RAID Calculator

Enter a RAID level, number of disks, and capacity per disk to calculate raw capacity, usable capacity, storage efficiency, and fault tolerance.

Quick Facts

Formula
Usable capacity = (disks − parity disks) × capacity per disk
Parity disks: 0 for RAID 0, 1 for RAID 5, 2 for RAID 6; RAID 1/10 mirror instead of using parity.
Minimum disks
RAID 0/1: 2 · RAID 5: 3 · RAID 6: 4 · RAID 10: 4 (even)
Each RAID level has a hard minimum disk count to function.

Your Results

Calculated
Raw capacity
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Total disk capacity (disks × size)
Usable capacity
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Storage available after redundancy
Storage efficiency
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Usable ÷ raw capacity
Fault tolerance
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Disks that can fail without data loss

Ready

Choose a RAID level and enter disk count and capacity per disk.

About the RAID Calculator

RAID (Redundant Array of Independent Disks) combines multiple physical disks into one logical volume to improve performance, add fault tolerance, or both. Different RAID levels trade raw disk capacity for protection against drive failure in different ways. This calculator applies the standard capacity formula for RAID 0, 1, 5, 6, and 10 so you can see exactly how many usable terabytes an array delivers and how many disks can fail before data is lost.

The formulas by RAID level

Let N be the number of disks and C be the capacity of each disk. Raw capacity is always N × C. Usable capacity and fault tolerance depend on the level:

  • RAID 0 (striping): usable = N × C. No redundancy — data is split evenly across every disk with no duplication, so losing any one disk loses the entire array. Minimum 2 disks.
  • RAID 1 (mirroring): usable = C. Every disk holds an identical copy, so usable space equals a single disk's capacity no matter how many disks you add. Tolerates all but one disk failing. Minimum 2 disks.
  • RAID 5 (single parity): usable = (N − 1) × C. One disk's worth of space is reserved for distributed parity data, which can reconstruct any single failed disk. Minimum 3 disks.
  • RAID 6 (dual parity): usable = (N − 2) × C. Two disks' worth of space go to parity, allowing the array to survive any two simultaneous disk failures. Minimum 4 disks.
  • RAID 10 (mirrored stripes, a.k.a. RAID 1+0): usable = (N ÷ 2) × C. Disks are mirrored in pairs, then the pairs are striped together. This gives half the raw capacity and tolerates one failure per pair. Requires an even number of disks, minimum 4.

Storage efficiency

Storage efficiency is simply usable capacity divided by raw capacity, expressed as a percentage. RAID 0 is 100% efficient but offers zero protection. RAID 5 efficiency rises as you add disks (a 3-disk array is 67% efficient; a 10-disk array is 90% efficient) because the one-disk parity overhead is spread across more disks. RAID 6 follows the same pattern with two disks of overhead. RAID 1 and RAID 10 are fixed at 50% efficiency because exactly half the raw capacity is a mirrored copy.

Choosing a RAID level

RAID 0 suits scratch space or temporary data where speed matters more than safety. RAID 1 suits small, critical volumes like boot drives. RAID 5 is a common general-purpose choice balancing capacity and single-drive protection, though rebuild times on very large disks raise the risk of a second failure during recovery. RAID 6 addresses that risk for large arrays by tolerating two failures. RAID 10 offers the fastest rebuilds and strong performance, at the cost of losing half the raw capacity to mirroring.

Frequently Asked Questions

How is usable RAID capacity calculated?
Usable capacity depends on the RAID level. RAID 0 (striping) uses all disks with no overhead, so usable capacity equals disk count times capacity per disk. RAID 1 (mirroring) uses one disk's capacity regardless of how many disks mirror it. RAID 5 reserves one disk's worth of space for parity, so usable capacity is (disks minus 1) times capacity per disk. RAID 6 reserves two disks for parity: (disks minus 2) times capacity per disk. RAID 10 mirrors pairs then stripes across them, giving half the raw capacity: (disks divided by 2) times capacity per disk.
How many disks can fail in each RAID level?
RAID 0 tolerates zero failed disks — any failure loses all data. RAID 1 tolerates all but one disk failing, since every disk holds a full copy. RAID 5 tolerates exactly one simultaneous disk failure. RAID 6 tolerates any two simultaneous disk failures. RAID 10 tolerates one failure per mirrored pair, so up to half the disks can fail as long as no complete pair is lost.
What is the minimum number of disks for each RAID level?
RAID 0 and RAID 1 need at least 2 disks. RAID 5 needs at least 3 disks. RAID 6 needs at least 4 disks. RAID 10 needs at least 4 disks and an even number, since it is built from mirrored pairs that are then striped together.

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