Compute usable storage.
Before you spend a dollar.
A precision tool for Synology operators. Enter your RAID configuration and read out usable capacity, parity overhead, and fault tolerance. All standardized against DSM's internal calculations.
One drive of parity across all disks. Good balance of speed, capacity, and safety.
What Is the Synology RAID Calculator?
The Synology RAID Calculator tells you exactly how much usable storage you'll get from any RAID configuration on your Synology NAS, before you spend money on drives or commit to a setup you can't easily change. Enter your RAID type, drive count, and drive capacity, and you'll instantly see usable storage, raw storage, storage efficiency, and how many drives you can lose without data loss.
It's built for Synology DiskStation and RackStation owners who want to compare configurations without digging through technical documentation. Whether you're setting up a DS223j with two drives or planning a large RS2423+ rack unit, the calculator handles every RAID type Synology supports: RAID 0, RAID 1, RAID 5, RAID 6, RAID 10, SHR, and SHR-2.
Home NAS users often find SHR vs RAID 5 confusing. Our RAID storage calculator makes the trade-offs visible at a glance. IT admins planning SMB deployments use it to size arrays for specific capacity targets. And anyone shopping for their first Synology can run "what-if" scenarios to figure out whether 4 drives or 6 drives makes more sense for their budget.
Want to know more about how the site works or who built it? Learn about our methodology and team.
Synology RAID Guide: Everything You Need to Know
RAID (Redundant Array of Independent Disks) is how your NAS turns multiple physical drives into a single logical storage pool. Synology supports seven RAID configurations, each with different trade-offs between capacity, redundancy, and rebuild speed. Here's what actually matters when choosing.
Choosing Between SHR and RAID 5
With all same-size drives, SHR and RAID 5 produce identical usable storage. Four 4TB drives in SHR gives you the same 12TB usable that RAID 5 does. The difference shows up when you mix drive sizes. SHR's proprietary algorithm allocates capacity intelligently across unequal drives, while RAID 5 wastes capacity by forcing all drives to match the smallest one.
For a home NAS where you'll slowly upgrade drives over time, SHR is the more practical long-term choice. For a business NAS where you buy all drives at once and prioritize interoperability, RAID 5 is simpler and works on any hardware. Read our in-depth SHR vs RAID 5 comparison for a full breakdown.
Understanding Fault Tolerance
Fault tolerance is how many drives can fail before you lose data. RAID 5 survives one drive failure; RAID 6 and SHR-2 survive two. This distinction matters most during RAID rebuild, the hours or days it takes to reconstruct the array after a failed drive is replaced. During rebuild, all remaining drives are under heavy load. A second failure during that window, on a RAID 5 array, means total data loss.
For arrays with 5 or more drives, or drives holding irreplaceable data, the case for RAID 6 or SHR-2 is strong. At 6 drives, RAID 6 costs you one extra drive of capacity (83% vs 100% of RAID 5's usable space), but doubles your tolerance for coincidental failures. Compare RAID 5 vs RAID 6 in detail here.
The 3-2-1 Backup Rule and RAID
RAID protects against drive hardware failure. It does not protect against accidental deletion, ransomware, fire, theft, or NAS controller failure. A drive failure in RAID 5 is a hardware event; a ransomware attack encrypts data on all drives simultaneously, RAID included.
The 3-2-1 rule means: 3 copies of your data, on 2 different media types, with 1 copy stored offsite. Synology's Hyper Backup app makes cloud backups straightforward, whether you're backing up to Backblaze B2, Amazon S3, or a second NAS at another location. RAID is the first line of defense; backup is the second. Learn why RAID is not a substitute for backups.
When to Upgrade or Expand Your NAS
Synology supports online RAID expansion, letting you add drives or swap in larger ones without downtime. SHR arrays let you add one drive at a time and automatically rebalance. RAID 5 and RAID 6 arrays can also expand online on most Synology models. The safest expansion strategy: always take a full backup first, then add the new drive, then let the NAS rebuild.
A good rule of thumb: plan to expand when your NAS hits 75–80% capacity. At that point, re-run this calculator with your expanded drive count to see how much headroom a new configuration gives you. Follow our step-by-step expansion guide for the full process.
