Why 1 TB ≠ 1024 GB
A “1 TB” drive holds exactly 1,000,000,000,000 bytes — the tera prefix in SI units means 10¹². Drive manufacturers label drives in these decimal terabytes because that’s literally how many bytes the drive contains.
Computers, on the other hand, measure storage in powers of two. For a long time the kilobyte unit was used loosely to mean 1024 bytes even though “kilo” elsewhere means 1000. To clean the ambiguity up, the IEC introduced new binary prefixes:
- Kibibyte (KiB) — 1024 bytes (2¹⁰).
- Mebibyte (MiB) — 1024 KiB ≈ 1.049 MB.
- Gibibyte (GiB) — 1024 MiB = 1,073,741,824 bytes.
The operating system uses the binary view internally — but displays it as “GB” on screen, which is where the confusion starts.
What a 1 TB drive actually shows
In practice, 1 TB (1,000,000,000,000 bytes) is about 931 GiB (999,653,638,144 bytes). The OS displays this as 931 GB.
That’s not the drive lying — it’s “1 TB” being a decimal number and “931 GB” being the same drive measured in binary. The drive is sold as “1 TB” because 1 TB sounds bigger than 931 GB and the trillion-byte claim is accurate.
Free-space target for a NAS
The filesystem benefits from headroom, and active workflows need working space to write, copy, and snapshot files. Leave at least 25% of any production NAS free. On a 1 TB drive, that works out to roughly 230–250 GB of free space — somewhere around 700 GB of useful storage at the 25% threshold.
If utilisation creeps above 75–80% on an Uplevel NAS, plan to either prune content or move up to a bigger drive before performance starts to degrade.