Free RAID Calculator
Calculate usable storage capacity, efficiency, and estimated costs for any RAID configuration. Works for Synology, QNAP, TrueNAS, Unraid, and any RAID array.
RAID Calculator Quick Reference
Don't want to use the calculator? Here are pre-calculated results for popular configurations:
RAID 5 / SHR Capacity Table
Formula: Usable = (N - 1) × Drive Size | Fault Tolerance: 1 drive
| Drives | Size | Raw | Usable | Efficiency | Est. Cost* |
|---|---|---|---|---|---|
| 3× | 8TB | 24TB | 16TB | 67% | $447 |
| 4× | 8TB | 32TB | 24TB | 75% | $596 |
| 5× | 8TB | 40TB | 32TB | 80% | $745 |
| 6× | 8TB | 48TB | 40TB | 83% | $894 |
| 4× | 12TB | 48TB | 36TB | 75% | $876 |
| 4× | 16TB | 64TB | 48TB | 75% | $1,196 |
| 6× | 12TB | 72TB | 60TB | 83% | $1,314 |
| 8× | 16TB | 128TB | 112TB | 88% | $2,392 |
*Estimated costs based on current WD Red Plus / Seagate IronWolf pricing (February 2026)
RAID 6 / SHR-2 Capacity Table
Formula: Usable = (N - 2) × Drive Size | Fault Tolerance: 2 drives
| Drives | Size | Raw | Usable | Efficiency | Est. Cost* |
|---|---|---|---|---|---|
| 4× | 8TB | 32TB | 16TB | 50% | $596 |
| 5× | 8TB | 40TB | 24TB | 60% | $745 |
| 6× | 8TB | 48TB | 32TB | 67% | $894 |
| 8× | 8TB | 64TB | 48TB | 75% | $1,192 |
| 6× | 12TB | 72TB | 48TB | 67% | $1,314 |
| 8× | 16TB | 128TB | 96TB | 75% | $2,392 |
RAID 10 Capacity Table
Formula: Usable = N/2 × Drive Size | Fault Tolerance: 1 per mirror pair
| Drives | Size | Raw | Usable | Efficiency | Est. Cost* |
|---|---|---|---|---|---|
| 4× | 8TB | 32TB | 16TB | 50% | $596 |
| 6× | 8TB | 48TB | 24TB | 50% | $894 |
| 8× | 8TB | 64TB | 32TB | 50% | $1,192 |
| 4× | 16TB | 64TB | 32TB | 50% | $1,196 |
| 8× | 16TB | 128TB | 64TB | 50% | $2,392 |
RAID 1 Capacity Table (2-Bay NAS)
Formula: Usable = Drive Size | Fault Tolerance: 1 drive
| Drives | Size | Raw | Usable | Efficiency | Est. Cost* |
|---|---|---|---|---|---|
| 2× | 4TB | 8TB | 4TB | 50% | $198 |
| 2× | 8TB | 16TB | 8TB | 50% | $298 |
| 2× | 12TB | 24TB | 12TB | 50% | $438 |
| 2× | 16TB | 32TB | 16TB | 50% | $598 |
| 2× | 20TB | 40TB | 20TB | 50% | $798 |
RAID Calculator Formulas
Understanding how each RAID level calculates usable capacity:
| RAID Type | Usable Capacity Formula | Example (4× 8TB) | Min Drives |
|---|---|---|---|
| RAID 0 | N × Size | 32TB (100%) | 2 |
| RAID 1 | Size | 8TB (25%) | 2 |
| RAID 5 / SHR | (N - 1) × Size | 24TB (75%) | 3 |
| RAID 6 / SHR-2 | (N - 2) × Size | 16TB (50%) | 4 |
| RAID 10 | (N / 2) × Size | 16TB (50%) | 4 (even) |
Where: N = Number of drives, Size = Capacity of smallest drive
RAID Comparison: Which Should You Choose?
| RAID | Min Drives | Efficiency | Fault Tolerance | Performance | Best For |
|---|---|---|---|---|---|
| RAID 0 | 2 | 100% | ❌ None | ⭐⭐⭐⭐⭐ | Temp files only |
| RAID 1 | 2 | 50% | 1 drive | ⭐⭐⭐⭐ | 2-bay NAS |
| RAID 5/SHR | 3 | 67-90% | 1 drive | ⭐⭐⭐⭐ | Most users |
| RAID 6/SHR-2 | 4 | 50-75% | 2 drives | ⭐⭐⭐ | Large arrays, critical data |
| RAID 10 | 4 | 50% | 1 per pair | ⭐⭐⭐⭐⭐ | Databases, VMs |
RAID Selection Guide
| Your Situation | Recommended RAID | Why |
|---|---|---|
| 2-bay NAS | RAID 1 / SHR | Only option with redundancy |
| Home user, 4-bay | RAID 5 / SHR | Best balance of capacity & protection |
| Plex media server | RAID 5 / SHR | Maximize storage, sequential reads |
| Small business | RAID 6 / SHR-2 | Survives 2 failures during rebuild |
| 6+ drive array | RAID 6 / SHR-2 | Long rebuild = higher failure risk |
| Database server | RAID 10 | Best random I/O performance |
| Surveillance NVR | RAID 5 / SHR | Sequential writes, capacity focus |
| Scratch disk / cache | RAID 0 | Speed priority, data is replaceable |
RAID Calculator for Different NAS Brands
Our calculator works for any NAS or RAID controller. Here's how different brands implement RAID:
Synology RAID Calculator
Synology offers SHR (Synology Hybrid RAID) - a flexible RAID that allows mixed drive sizes. SHR provides RAID 5-equivalent protection with easier expansion. SHR-2 provides RAID 6-equivalent dual-drive protection.
