RAID 6 Capacity Calculator
Calculate usable capacity of a RAID 6 array accounting for dual-disk parity.
Inputs
Usable Capacity
8,000.00GB
Raw Capacity
12,000.00GB
Parity Overhead
4,000.00GB
Storage Efficiency
66.67%
Fault Tolerance
Tolerates 2 simultaneous disk failures
Step by step
Disk count (n)
6
= 6
Disk size
2000 GB
= 2000 GB
RAID 6 usable capacity
(n - 2) × size = (6 - 2) × 2000
= 8000 GB
Dual-parity overhead
2 × size = 2 × 2000
= 4000 GB
Storage efficiency
(n-2)/n × 100 = 4/6 × 100
= 66.67%
How it works
RAID 6 extends RAID 5 by adding a second independent parity block, reserving the equivalent of two disks for parity data distributed across all members. This allows any two disks to fail simultaneously without data loss, making it safer for large arrays where the probability of a second failure during rebuild is non-trivial. The minimum disk count is 4.
Formulas
RAID 6 usable capacity
usable = (n − 2) × disk_size
- n
- number of disks
- S
- capacity per disk
Storage efficiency
efficiency = (n − 2) / n × 100%
- n
- number of disks
Frequently Asked Questions
When should I choose RAID 6 over RAID 5?
Use RAID 6 for arrays with large-capacity drives (4TB+) where rebuild times are long and a second failure during rebuild would be catastrophic, or for arrays with more than 8 disks where statistical likelihood of correlated failure increases.
What is the write penalty for RAID 6?
RAID 6 has a higher write penalty than RAID 5 because each write operation must update two parity blocks instead of one — typically requiring 6 I/O operations per logical write vs 4 for RAID 5.