RAID Storage Capacity Calculator
Estimate your raw and usable storage based on RAID level, drive count, and per-drive capacity.
What This RAID Storage Calculator Does
RAID is great for performance, availability, and uptime, but it can be confusing when you try to estimate usable capacity. Raw disk size is not the same as usable storage. This calculator helps you quickly estimate how much space you will actually get after RAID overhead and optional filesystem reserve.
Whether you are planning a home NAS, a small business file server, or a virtualization host, this tool gives a practical first-pass estimate before you buy drives.
How to Use the Calculator
- Select your RAID level (0, 1, 5, 6, or 10).
- Enter how many drives you plan to install.
- Enter the capacity of each drive (for example, 4 TB, 8 TB, or 18 TB).
- Optionally set a reserve percentage to account for filesystem overhead.
- Click Calculate to see raw capacity, usable capacity, and efficiency.
RAID Levels Explained (Quick Reference)
RAID 0
RAID 0 stripes data across drives for speed and full capacity usage, but there is no redundancy. If one drive fails, the entire array is lost.
- Minimum drives: 1
- Usable formula: N × drive size
- Fault tolerance: none
RAID 1
RAID 1 mirrors data. In this calculator, multi-drive RAID 1 is treated as full mirroring, where usable capacity is equal to one drive.
- Minimum drives: 2
- Usable formula: 1 × drive size
- Fault tolerance: can survive multiple failures as long as one mirror copy remains
RAID 5
RAID 5 uses distributed parity to protect against one drive failure while keeping good storage efficiency.
- Minimum drives: 3
- Usable formula: (N − 1) × drive size
- Fault tolerance: 1 failed drive
RAID 6
RAID 6 uses double parity and can survive two simultaneous drive failures. It is popular for larger arrays where rebuild risk is a concern.
- Minimum drives: 4
- Usable formula: (N − 2) × drive size
- Fault tolerance: 2 failed drives
RAID 10
RAID 10 combines mirroring and striping for high performance and solid resilience, often preferred for databases and virtualization workloads.
- Minimum drives: 4 (even count recommended)
- Usable formula: (N / 2) × drive size (rounded down for odd drive counts)
- Fault tolerance: at least 1 drive; potentially more if failures are in different mirror pairs
Important Planning Notes
- RAID is not backup. It protects availability, not against accidental deletion or ransomware.
- Rebuild times matter. Larger disks can take a long time to rebuild after a failure.
- Use matching drives when possible. Mixed sizes usually get limited by the smallest disk.
- Leave free space. Arrays often perform better when not filled to 100%.
Example Scenario
Suppose you install 6 drives × 10 TB in RAID 6:
- Raw capacity: 60 TB
- RAID 6 parity overhead: 20 TB equivalent
- Usable before reserve: 40 TB
- With 5% reserve: 38 TB estimated usable
This gives you resilience to two drive failures while still retaining a substantial amount of usable capacity.
Final Thoughts
A RAID storage calculator helps you avoid surprises and compare RAID choices quickly. Use it early in your planning process, then validate against your exact controller, NAS software, and filesystem behavior for production deployments.