RAID 5 Capacity Calculator
Calculate usable capacity of a RAID 5 array accounting for single-disk parity.
Inputs
Usable Capacity
6,000.00GB
Raw Capacity
8,000.00GB
Parity Overhead
2,000.00GB
Storage Efficiency
75.00%
Fault Tolerance
Tolerates 1 disk failure
Step by step
Disk count (n)
4
= 4
Disk size
2000 GB
= 2000 GB
RAID 5 usable capacity
(n - 1) × size = (4 - 1) × 2000
= 6000 GB
Raw capacity
n × size = 4 × 2000
= 8000 GB
Parity overhead
1 × size = 2000
= 2000 GB
Storage efficiency
(n-1)/n × 100 = 3/4 × 100
= 75%
How it works
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
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.