Vendor neutral storage math
Size your storage before you buy the drives.
Every RAID calculator on the internet belongs to a hardware vendor, so each one models exactly one scheme. Synology's models SHR. TrueNAS-flavoured tools model RAIDZ. Controller vendors model the classic levels. No manufacturer tool compares all three for the same physical drives, which is the one comparison you need with a shopping cart open.
- Layouts compared
- 11
- NAS models
- 48
- Pages
- 141
| Layout | Usable | As reported | Survives |
|---|---|---|---|
| Synology SHR | 48 TB | 43.66 TiB | 1 drive |
| Synology SHR-2 | 32 TB | 29.1 TiB | 2 drives |
| ZFS RAIDZ1 | 48 TB | 43.66 TiB | 1 drive |
| ZFS RAIDZ2 | 32 TB | 29.1 TiB | 2 drives |
| RAID 5 | 48 TB | 43.66 TiB | 1 drive |
| RAID 6 | 32 TB | 29.1 TiB | 2 drives |
| RAID 10 | 32 TB | 29.1 TiB | 1 drive |
Now make them mismatched. Drives of 4, 8, 8 and 12 TB give 12 TB under RAID 5 and 20 TB under Synology SHR, a gap of 8 TB. Standard RAID and every ZFS vdev truncate each member to the smallest drive and strand the rest.
Start with the arithmetic
- RAID capacity calculator SHR, SHR-2, RAIDZ1/2/3 and RAID 0/1/5/6/10 for the same drives, with mixed sizes. Open →
- Power draw calculator Watts per drive and per board, turned into a real annual electricity bill. Open →
- UPS runtime calculator How long a battery actually holds your array up, and why VA is not watts. Open →
- Drive failure probability Annualised failure rates across your drive count over one, three and five years. Open →
- 3-2-1 backup sizing How much storage three copies of your data really needs, with growth and versions. Open →
- Rack unit space calculator U consumed against cabinet height, with the exact 1U equals 44.45 mm conversion. Open →
- Network throughput calculator Whether your link or your array is the bottleneck, and how long a transfer takes. Open →
The enclosure is the cheap part
An eight-bay NAS is a big purchase on its own. The eight drives that go in it are $200 to $500 each, so $2,000 to $4,000 in a single order. And unlike the box, drives fail and get upgraded, so that spend repeats every few years.
Which is why the drive pages here get the same care as the calculators. Two requirements are not negotiable: CMR recording, never SMR, because shingled drives can take days to rebuild an array, and a published NAS workload rating of at least 180 TB per year.
| Build | Enclosure budget | Drives budget | Drive share |
|---|---|---|---|
| 2 bay, 8 TB drives | Under $500 | Under $750 | 66% |
| 4 bay, 8 TB drives | Under $1,000 | Under $1,500 | 66% |
| 4 bay, 20 TB drives | Under $1,000 | Under $3,500 | 82% |
| 8 bay, 16 TB drives | Under $2,500 | Under $5,000 | 77% |
| 8 bay, 20 TB drives | Under $2,500 | Under $7,500 | 83% |
Budgets are rounded up from the catalog prices on our comparison pages. Prices change often, so check the current one on Amazon. As an Amazon Associate we earn from qualifying purchases.
Then buy the hardware
- Best NAS hard drives Where the money in any build actually goes. CMR against SMR, workload ratings, price per terabyte. Read →
- Best NAS for home Two, four and eight bay enclosures, with an honest note on who should not buy the expensive one. Read →
- Best NAS for Plex Why Intel Quick Sync decides this category and AMD embedded parts do not transcode. Read →
- Best UPS for a NAS Pure sine wave, real runtime, and why the goal is a clean shutdown rather than working through an outage. Read →
All hardware guides · Specs for 48 NAS models · Reference charts · Head to head comparisons · Guides
Why your 20 TB drive says 18.2 TB
Nothing is missing and nobody cheated you. Drive makers sell decimal terabytes, where one TB is exactly 1,000,000,000,000 bytes. Your operating system reports binary tebibytes, where one TiB is 2 to the power of 40 bytes, or 1,099,511,627,776. Divide one by the other and you get 0.909495, so a 20 TB drive genuinely holds 18.19 TiB of the same bytes.
The practical effect is a fixed 9.05 percent apparent shrink at every capacity, before any filesystem overhead. Windows and most NAS interfaces simply label a tebibyte with the letters "TB". On top of that, ext4 and Btrfs reserve roughly one to two percent for metadata, and ZFS reserves 1/64 of the pool as slop space. Those are real, and they are much smaller than the unit difference.
The full conversion chart covers every capacity from 1 TB to 100 TB, and the RAID calculator shows both units side by side for whatever drives you own.
RAID is not a backup. It protects against a drive dying. It does nothing about accidental deletion, ransomware, a controller writing garbage to every member, theft, fire or flood, because every one of those reaches all drives at once. Size three copies with the 3-2-1 backup calculator and keep one of them off site.
