Buy four 8 TB drives expecting 32 TB of space and you will be surprised: RAID redundancy, filesystem overhead, and the marketing-versus-real gap between advertised and formatted capacity all take their cut. Before you buy drives for a NAS, it helps to calculate what you will actually be able to store, so you size the array right the first time.
Two haircuts before RAID even applies
First, drive makers count a terabyte as 1,000,000,000,000 bytes, while operating systems count in binary (1 TiB = 1,099,511,627,776 bytes). That difference means an “8 TB” drive shows as roughly 7.3 TiB in your NAS. Expect to lose about 7–9% here on every drive before anything else happens.
Second, the filesystem itself reserves a slice for metadata, snapshots, and housekeeping—often another few percent. So even a single drive with no RAID nets meaningfully less than its label.
Then RAID takes its share
Redundancy is the big one. Each RAID level trades raw capacity for fault tolerance in a predictable way:
| RAID level | Usable formula | 4x 8 TB raw | Approx. usable* | Drives you can lose |
|---|---|---|---|---|
| RAID 0 | n x size | 32 TB | ~28.8 TB | 0 |
| RAID 1 (mirror) | size (of one) | 8 TB pair | ~7.2 TB | 1 per pair |
| RAID 5 | (n-1) x size | 24 TB | ~21.6 TB | 1 |
| RAID 6 | (n-2) x size | 16 TB | ~14.4 TB | 2 |
| RAID 10 | (n/2) x size | 16 TB | ~14.4 TB | 1–2 (per pair) |
*Usable figures already trim ~10% for the binary conversion and filesystem overhead.
Worked example
Say you want RAID 5 across four 8 TB drives. RAID 5 usable is (n-1) x size = 3 x 8 = 24 TB raw. Apply the ~10% real-world haircut and you land near 21.6 TiB usable. If you instead chose RAID 6 for two-drive safety, you would drop to (4-2) x 8 = 16 TB raw, about 14.4 TiB usable—the price of surviving a second failure during a rebuild.
Plan for growth and the 80% rule
Do not size to fill the array to the brim. Most filesystems slow down and fragment as they pass 80–90% full, and snapshots need free space to function. Treat about 80% of the usable figure as your practical working capacity. In the RAID 5 example above, that means planning around 17 TB of real data, not 21.6.
Also think ahead about expansion. Some systems let you swap drives one at a time for larger ones and grow the array; others require rebuilding from scratch. Knowing your NAS’s expansion model up front saves a painful migration later.
Frequently asked questions
Why does my new NAS show less space than the drives’ labels?
Two reasons stack up: the decimal-versus-binary counting difference (about 7–9%) and RAID redundancy reserving whole drives’ worth of capacity for fault tolerance. Both are normal and expected, not a defect.
Can I mix different drive sizes in one array?
You can, but traditional RAID treats every drive as the size of the smallest one, wasting the extra capacity on larger disks. Some NAS-specific hybrid schemes use the surplus more efficiently—check whether your unit supports that before mixing sizes.
Bottom line
Real usable capacity = raw drives, minus the ~10% binary-and-filesystem haircut, minus whatever RAID reserves for redundancy, and then treat 80% of that as your comfortable ceiling. Run the numbers for your chosen RAID level before buying so you end up with the space you actually need rather than a surprise shortfall.
