RAID Calculator

What is a RAID Calculator?

A RAID calculator is a tool that estimates the usable storage capacity, fault tolerance, and storage efficiency of a RAID array, based on the number of disks, the size of each disk, and the RAID level selected. It removes the guesswork of manually working out how parity, mirroring, or striping affects your final available space.

Raw capacity vs. usable capacity — what’s the difference?

Raw capacity is simply the total size of all disks added together, before any RAID overhead. Usable capacity is what’s actually available for storing files once the RAID level’s redundancy requirements — mirroring or parity — have been accounted for. Two arrays with identical raw capacity can offer very different usable capacity and protection depending on the RAID level chosen.

Why Use a RAID Calculator?

Using a RAID calculator before building an array saves money and prevents data-loss surprises in several practical ways:

• Avoid overestimating capacity — know exactly how much space parity or mirroring will consume
• Compare RAID levels side by side — see the capacity and protection trade-off before committing
• Plan fault tolerance — understand how many disk failures your array can survive
• Budget for drives accurately — know how many disks you need to hit a target usable capacity
• Avoid under-protected setups — don’t accidentally choose a RAID level with no redundancy for critical data
• Plan for growth — estimate usable capacity for future expansion scenarios
• Understand storage efficiency — see what percentage of raw capacity you’ll actually keep

How Does the Calculator Work?

Webequa’s calculator uses your disk configuration and RAID level to estimate capacity and protection. Here’s what you’ll need:

• Number of disks – how many physical drives are in the array
• Disk size – the capacity of each individual disk (in TB or GB)
• RAID level – RAID 0, 1, 5, 6, or 10
• Disk speed (optional) – used only for rough rebuild-time estimates, not capacity

Once entered, the calculator computes total raw capacity, applies the redundancy formula for the selected RAID level, and returns usable capacity, storage efficiency, and fault tolerance.

Formula

Webequa’s calculator uses the standard RAID capacity formulas used by storage administrators and system builders.

Step 1: Calculate Raw Capacity

Raw Capacity = Number of Disks (N) × Disk Size

Step 2: Calculate Usable Capacity (by RAID level)

RAID 0  (striping, no redundancy):
  Usable Capacity = N × Disk Size

RAID 1  (mirroring):
  Usable Capacity = (N ÷ 2) × Disk Size

RAID 5  (single parity):
  Usable Capacity = (N − 1) × Disk Size

RAID 6  (double parity):
  Usable Capacity = (N − 2) × Disk Size

RAID 10 (mirrored + striped):
  Usable Capacity = (N ÷ 2) × Disk Size

Step 3: Calculate Storage Efficiency

Storage Efficiency (%) = (Usable Capacity ÷ Raw Capacity) × 100

Step 4: Determine Fault Tolerance

RAID 0  → 0 disks (no drive can fail without data loss)
RAID 1  → 1 disk per mirrored pair
RAID 5  → 1 disk, array-wide
RAID 6  → 2 disks, array-wide
RAID 10 → 1 disk per mirrored pair (can tolerate multiple failures if in different pairs)

Variable explanations:

• N (Number of Disks) — total physical drives in the array
• Disk Size — the capacity of a single drive, assumed identical across all drives in the array
• Parity — extra data written to allow reconstruction of a failed disk’s contents (used in RAID 5 and 6)
• Mirroring — writing identical data to two disks so one can fail without data loss (used in RAID 1 and 10)
• Storage Efficiency — the percentage of raw capacity that remains usable after redundancy overhead

Assumptions:

• All disks in the array are the same size (mixed-size arrays are typically limited to the smallest disk’s capacity per member)
• RAID 5 requires a minimum of 3 disks; RAID 6 requires a minimum of 4 disks; RAID 10 requires an even number of disks, minimum 4
• Fault tolerance figures assume a properly configured, healthy array with no disks already degraded
• Calculations reflect theoretical capacity; actual formatted capacity is typically 5–10% lower due to filesystem overhead

Note: RAID is not a backup. Even with high fault tolerance, RAID protects against drive failure — it does not protect against accidental deletion, ransomware, or site-level disasters. A separate backup strategy is still required.

How to Use This Calculator

1. Enter the number of disks in your planned or existing array.
2. Enter the size of each individual disk.
3. Select the RAID level you want to calculate (RAID 0, 1, 5, 6, or 10).
4. Click Calculate.
5. Review usable capacity, storage efficiency, and fault tolerance.
6. Adjust disk count or RAID level to compare different configurations.

Tip: If you’re unsure which RAID level to pick, compare RAID 5 and RAID 10 side by side for the same disk count — RAID 10 gives better performance and fault tolerance, while RAID 5 gives more usable capacity.

Example Calculations

Example 1: RAID 5 — Small Business NAS

Configuration:

Step 1: Raw capacity:

4 × 4 TB = 16 TB

Step 2: Usable capacity:

(4 − 1) × 4 TB = 3 × 4 TB = 12 TB

Step 3: Storage efficiency:

(12 ÷ 16) × 100 = 75%

Step 4: Fault tolerance:

1 disk (array remains online if any single disk fails)

Result: 16 TB raw becomes 12 TB usable at 75% efficiency, tolerating 1 disk failure.

Example 2: RAID 10 — Performance Server

Configuration:

Step 1: Raw capacity:

6 × 2 TB = 12 TB

Step 2: Usable capacity:

(6 ÷ 2) × 2 TB = 3 × 2 TB = 6 TB

Step 3: Storage efficiency:

(6 ÷ 12) × 100 = 50%

Step 4: Fault tolerance:

1 disk per mirrored pair — up to 3 disks total if each failure is in a different pair

Result: 12 TB raw becomes 6 TB usable at 50% efficiency, but with stronger fault tolerance and better read/write performance than RAID 5.

