Array details
Use equal-capacity disks. Mixed arrays are limited by the smallest disk.
Free storage capacity planner
RAID Calculator
A RAID calculator estimates usable storage after mirroring, parity, and hot-spare overhead. Compare RAID 0, 1, 5, 6, and 10 capacity, fault tolerance, efficiency, and cost before buying disks.
Usable capacity
24 TB
Raw installed
32 TB
Parity, mirror & spare overhead
8 TB
Storage efficiency
75%
Active disks
4 of 4
Total disk cost
$720.00
Cost per usable TB
$30.00
Capacity formula
(4 active disks − 1 parity) × 8 TB
Manufacturer capacity uses decimal units. Filesystem formatting, metadata, and operating-system unit conventions can make displayed capacity lower. RAID is not a backup.
Plan a RAID array in four steps
The calculator separates installed capacity from storage that remains available for files.
1. Choose RAID
Match the protection and performance profile to your workload.
2. Size disks
Enter equal disk count and the smallest usable disk capacity.
3. Reserve spares
Set aside disks for automatic replacement after a failure.
4. Compare
Review capacity, overhead, efficiency, protection, and price.
RAID capacity questions
How is usable RAID capacity calculated?
Usable capacity equals disk capacity multiplied by the disks available for data. RAID 5 reserves one disk-equivalent for parity, RAID 6 reserves two, and RAID 1 or 10 reserves half or more for mirrors.
Do hot spares add usable storage?
No. A hot spare waits unused until an active disk fails, so it counts toward purchase cost and raw installed capacity but not usable capacity.
What is the difference between RAID 5 and RAID 6?
RAID 5 uses one disk-equivalent of parity and survives one disk failure. RAID 6 uses two disk-equivalents of parity and survives two disk failures, trading capacity for protection.
Why does RAID 10 need an even number of disks?
RAID 10 creates mirrored disk pairs and stripes data across those pairs. Every active disk therefore needs a matching mirror, which requires an even disk count.
Is RAID a backup?
No. RAID improves availability after certain disk failures, but it does not protect against deletion, corruption, ransomware, theft, or site loss. Keep separate tested backups.
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