QNAP RAID Calculator
QNAP uses standard RAID levels (0, 1, 5, 6, 10). For mixed drives, use JBOD. Same capacity formulas as traditional RAID.
TrueNAS / FreeNAS RAID Calculator
TrueNAS uses ZFS with RAID-Z levels: RAID-Z1 ≈ RAID 5, RAID-Z2 ≈ RAID 6, RAID-Z3 = 3-drive parity. Same capacity formulas apply.
Unraid Calculator
Unraid uses a unique parity system. Single parity = RAID 5-like capacity. Dual parity = RAID 6-like capacity. Allows mixed drive sizes like SHR.
RAID Rebuild Time Estimates
Larger drives take longer to rebuild. During rebuild, the array is vulnerable to additional failures - this is why RAID 6/SHR-2 is recommended for large drives.
| Drive Size | Estimated Rebuild Time | Recommendation |
|---|---|---|
| 4TB | 8-12 hours | RAID 5 / SHR OK |
| 8TB | 16-24 hours | RAID 5 / SHR OK |
| 12TB | 24-36 hours | Consider RAID 6 / SHR-2 |
| 16TB | 36-48 hours | Recommend RAID 6 / SHR-2 |
| 20TB+ | 48-72+ hours | Strongly recommend RAID 6 / SHR-2 |
Frequently Asked Questions
What is a RAID calculator?
A RAID calculator is a tool that calculates the usable storage capacity of a RAID array based on the number of drives, drive capacity, and RAID level. It helps you plan storage systems by showing how much space you'll have after accounting for redundancy overhead.
How do I calculate RAID 5 capacity?
RAID 5 capacity = (Number of drives - 1) × smallest drive size. For example, 4 drives × 8TB in RAID 5 = (4-1) × 8TB = 24TB usable. One drive's worth of space is used for parity data that enables recovery if a drive fails.
What is SHR on Synology?
SHR (Synology Hybrid RAID) is Synology's flexible RAID system that allows mixing different drive sizes while optimizing capacity. It provides RAID 5-equivalent protection (1-drive fault tolerance). SHR-2 provides RAID 6-equivalent protection (2-drive fault tolerance).
Which RAID is best for NAS?
RAID 5 or SHR is best for most NAS users. It provides good usable capacity (75%+ with 4 drives) with single-drive fault tolerance. For critical data or large arrays (6+ drives), use RAID 6 or SHR-2 for two-drive fault tolerance during rebuilds.
What is the difference between RAID 5 and RAID 6?
RAID 5 uses single parity (survives 1 drive failure), while RAID 6 uses dual parity (survives 2 drive failures). RAID 5: (N-1) × size. RAID 6: (N-2) × size. RAID 6 is recommended for large arrays where rebuild times are long.
Can I mix different size drives in RAID?
Traditional RAID (5, 6, 10) requires same-size drives - larger drives are truncated to match the smallest. Synology SHR and Unraid allow mixed drive sizes and optimize capacity automatically. This flexibility is a major advantage of SHR.
Is RAID a backup?
No, RAID is NOT a backup. RAID protects against drive failure only. It does NOT protect against: accidental deletion, ransomware, fire/theft, controller failure, or file corruption. Always maintain separate backups (cloud, external drive, or another NAS) regardless of RAID level.
How much storage do I lose with RAID?
Storage overhead depends on RAID level: RAID 0 = 0% loss (no protection), RAID 1 = 50% loss, RAID 5 = 1 drive's worth (25% with 4 drives), RAID 6 = 2 drives' worth (33% with 6 drives), RAID 10 = 50% loss. This 'lost' capacity provides data protection.
What is RAID 10?
RAID 10 combines mirroring (RAID 1) and striping (RAID 0). Data is mirrored across pairs, then striped for performance. RAID 10 offers the best read/write speeds with 50% capacity and can survive 1 drive failure per mirror pair. Requires minimum 4 drives (even number).
How long does RAID rebuild take?
RAID rebuild time depends on drive size: 4TB ≈ 8-12 hours, 8TB ≈ 16-24 hours, 12TB ≈ 24-36 hours, 16TB+ ≈ 36-48+ hours. During rebuild, the array is vulnerable to another failure. This is why RAID 6/SHR-2 is recommended for large drives.
Related Resources
- Synology Compatible Hard Drives - Full drive database with pricing
- Best Synology NAS 2026 - Complete buying guide
- DS925+ vs DS923+ Comparison - 4-bay NAS showdown
- DSM 7.3 Drive Support - Third-party drive compatibility
- WD Red Plus Review - Best value NAS drives
Last Updated: February 2026 | RAID Calculator v2.0 | Drive prices updated regularly