RAID Level Reference Chart

Capacity, Redundancy, and Performance by RAID Level

Choosing a RAID Level by Priority

Understanding Your Results

• Raw capacity — the total combined size of every disk before any RAID overhead
• Usable capacity — the actual storage available for files after redundancy is applied
• Storage efficiency — the percentage of raw capacity retained as usable space
• Fault tolerance — the number of simultaneous disk failures the array can survive without data loss

Always check your storage controller or NAS documentation, since some systems reserve additional space for filesystem metadata, snapshots, or spare drives.

Factors That Affect Real-World Capacity and Protection

Several real-world factors can affect the numbers beyond the basic formula:

• Filesystem overhead — formatting typically reduces usable space by a further 5–10%
• Hot spares — a reserved standby disk reduces usable capacity but speeds up recovery after a failure
• Rebuild time — larger disks take longer to rebuild after a failure, increasing risk of a second failure during rebuild
• Mixed disk sizes — arrays are usually limited to the capacity of the smallest disk per member, wasting space on larger drives
• Controller overhead — hardware and software RAID controllers may reserve small amounts of capacity for metadata
• URE (Unrecoverable Read Error) risk — very large disks in RAID 5 carry a higher statistical risk of a second error during rebuild, which is why RAID 6 is often preferred for large arrays

Benefits of Using a RAID Calculator

1. Free and instant results
2. Eliminates guesswork from storage planning
3. Compares usable capacity across RAID levels instantly
4. Shows fault tolerance alongside capacity, not just space
5. Helps size a NAS or server build before purchasing disks
6. Saves money by avoiding over-buying disks for a target capacity
7. Prevents under-protected setups for critical data
8. Useful for home labs, small business NAS, and enterprise servers
9. Helps compare performance trade-offs between RAID levels
10. Free to use as often as needed with no registration

Limitations

• Assumes identical disk sizes — mixed-size arrays require manual adjustment
• Does not include filesystem or metadata overhead — actual formatted capacity will be slightly lower
• Does not model rebuild time or performance under load — these depend on controller, disk type, and array size
• Does not account for hot spares — add a spare disk manually if your setup includes one
• RAID is not a backup — the calculator estimates capacity and fault tolerance only, not full data protection

Disclaimer: Results are estimates based on standard RAID capacity formulas. Actual usable capacity may vary depending on controller, filesystem, and configuration. Always verify final capacity in your RAID controller or NAS software before deployment.

Common Mistakes

1. Assuming raw disk capacity equals usable capacity
2. Choosing RAID 0 for important data with no separate backup
3. Using mismatched disk sizes and expecting full capacity from the larger drives
4. Not accounting for a hot spare when planning total drive purchases
5. Choosing RAID 5 for very large, high-capacity drives without considering rebuild risk
6. Forgetting that RAID 6 requires a minimum of 4 disks
7. Assuming RAID 10 needs an odd number of disks (it requires an even number)
8. Not leaving room for filesystem overhead when sizing for a specific target capacity
9. Confusing fault tolerance with backup protection
10. Overlooking rebuild time when choosing between RAID 5 and RAID 6 for large arrays

Tips for Accurate Results

1. Always use identical disk models and sizes within a single array where possible
2. For large-capacity drives (8TB+), lean toward RAID 6 or RAID 10 over RAID 5 to reduce rebuild risk
3. Factor in a hot spare disk if fast automatic recovery matters for your setup
4. Reserve 5–10% additional capacity in your planning for filesystem overhead
5. Use RAID 10 when performance and protection both matter more than raw capacity
6. Use RAID 6 for large arrays where a second disk failure during rebuild is a realistic risk
7. Never treat RAID as your only protection — pair it with a proper backup strategy
8. Recalculate usable capacity whenever you add or replace disks in an existing array
9. Check your specific RAID controller’s documentation, since some reserve extra capacity for internal use
10. Test failure scenarios (where possible) before deploying an array for critical production data

Why Trust Webequa?

• ✔ Accurate, industry-standard formulas
• ✔ Verified calculations
• ✔ Expert-reviewed information
• ✔ Privacy-first — no data stored
• ✔ No registration required
• ✔ Mobile-friendly design
• ✔ Fast loading, no clutter
• ✔ Free forever
• ✔ Regularly updated content
• ✔ Accessible worldwide

Related Calculators

• Storage Cost Calculator
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• Bandwidth Calculator
• Server Sizing Calculator
• Data Transfer Time Calculator
• File Size Converter
• Cloud Storage Cost Calculator
• IOPS Calculator
• Unit Converter
• Percentage Calculator
• Network Speed Calculator
• Uptime Percentage Calculator

Scientific and Trade References

• Storage Networking Industry Association (SNIA) — RAID Standards and Terminology
• National Institute of Standards and Technology (NIST) — Storage System Security Guidance
• Seagate — Understanding RAID Levels
• Western Digital — RAID Configuration Guide
• Synology — RAID Types and Storage Calculator Reference
• IBM — RAID Levels and Data Protection Overview

Conclusion

Getting your usable storage capacity right before you buy drives saves money, prevents under-protected setups, and helps you choose the right balance of performance and redundancy. A RAID calculator gives you a fast, accurate estimate of exactly how much usable space and fault tolerance your array will have — based on your disk count, disk size, and chosen RAID level. Webequa’s calculator handles all the maths automatically so you can plan your storage build with confidence.

Try Webequa’s free RAID calculator today and make sure your next storage array starts with exactly the right configuration.