copy a mirror, unlike parity-based levels.
  • RAID 5 becomes riskier with very large drives because rebuild times grow, increasing the chance of a second failure during rebuild — many admins choose RAID 6 for large arrays.
  • RAID is not a backup. Keep a separate backup copy for protection against deletion, corruption or theft.

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Note: This calculator shows theoretical usable capacity based on drive count and size. Actual formatted capacity is typically slightly lower due to filesystem overhead, and some controllers reserve additional space for hot spares or metadata.

Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team