RAID & Synology SHR Calculator: Real Usable Capacity
This RAID usable capacity calculator shows how much storage you'll actually get after redundancy, filesystem overhead, and NAS system partitions, for standard RAID levels and Synology SHR. Enter drive count, size, and RAID type to see real usable capacity, fault tolerance, and rebuild risk.
Most RAID calculators show theoretical capacity, raw drive count times drive size minus parity. In reality, usable storage is further reduced by binary conversion, NAS system partitions, filesystem metadata reserves, and snapshot allocation.
Select your NAS ecosystem, drives, and RAID type to see what you'll actually end up with, including every deduction layer, with sources cited where available.
Synology SHR Calculator: What Is SHR and How Does It Work?
▲ HideSynology Hybrid RAID (SHR) is Synology's proprietary RAID implementation designed for home and small business NAS users who want to mix drive sizes without wasting capacity. Unlike standard RAID 5, which requires all drives to be identical: SHR automatically optimises usable storage across drives of different sizes by splitting them into matching capacity tiers internally.
SHR is available exclusively on Synology NAS running DSM. If you are using a QNAP, UGREEN, or any other brand, use RAID 5 instead. See our Best Synology NAS guide for model recommendations.
SHR vs RAID 5: Key Differences
| SHR-1 | RAID 5 | SHR-2 | |
|---|---|---|---|
| Mixed drive sizes | ✅ Yes | ❌ No, all must match | ✅ Yes |
| Minimum drives | 2 | 3 | 4 |
| Parity drives | 1 | 1 | 2 |
| Fault tolerance | 1 drive failure | 1 drive failure | 2 drive failures |
| Usable (4× 4TB identical) | ~12 TB | ~12 TB | ~8 TB |
| Usable (2+4+4+8TB mixed) | ~10 TB | ~6 TB (wastes capacity) | ~6 TB |
| Supported on | Synology only | Universal | Synology only |
| Drive expansion | One at a time, any larger size | Replace all drives | One at a time, any larger size |
When to Choose SHR
- You are starting with drives you already own (different sizes): SHR uses every GB efficiently
- You want to expand capacity gradually by replacing drives one at a time as budget allows
- You are buying a Synology NAS (SHR is Synology-exclusive: QNAP and others use RAID 5)
When RAID 5 May Be Better
- You are buying all drives at once, same size, both give identical usable capacity
- You may migrate to a non-Synology NAS in future: RAID 5 is universally readable
SHR-2: Double Parity
SHR-2 adds a second parity drive, protecting against simultaneous failure of any two drives. Requires a minimum of 4 drives. Recommended for arrays with 4+ large drives (12TB+) where a rebuild takes 12+ hours, the longer the rebuild, the higher the risk of a second failure during that window. Use our RAID Rebuild Risk Calculator to see whether SHR-2 is warranted for your drive sizes.
Enter values from your NAS to override estimated figures. Leave blank to use defaults.
TB vs TiB: Why the Gap?
Drive manufacturers use decimal terabytes: 1 TB = 1,000,000,000,000 bytes. Operating systems display binary tebibytes: 1 TiB = 1,099,511,627,776 bytes. The difference means a 4 TB drive appears as approximately 3.64 TiB, about 6.8% less than the label. This is universal across every drive brand.
How We Calculate
This calculator processes your configuration through seven layers:
- Binary conversion , converts advertised TB to real TiB
- Hot spare deduction , removes spare drives from the RAID array
- RAID parity , applies the correct formula for your RAID type
- System overhead , deducts NAS OS reserved space where vendor-published figures are available (labeled with source)
- Filesystem metadata , deducts Btrfs/ext4/ZFS/XFS metadata reserves (labeled as estimated)
- Snapshot reserve , deducts pre-allocated snapshot space
- Headroom , shows recommended free space buffer
Values marked A are vendor-published with a cited source. Values marked B are community-observed estimates. Values shown as "varies" should be entered manually in Exact Mode for precision.
Overhead figures last reviewed: 20 March 2026
AU NAS Drive Pricing Reference (early 2026)
NAS-grade drives available from Australian retailers. Check Staticice, Mwave, and PLE for current pricing.
