RAID HDD Capacity Calculator
Use this calculator to estimate usable storage, parity overhead, and fault tolerance for common RAID arrays.
Why a RAID HDD calculator matters
RAID planning is one of those tasks that seems simple until hardware is already purchased. You might assume “4 drives x 8 TB = 32 TB usable,” only to discover parity, mirroring, or hot spares have reduced available capacity more than expected. A RAID HDD calculator helps you make sizing decisions before buying disks, choosing a NAS, or migrating storage.
This page gives you a practical estimate of usable space, efficiency, and drive-failure tolerance for common RAID levels. It is useful for home labs, small business file servers, media archives, backup targets, and virtualization hosts.
RAID levels covered in this calculator
RAID 0
RAID 0 stripes data across drives for performance and full capacity use. However, there is no redundancy. If one drive fails, all array data is lost.
RAID 1
RAID 1 mirrors data. Capacity is roughly half of the active drives because each block is duplicated. Redundancy is strong, but storage efficiency is lower.
RAID 5
RAID 5 uses distributed single parity and can tolerate one drive failure. Usable capacity is approximately (active drives - 1) × drive size.
RAID 6
RAID 6 uses dual parity and can tolerate two simultaneous drive failures. Usable capacity is approximately (active drives - 2) × drive size.
RAID 10
RAID 10 combines mirroring and striping. It requires an even number of drives and provides strong performance plus redundancy. Usable capacity is about half of the active drives.
How the calculator computes values
- Installed raw capacity: total drives × drive size.
- Active pool raw capacity: (total drives - hot spares) × drive size.
- Usable capacity: depends on RAID level parity/mirroring rules.
- Overhead: active raw capacity - usable capacity.
- Efficiency: usable capacity ÷ active raw capacity.
The tool also shows an approximate TiB figure because operating systems often display binary units (TiB) while drive vendors market decimal units (TB).
Example planning scenarios
Example 1: Home NAS media server
A 4-drive setup with 8 TB disks in RAID 5 gives roughly 24 TB usable before filesystem overhead. This is a common balance of capacity and basic fault tolerance for home media and backups.
Example 2: Business documents and shared folders
A 6-drive setup with 12 TB disks in RAID 6 gives about 48 TB usable, while allowing two drives to fail. RAID 6 is often preferred for larger arrays where rebuild stress and risk are higher.
Example 3: Virtualization host
An 8-drive RAID 10 can provide good random I/O and redundancy. While only half the active raw capacity is usable, performance consistency and rebuild behavior can be favorable for VMs.
Important caveats before purchase
- RAID is not backup: it protects availability, not accidental deletion, ransomware, or site loss.
- Mixed drive sizes: many arrays effectively use the smallest drive size across all disks.
- Controller differences: hardware RAID, software RAID, and ZFS-style systems may calculate space differently.
- Rebuild time: large disks can take a long time to rebuild; choose redundancy level accordingly.
- Keep free space: plan to leave headroom (often 10–20%) to maintain performance and manage snapshots.
Quick RAID selection guide
- Need max speed and capacity, no safety: RAID 0.
- Need simple mirroring: RAID 1.
- Need good capacity efficiency + 1-drive protection: RAID 5.
- Need stronger protection for bigger arrays: RAID 6.
- Need performance + redundancy for random workloads: RAID 10.