Aaron Graves, PhDude (Replica)

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nas raid calculator

Posted on February 16, 2026 by Admin

NAS RAID Capacity Calculator

Use this tool to estimate raw capacity, usable capacity, and fault tolerance for common NAS RAID levels.

Example: 8 for 8TB disks. Supports decimal values.
RAID 5 requires at least 3 active drives and can survive one drive failure.
Hot spares are reserved and do not count toward usable capacity.
Note: This is an estimate for planning. Real-world usable space depends on your NAS OS (Synology, QNAP, TrueNAS, etc.), formatting, snapshots, and reserved system partitions.

Why a NAS RAID calculator matters before you buy drives

Many people shop for a NAS by multiplying disk count by disk size. On paper, 4 drives × 8TB sounds like 32TB. In practice, your usable space is lower once you account for parity, mirroring, hot spares, and filesystem overhead. A NAS RAID calculator helps you avoid under-sizing your storage and prevents expensive surprises after setup day.

If you are building a home media server, business backup target, or surveillance archive, capacity planning is not just about today. It is about growth over the next 12 to 36 months. A good estimate now helps you choose the right RAID level and expansion path without rebuilding your entire array too soon.

Key terms to understand

  • Raw capacity: Total disk size before RAID overhead.
  • Usable capacity: Space available for data after RAID protection is applied.
  • Active drives: Drives participating in the array (total drives minus hot spares).
  • Fault tolerance: How many drives can fail before data loss risk becomes critical.
  • Overhead: Space consumed by filesystem metadata, snapshots, and system reservations.

How this NAS RAID calculator works

The calculator uses simple, practical formulas used for planning most consumer and SMB NAS deployments:

  • RAID 0: Usable drives = active drives
  • RAID 1: Usable drives = 1 (all-drive mirror model)
  • RAID 5: Usable drives = active drives - 1
  • RAID 6: Usable drives = active drives - 2
  • RAID 10: Usable drives = active drives / 2 (active drives must be even)
  • JBOD: Usable drives = active drives (no parity protection)

After computing usable capacity, the tool applies your overhead percentage so you can see a more realistic final figure. It also shows both TB and TiB, since operating systems often report capacities in binary units (TiB), which appear lower than drive marketing labels (TB).

RAID level quick guide for NAS users

RAID 0

Best for speed and full capacity, but no redundancy. If one drive fails, the array fails. Usually not recommended for important NAS data.

RAID 1

Mirrors data for high safety. Capacity efficiency is low, but resilience is strong. Common for two-bay NAS units where simplicity matters.

RAID 5

A popular balance of capacity and protection. You lose the equivalent of one drive to parity and can survive a single drive failure.

RAID 6

Adds dual parity, sacrificing more capacity but improving protection during rebuilds and against a second failure. Great for larger arrays with bigger disks.

RAID 10

Combines mirroring and striping. Very strong performance and good resilience, but only 50% capacity efficiency.

JBOD / Linear

Concatenates or separates disks without parity. Easy to understand but no RAID fault tolerance. Better for non-critical workloads or archive tiers with external backups.

Example planning scenarios

Home media NAS

If you use 4 × 12TB drives in RAID 5, raw capacity is 48TB. Usable RAID capacity is roughly 36TB before overhead. After a 7% overhead assumption, the estimate is around 33.48TB.

Small business file server

With 8 × 10TB in RAID 6 and one hot spare, active drives are 7. RAID 6 usable drives become 5, giving around 50TB before overhead. This setup trades capacity for safer rebuild behavior.

Performance-focused VM storage

With 6 × 4TB in RAID 10, usable capacity is about 12TB before overhead. You give up capacity, but gain strong IOPS behavior and predictable latency.

Capacity is only part of the decision

A NAS RAID calculator answers “how much,” but not “how safe” or “how fast” in every edge case. Keep these realities in mind:

  • Rebuild times increase significantly with larger drives.
  • During rebuild, performance drops and failure risk increases.
  • RAID is not a backup strategy; keep versioned off-device backups.
  • Using mixed drive sizes usually limits usable size to the smallest drive (unless using flexible vendor-specific layouts).

Best practices when using a RAID capacity calculator

  • Plan at least 20–30% free space to avoid performance degradation.
  • Consider adding a hot spare for mission-critical environments.
  • Prefer RAID 6 over RAID 5 on large arrays and high-capacity drives.
  • Match drive models and firmware when possible for stability.
  • Validate your plan against vendor documentation before purchase.

Final thoughts

This NAS RAID calculator gives you a fast, practical estimate for real-world storage planning. Use it to compare RAID options, budget disk purchases, and avoid common mistakes. Then combine the estimate with your backup policy, performance requirements, and growth expectations to design a NAS that lasts.

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