How many disk do I need for RAID 5?

Determining how many disks are needed for a RAID 5 configuration depends on the desired storage capacity and level of fault tolerance. RAID 5 requires a minimum of 3 disks, but more disks can be added to increase capacity and resilience against disk failures.

What is RAID 5?

RAID 5 is a storage technology that uses disk striping with distributed parity to provide fault tolerance and improve performance. Data is striped across all the disks in the array, with parity information calculated and written across the disks as well. If one disk fails, the parity information can be used to reconstruct the lost data on a replacement disk.

The key advantages of RAID 5 include:

  • Improved read performance – Data is spread across multiple disks to allow concurrent disk reads.
  • Fault tolerance – The array can withstand a single disk failure without data loss.
  • Efficient use of disks – Only one disk worth of space is used for parity, so an n disk array has n-1 disks worth of usable space.

Minimum Disks for RAID 5

The minimum number of disks required for RAID 5 is 3. This provides capacity for 2 disks worth of data with 1 disk worth of parity. For example, with 3 x 1 TB disks:

  • 2 TB usable storage
  • 1 TB parity information

With only 2 disks, RAID 5 would have no fault tolerance since the loss of one disk would also mean the loss of all data. At least 3 disks are needed to provide redundancy and protection against a single disk failure.

Adding More Disks

Additional disks can be added to a RAID 5 array to increase usable capacity and fault tolerance:

  • 4 disks – Capacity of 3 disks, can withstand 1 disk failure
  • 5 disks – Capacity of 4 disks, can withstand 1 disk failure
  • 6 disks – Capacity of 5 disks, can withstand 1 disk failure

In general, for an n disk RAID 5 array:

  • Usable capacity = (n – 1) disks
  • Can withstand 1 disk failure

Adding more disks improves storage efficiency as less capacity needs to be used for parity. It also improves fault tolerance as the array can handle an increasing number of disk failures.

Factors to Consider

When determining the number of disks needed for a RAID 5 configuration, key considerations include:

  • Storage capacity needed – The total usable capacity required after accounting for parity overhead.
  • Disk size – Larger disks provide greater individual capacity.
  • Number of disk failures to tolerate – More disks means tolerating more failures, but cost rises.
  • Cost – The overall storage budget and cost per disk.
  • Performance needs – More disks can enhance performance via striping.

If available, using higher capacity disks can require fewer disks to meet capacity requirements. But larger arrays with more disks can deliver greater performance and fault tolerance.

RAID 5 Calculator

To determine the number of disks needed, a RAID 5 calculator is helpful. Here is an example calculator:

Inputs Values
Desired Capacity 8 TB
Disk Size 2 TB

Based on a desired capacity of 8 TB, and 2 TB disks, the calculator determines that 5 disks are needed:

  • With 5 x 2 TB disks, capacity = 4 disks x 2 TB = 8 TB
  • The 5 disk configuration can withstand 1 failed disk.

By inputting different capacity, disk size, and failure requirements, the optimal number of disks can be determined.


In summary, the number of disks needed for a RAID 5 configuration depends primarily on the desired usable capacity and level of fault tolerance:

  • Minimum 3 disks required for any RAID 5
  • Each additional disk adds usable capacity and tolerance for an additional disk failure
  • Larger capacity disks require fewer disks to meet capacity needs
  • Performance and overall budgets are also considerations

To find the right number of disks, factors like capacity, disk size, failure tolerance, performance, and cost constraints should be taken into account. RAID 5 calculators are helpful for playing with different configurations to select the optimal number of disks.