Two-Bay NAS Capacity Calculator (RAID 1 / SHR)
If you're buying drives for a two-bay NAS, the total capacity printed on the boxes isn't the amount of storage you'll have when you enable drive redundancy.
Enter the advertised capacities of two drives below. This calculator estimates the mirrored capacity of RAID 1 or two-drive Synology Hybrid RAID (SHR-1). It starts with the two 4 TB WD Red Plus drives I use in my Synology DS223.
Use the drive sizes on the labels (for example, 4 or 8 TB). Results update as you type.
Enter a positive capacity in TB for both drives.
About 4 TB is used for the redundant copy. No capacity is left unused because the drives match.
Estimate only: These figures are before system partitions, filesystem metadata and other overhead. DSM may show a slightly smaller usable volume.
Why two 4 TB drives don't give me 8 TB
My DS223 uses two 4 TB WD Red Plus drives in SHR. With only two drives, SHR-1 provides approximately the same usable capacity as RAID 1: one drive's worth of data, protected by a redundant copy on the other drive.
So the two drives add up to 8 TB of raw capacity, but the mirrored storage is approximately 4 TB, not 8 TB. If either drive fails, the remaining drive should still contain the data while I replace the failed one. This protects against a single drive failure; it doesn't make the NAS immune to data loss.
Why DSM shows roughly 3.6 instead of 4 TB
Drive manufacturers measure capacity in decimal terabytes: 1 TB is 1,000,000,000,000 bytes. Storage software commonly uses binary units: 1 TiB is 1,099,511,627,776 bytes.
That means a drive sold as 4 TB contains approximately 3.64 TiB of storage. Synology's interface may label its binary calculation as TB, which can make the difference confusing. The system and filesystem also reserve some capacity, so the amount available for files can be lower again.
What if the drives are different sizes?
With RAID 1 or SHR-1 using exactly two drives, mirrored capacity is limited by the smaller drive. A 4 TB drive paired with an 8 TB drive still gives about 4 TB of mirrored capacity (3.64 TiB), before overhead. The extra 4 TB on the larger drive cannot be used for mirrored storage in this two-drive arrangement.
SHR can make more efficient use of mixed drive sizes in larger arrays, but adding a bigger second disk to a two-bay mirrored NAS doesn't increase its usable capacity.
Redundancy is not a backup
Mirroring helps when a drive fails. It doesn't protect you against accidentally deleting files, ransomware, a damaged or stolen NAS, or a problem affecting both drives.
My DS223 is also where I keep backups from my Docker servers. The mirrored disks protect the NAS against one failed drive, but they are not, by themselves, a separate backup of everything stored on the NAS. I've written more about this in How I Back Up My Homelab.
For more detail about how I actually use the NAS, see my Synology DS223 homelab review. Synology also has an official RAID calculator for more drive bays and RAID types.
The drives I use
Both drives in my DS223 are WD Red Plus 4 TB models (WD40EFPX), as confirmed in Synology DSM. If you're looking for a similar drive, the WD40EFZX is another 4 TB model in the WD Red Plus range. I haven't personally used that particular model, so check the specifications and current price before buying.
Check the WD Red Plus 4 TB (WD40EFZX) on Amazon UK — Affiliate disclosure: If you buy through this link, I may earn a small commission at no additional cost to you.