RAID 5 Capacity Calculator
Calculate usable capacity of a RAID 5 array accounting for single-disk parity.
About RAID 5 Capacity Calculator
RAID 5 distributes parity data equivalent to one disk's capacity across all member disks, so usable capacity is (n − 1) × disk_size. This provides single-disk fault tolerance — any one disk can fail without data loss, but a second simultaneous failure causes total array loss. RAID 5 requires a minimum of 3 disks and offers a good balance between capacity efficiency and redundancy for most workloads.
Formulas
RAID 5 usable capacity
usable = (n − 1) × disk_size
- n
- number of disks in the array
- S
- capacity of each individual disk
Storage efficiency
efficiency = (n − 1) / n × 100%
- n
- number of disks
Worked example
With the default values below, RAID 5 Capacity Calculator returns Usable Capacity of 6,000.00. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of Disksminimum 3
- 4
- Size per Diskminimum 0.001 GB
- 2,000 GB
Results
- Usable Capacity
- 6,000.00 GB
- Raw Capacity
- 8,000.00 GB
- Parity Overhead
- 2,000.00 GB
- Storage Efficiency
- 75.00%
- Fault Tolerance
- Tolerates 1 disk failure
Step by step
- 1Disk count (n)= 4
- 2Disk size= 2000 GB
- 3RAID 5 usable capacity= 6000 GB
- 4Raw capacity= 8000 GB
- 5Parity overhead= 2000 GB
- 6Storage efficiency= 75%
Assumptions and limits
- The result is derived only from the 2 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 2 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 many disks does RAID 5 need at minimum?
RAID 5 requires at least 3 disks: n − 1 disks worth of usable capacity with 1 disk worth of parity distributed across all members.
What happens if two disks fail simultaneously in RAID 5?
RAID 5 only tolerates a single disk failure. If a second disk fails before the array is rebuilt, all data is lost. For dual-disk fault tolerance, use RAID 6 instead.
Is RAID a substitute for backups?
No. RAID protects against individual disk failure but not against accidental deletion, corruption, ransomware, or site-level disasters — separate backups are still required.