RAID Calculator

Find usable storage capacity and fault tolerance for RAID 0, 1, 5, 6, and 10 configurations from drive count and size.

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RAID level

Usable capacity

12.00 TB

Total raw 16.00 TB
Usable storage 12.00 TB
Storage efficiency 75%
Parity / mirror drives 1
Drives that can fail 1

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Storage Arrays

RAID 0, 1, 5, 6, and 10 capacity, fault tolerance, and efficiency

A RAID calculator shows how much usable storage a disk array provides and how many drives can fail before data is lost. It is a practical planning tool for home NAS builds, server storage design, and capacity estimation without doing the maths for each RAID level by hand.

How each RAID level uses drive capacity

RAID stands for Redundant Array of Independent Disks. Each level trades raw capacity for a different mix of performance, redundancy, and minimum drive count. The usable capacity depends on how many drives are dedicated to parity or mirroring versus actual data storage.

  • RAID 0 — striping across all drives. Usable = all drives. No fault tolerance: any single drive failure loses all data.
  • RAID 1 — full mirror. Usable = one drive size regardless of drive count. All but one drive may fail.
  • RAID 5 — distributed parity. Usable = (n − 1) drives. One drive may fail. Minimum 3 drives.
  • RAID 6 — double parity. Usable = (n − 2) drives. Two drives may fail simultaneously. Minimum 4 drives.
  • RAID 10 — mirrored stripes. Usable = half the raw capacity. One drive per mirror pair may fail. Requires even drive count.

Choosing the right RAID level

RAID 5 is the most common choice for general file storage where capacity efficiency and single-drive redundancy are both important. RAID 6 adds a second layer of protection at the cost of one more drive, which matters most during the rebuild window after a drive failure — when a second drive is at elevated risk.

RAID 10 has higher rebuild speed and better write performance than parity-based arrays, making it a preferred choice for databases and high-write workloads. RAID 0 provides maximum usable capacity with no overhead but should only be used where the data is either expendable or backed up elsewhere.

RAID is not a backup. A hardware failure, ransomware attack, or accidental deletion affects all drives in the array simultaneously. Regular backups stored separately are still essential regardless of RAID level.

Frequently asked questions

Does RAID replace backups?

No. RAID protects against drive hardware failure, not against data loss from accidental deletion, ransomware, fire, theft, or controller failure. The 3-2-1 backup rule — three copies, two media types, one offsite — applies even when RAID is in use.

 Can I mix different drive sizes in a RAID array?

Most RAID controllers allow mixed drive sizes, but usable capacity is based on the smallest drive in the array. All drives behave as if they are the same size as the smallest one. Using identically-sized drives maximises usable capacity.

 What happens during a RAID rebuild?

When a failed drive is replaced, the controller reconstructs the missing data from the remaining drives and writes it to the new drive. During this rebuild the array is in a degraded state with no redundancy. RAID 5 rebuilds are slower and put stress on remaining drives; RAID 6 keeps one layer of protection during a rebuild.

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