RAID Capacity & Redundancy Calculator
Estimate raw capacity, usable capacity, efficiency, and drive-failure tolerance for common RAID levels.
What this RAID array calculator helps you figure out
A RAID array calculator gives you a quick planning view before you buy hardware or deploy a server. Instead of guessing, you can estimate four practical outcomes: total raw capacity, RAID-usable capacity, post-filesystem usable capacity, and how many drives can fail before data loss becomes likely.
This is useful for home NAS builds, virtualization hosts, media servers, backup targets, and SMB storage. Capacity planning mistakes can be expensive, especially when parity overhead and hot spares are ignored. With a few inputs, you can compare multiple RAID levels and choose a layout that balances speed, cost, and resiliency.
RAID levels covered in this tool
RAID 0 (striping)
RAID 0 combines all selected disks for maximum throughput and full capacity usage. It has no redundancy. A single drive failure means total array failure. Use it only for non-critical workloads or scratch space.
RAID 1 (mirroring)
RAID 1 duplicates data across drives. In this calculator, RAID 1 is modeled as a single mirror set, where usable capacity equals one drive size regardless of mirror member count. This gives strong redundancy, but poor capacity efficiency.
RAID 5 (single parity)
RAID 5 offers a balanced option for many environments. Capacity is roughly
(active drives - 1) × drive size. It can survive one drive failure.
Rebuild times on large disks can be long, so plan accordingly.
RAID 6 (dual parity)
RAID 6 reserves the equivalent of two drives for parity:
(active drives - 2) × drive size. It tolerates two simultaneous
drive failures and is often preferred for large-capacity arrays.
RAID 10 (striped mirrors)
RAID 10 requires an even number of active drives and combines mirroring with striping.
Capacity is (active drives ÷ 2) × drive size. It generally provides excellent
random I/O and rebuild behavior, with strong fault tolerance depending on which drives fail.
How the calculator performs its estimates
- Installed raw capacity: total drives × drive size.
- Active RAID drives: installed drives − hot spares.
- RAID-usable capacity: determined by RAID level formula.
- Estimated practical capacity: RAID-usable × (1 − overhead%).
- Efficiency: RAID-usable compared to total installed raw capacity.
The numbers are estimates, not a replacement for controller-specific documentation. Vendors can implement metadata and reserve space differently.
Important real-world considerations
1) Drive size mixing
Most RAID systems effectively use the size of the smallest drive in the group. If you mix a 10 TB drive with 12 TB drives, the extra capacity on larger disks may be unusable.
2) Rebuild windows and URE risk
As drive sizes increase, rebuild times increase too. During rebuild, performance is lower and risk is higher. RAID 6 or RAID 10 can reduce exposure compared with RAID 5 for large arrays.
3) RAID is not backup
RAID protects availability during drive failures. It does not protect against accidental deletion, ransomware, silent corruption, software bugs, or catastrophic events. Always pair RAID with a backup plan.
Example planning scenarios
- Media NAS (8 × 12 TB, RAID 6, 1 hot spare): good balance between usable capacity and safer dual-parity resilience.
- VM host (6 × 3.84 TB SSD, RAID 10): favors IOPS consistency and faster rebuild characteristics.
- Scratch workspace (4 × 2 TB, RAID 0): maximum speed/capacity with no redundancy, suitable only when data is disposable.
Quick RAID selection guide
- Choose RAID 0 only for non-critical performance-focused workloads.
- Choose RAID 1 when simplicity and strong mirroring matter more than capacity.
- Choose RAID 5 for smaller arrays where one-drive tolerance is acceptable.
- Choose RAID 6 for larger HDD pools where two-drive tolerance is preferred.
- Choose RAID 10 for high-performance workloads needing strong redundancy.
Final note
Capacity math is only one part of storage design. Also consider workload type, controller quality, filesystem (ZFS, XFS, ext4, NTFS, etc.), backup strategy, and recovery objectives. Use this RAID array calculator as your starting point, then validate against your hardware vendor’s official sizing guidance before production deployment.