Who Should Use This Calculator?
This tool is for anyone making storage decisions on a Synology NAS, from first-time buyers to experienced homelab engineers.
Home NAS users use it to decide between RAID configurations before buying drives. If you're setting up a Synology DS923+ and wondering whether 4 × 4TB in SHR or 4 × 4TB in RAID 5 gives you more space, the answer is they're identical, but now you know why. If you're comparing SHR vs RAID 6, the calculator shows exactly what two-drive fault tolerance costs you in usable capacity.
Synology shoppers use it during the buying process. Before committing to a 4-bay vs 6-bay unit, run the numbers. Six 8TB drives in RAID 6 gives you 32TB usable, equivalent to a 4-bay unit maxed out at RAID 5 with 8TB drives. Knowing this upfront prevents expensive mistakes.
IT admins and MSPs planning SMB NAS deployments use it for quick capacity planning during client consultations. Comparing a 4-bay RAID 5 array against a 6-bay RAID 6 configuration takes seconds, not spreadsheet work.
Homelab enthusiasts running TrueNAS migrations or Synology DSM upgrades use it to verify their expected capacity before reorganizing pools.
Ready to start? Try the calculator above or read our RAID guides to learn more about Synology storage best practices.
Frequently Asked Questions
For home users with mixed drive sizes, SHR (Synology Hybrid RAID) is the best choice as it maximizes usable storage across drives of different capacities. For home users with all same-size drives who want simplicity, SHR or RAID 5 is ideal. Business users with critical data should use RAID 6 or SHR-2 for two-drive fault tolerance. RAID 0 is only for non-critical temporary data where maximum performance is needed.
In RAID 5 with four identical drives, your usable storage equals three drives worth of capacity. For example, four 4TB drives in RAID 5 gives 12TB usable (75% efficiency). Four 8TB drives give 24TB usable. RAID 5 uses one drive equivalent for distributed parity across all drives, enabling recovery if any single drive fails.
With same-size drives, SHR and RAID 5 deliver identical usable storage. The key difference is drive flexibility: SHR automatically optimizes storage when you mix drives of different sizes, while RAID 5 uses only the smallest drive's capacity from all drives (wasting space). Additionally, SHR allows you to start with fewer drives and expand easily. For same-size drives, there is functionally no difference in performance or reliability.
RAID 6 tolerates up to two simultaneous drive failures. It uses two drive equivalents for dual parity rather than the single parity of RAID 5. This makes RAID 6 more resilient during the rebuild period after a first drive failure. A time when a second drive failure is statistically more likely due to the stress of reading all other drives during rebuild.
No: RAID is not a backup. RAID protects against hardware failure of one or more drives but does not protect against accidental file deletion, ransomware, fire, theft, software corruption, or NAS controller failure. Always maintain a separate backup using the 3-2-1 rule: 3 copies of your data, on 2 different media types, with 1 copy stored offsite or in the cloud.
RAID 5 requires a minimum of 3 drives. With 3 drives, one drive equivalent is used for parity, leaving 2 drives worth of usable storage (66.7% efficiency). As you add more drives, efficiency improves: 4 drives = 75%, 5 drives = 80%, 6 drives = 83.3%. Synology NAS units range from 1-bay (J-series) to 60-bay rack units, accommodating everything from basic RAID 1 to large RAID 6 arrays.
Yes, Synology supports online RAID expansion for most configurations. SHR arrays can expand by adding new drives one at a time. The NAS automatically optimizes storage allocation. RAID 5 and RAID 6 arrays can also expand online. RAID 1 can expand by upgrading to larger drives one at a time (replace, rebuild, repeat). RAID 0 and RAID 10 have limited online expansion support. Always ensure you have a fresh backup before any RAID expansion.
Both SHR-2 and RAID 6 provide two-drive fault tolerance, but SHR-2 has the same mixed-drive advantage as SHR-1. With same-size drives and four or more drives, they deliver identical usable storage. SHR-2 requires a minimum of 4 drives and is available on higher-end Synology NAS models (XS, XS+, and some Plus series). RAID 6 requires a minimum of 4 drives and follows standard RAID 6 math.
Synology RAID Calculator Team
Free, accurate storage planning tools for Synology NAS users.