| Drive | Capacity | AU retail range | Stocked at |
|---|---|---|---|
| Seagate IronWolf | 4 TB | $135-$165 | Mwave, PLE, Amazon AU |
| Seagate IronWolf | 6 TB | $170-$210 | Mwave, PLE, Amazon AU |
| Seagate IronWolf | 8 TB | $215-$265 | Mwave, PLE, Scorptec |
| Seagate IronWolf | 12 TB | $330-$380 | Mwave, PLE, Amazon AU |
| Seagate IronWolf | 16 TB | $440-$510 | Mwave, PLE, Scorptec |
| WD Red Plus | 4 TB | $130-$165 | Mwave, PLE, Amazon AU |
| WD Red Plus | 8 TB | $225-$270 | Mwave, PLE, Amazon AU |
| WD Red Plus | 12 TB | $330-$395 | Mwave, PLE, Scorptec |
SHR vs RAID 1 vs RAID 5: AU Home User Decision Guide
| RAID type | Min drives | Usable (4× 4 TB) | Best for | Key trade-off |
|---|---|---|---|---|
| RAID 1 (Mirror) | 2 | 4 TB (50%) | 2-bay NAS, critical data, simple setup | Wastes half raw capacity |
| RAID 5 | 3 | 12 TB (75%) | 4-bay NAS with identical drives, best capacity/protection ratio | URE risk on large drives (>8 TB, consider RAID 6) |
| RAID 6 / SHR-2 | 4 | 8 TB (50%) | Large drives (12 TB+), critical data, tolerates 2 simultaneous failures | Two drives of parity overhead |
| SHR (Synology) | 2 | ~12 TB with identical drives | Mixed drive sizes: Synology-specific | Same as RAID 5 with identical drives; smarter with mixed |
Named Example: DS925+ with 4× 4 TB IronWolf (RAID 5 / SHR)
A Synology DS925+ with 4× Seagate IronWolf 4 TB in RAID 5 or SHR gives approximately 10.7 TB usable after binary conversion, Btrfs metadata, and Synology system overhead. Drive cost at Mwave: approximately $135 × 4 = $540 AUD for drives only.
ACL warranty note: Seagate IronWolf drives from Australian retailers carry a 3-year warranty under the Australian Consumer Law, serviced locally. If a drive fails during a rebuild, you'll replace it before a warranty return clears, keeping a cold spare on the shelf prevents extending your degraded window by however long AU shipping takes (typically 2-7 business days).
Frequently Asked Questions
Why is this different from other RAID calculators?
Most RAID calculators only calculate parity overhead. This tool also accounts for binary conversion (TB to TiB), NAS system partitions, filesystem metadata, snapshot reserves, and recommended headroom, giving a realistic picture of what you'll actually have available.
Why does my NAS show less space than the drives are labelled?
Drive manufacturers use decimal terabytes (1 TB = 1,000 GB) while your NAS operating system displays binary tebibytes (1 TiB = 1,024 GiB). A 4 TB drive appears as approximately 3.64 TiB, about 6.8% less. This happens with every drive on every NAS brand.
How accurate are the system overhead values?
Where the NAS vendor has published specific overhead figures (in documentation, FAQs, or their own calculators), this tool uses those figures and cites the source. These are marked with ᴬ in the output. Where no official figure exists, the tool shows "varies" and recommends Exact Mode, where you can enter your NAS-reported values.
What is SHR and how does it differ from RAID 5?
SHR (Synology Hybrid RAID) provides the same single-drive redundancy as RAID 5, but handles mixed drive sizes more efficiently. With identical drives, SHR and RAID 5 produce the same usable capacity. With mixed drives, SHR creates multiple RAID arrays across size tiers to use more of each drive's capacity.
Why does QNAP show so much reserved space?
QNAP reserves approximately 70 GB per storage pool and 20 GB per static volume for system data. Additionally, QNAP enables a 20% snapshot reserve by default on storage pools. This snapshot reserve is the largest factor and can be adjusted (0-50%) in QTS under Storage & Snapshots → Storage Pool → Management.
What is the "headroom" recommendation?
The headroom slider shows how much free space to leave on your NAS for optimal performance. Running near 100% capacity can cause degraded write performance and prevent snapshot operations. 10-20% free space is generally recommended, with ZFS-based systems (TrueNAS) benefiting from at least 20%.
Can I share my calculation?
Yes. Every configuration generates a unique permalink URL that you can bookmark, share on forums, or embed in blog posts. The "Copy for forum" and "Cite this result" buttons generate formatted text ready to paste.
How often is the data updated?
The NTKIT editorial team reviews ecosystem overhead figures, firmware changes, and source documentation periodically. The last review date is shown on this page.
What NAS drives are available in Australia and how much do they cost?
The main NAS-grade drives sold through Australian retailers (Mwave, PLE, Scorptec, Amazon AU) are the Seagate IronWolf and WD Red Plus. As of early 2026: IronWolf 4 TB ~$135-$165, IronWolf 8 TB ~$215-$265, IronWolf 12 TB ~$330-$380, IronWolf 16 TB ~$440-$510. WD Red Plus is priced similarly. Use Staticice to compare current pricing across AU retailers before buying.
How does Australian Consumer Law (ACL) affect NAS drive warranty claims?
NAS drives purchased from Australian retailers carry a manufacturer warranty under the ACL, typically 2 years for WD Red Plus and 3 years for Seagate IronWolf. Warranty claims are handled locally; you don't need to ship drives internationally. However, replacement takes days to weeks. If you're running a degraded RAID array while waiting, your data is at risk for the entire window. Keeping a cold spare (a replacement drive on the shelf) eliminates the wait, the warranty handles the cost, the spare eliminates the downtime.
Free Download: NAS RAID Planning Checklist
A 1-page worksheet covering drive count, RAID type selection, usable capacity, and rebuild risk, fill it in before you buy drives or build a NAS.