What this RAID configuration calculator does
This calculator estimates the usable storage you can expect from common RAID levels: RAID 0, RAID 1, RAID 5, RAID 6, and RAID 10. It also shows capacity overhead, a simple fault-tolerance summary, and efficiency percentages so you can quickly compare tradeoffs between speed, resilience, and total usable space.
It is designed for practical planning: home NAS builds, lab servers, small office file storage, and virtualization hosts. Just enter your drive count, drive size, and any hot spares, then review the output before you buy hardware or build your array.
RAID levels at a glance
RAID 0 (Striping)
RAID 0 uses all drives for performance and full capacity. There is no redundancy. If one drive fails, all data in the array is lost.
- Minimum drives: 2
- Usable capacity: all active drives
- Fault tolerance: none
RAID 1 (Mirroring)
RAID 1 duplicates data across drives. In this calculator, RAID 1 is treated as a full mirror set (all active drives mirror the same data), which maximizes protection but significantly reduces usable capacity.
- Minimum drives: 2
- Usable capacity: equivalent to one drive
- Fault tolerance: multiple drive failures possible as long as one mirror copy survives
RAID 5 and RAID 6 (Parity RAID)
These are common capacity-efficient choices for NAS systems. RAID 5 uses one drive's worth of parity, while RAID 6 uses two drives' worth. RAID 6 offers stronger failure protection and is often preferred for larger arrays and larger drives.
- RAID 5 minimum drives: 3
- RAID 6 minimum drives: 4
- Typical use: balanced capacity and protection
RAID 10 (Mirrors + Striping)
RAID 10 combines mirroring and striping for strong performance and good resilience, often used in databases and virtualization. It requires an even number of active drives.
- Minimum drives: 4
- Usable capacity: 50% of active array capacity
- Fault tolerance: at least one drive; sometimes more, depending on failure pattern
How to use the calculator effectively
For realistic planning, keep these points in mind:
- Use the smallest drive size if your drives are mixed.
- Add at least one hot spare for business-critical systems.
- Leave reserve capacity (for example 10–20%) to reduce fragmentation and keep performance stable.
- Compare both array efficiency and overall efficiency when hot spares are involved.
Example outcomes
Example 1: 4 × 8TB in RAID 5
Raw installed capacity is 32TB. RAID 5 uses one drive equivalent for parity, so usable capacity is about 24TB before filesystem overhead. This is a common home and SMB NAS setup.
Example 2: 8 × 4TB in RAID 10
Raw installed capacity is 32TB. RAID 10 gives approximately 16TB usable. You sacrifice capacity for stronger write performance and better fault tolerance behavior under many failure scenarios.
Important limitations and assumptions
This tool provides planning estimates, not vendor-certified numbers. Real-world capacity can differ due to filesystem metadata, controller implementation, GiB/TiB display conventions, and partition alignment.
- Assumes same-size drives for clean RAID math.
- Does not model rebuild time, URE risk, or controller-specific constraints.
- RAID 1 is represented as a full mirror set for simplicity.
- RAID is not a substitute for backups.
RAID is not backup
RAID protects against hardware failure, not accidental deletion, ransomware, corruption, fire, or theft. Always pair RAID with a tested backup strategy (3-2-1 is a good starting point: three copies, two media types, one offsite/offline copy).
Final thoughts
The best RAID level depends on your real goal: maximum capacity, best throughput, strongest uptime, or balanced risk. Use this calculator to size your array quickly, then validate against your controller documentation and recovery plan before deployment.