RAID 5 Capacity Calculator
Estimate raw capacity, usable capacity, parity overhead, and post-format usable space.
How RAID 5 capacity works
RAID 5 stripes data across multiple drives and distributes parity information across the array. The key trade-off is simple: you gain fault tolerance against a single drive failure, but you sacrifice the equivalent capacity of one drive for parity.
In practice, RAID 5 usable capacity is usually calculated as: (number of drives - 1) × smallest drive size. If every drive is the same size, the math is straightforward and this calculator gives you a quick answer.
RAID 5 space formula
Base calculation
- Raw capacity: number of drives × per-drive size
- Parity overhead: size of one drive
- Usable capacity: (number of drives - 1) × per-drive size
After formatting and system overhead
Storage vendors market drive sizes in decimal units (TB/GB), while many operating systems show binary units (TiB/GiB). On top of that, filesystems and snapshots consume extra space. That is why your NAS or server may report less than your headline RAID 5 number.
Use the overhead field in the calculator to estimate real-world available space after formatting, metadata, and reserved blocks.
Example RAID 5 calculations
Example 1: Home NAS
4 drives of 4 TB each:
- Raw: 16 TB
- Usable: 12 TB
- Parity: 4 TB
Example 2: Small office backup array
8 drives of 12 TB each:
- Raw: 96 TB
- Usable: 84 TB
- Parity: 12 TB
Important planning notes before you build
- Minimum drive count: RAID 5 requires at least 3 disks.
- Mixed drive sizes: Effective size is limited by the smallest drive in the array.
- One-drive fault tolerance: RAID 5 survives one drive failure, not two.
- Rebuild risk: Large drives take longer to rebuild, increasing stress on remaining disks.
- RAID is not backup: Always keep independent backups for ransomware, accidental deletion, and corruption.
RAID 5 vs RAID 6 for capacity
If you are choosing between RAID levels, capacity is one factor:
- RAID 5: loses 1 drive worth of capacity, tolerates 1 failed drive.
- RAID 6: loses 2 drives worth of capacity, tolerates 2 failed drives.
For large arrays or high-capacity disks, many admins prefer RAID 6 due to better resiliency during long rebuild windows.
Frequently asked questions
Why does my NAS show less space than this calculator?
Most often it is decimal vs binary unit conversion plus filesystem overhead and snapshots. This is normal and expected.
Can I use different-size drives in RAID 5?
Yes, but the array typically behaves as if all drives were the size of the smallest one. Extra capacity on larger disks is unused.
Is RAID 5 good for media storage?
It can be, especially for read-heavy workloads and moderate array sizes. For mission-critical data or very large drives, RAID 6 or RAID 10 may be safer options.
Bottom line
A RAID 5 space calculator helps you estimate usable capacity quickly, but your final available storage is always a little lower once you account for formatting, metadata, and operational headroom. Use this tool for planning, then confirm expected capacity inside your specific NAS, server, or RAID controller before deployment.