Why Your 1 TB Drive Shows Up as 931 GB
Drive makers count a terabyte as a trillion bytes. Windows counts it in powers of two. Both are right; the labels are the problem.
23 February 2026 · 5 min read · Technology
You buy a 1 TB drive, plug it in, and the operating system reports 931 GB. Nothing is missing — two definitions of "gigabyte" are in play.
Decimal versus binary
Computers address memory in powers of two, so early programmers borrowed the SI prefix "kilo" for 1,024 rather than 1,000. Close enough at small sizes; the gap compounds badly at large ones.
| Prefix | Decimal (SI) | Binary (IEC) | Gap |
|---|---|---|---|
| kilo | 1,000 | 1,024 (kibi) | 2.4% |
| mega | 1,000,000 | 1,048,576 (mebi) | 4.9% |
| giga | 10⁹ | 1,073,741,824 (gibi) | 7.4% |
| tera | 10¹² | 1,099,511,627,776 (tebi) | 10.0% |
Doing the sum
A "1 TB" drive holds 1,000,000,000,000 bytes. Divide by 1,073,741,824 and you get 931.32 GiB — which Windows displays as "931 GB" because it uses the old label for the binary value.
macOS and most Linux distributions switched to decimal years ago, which is why the same drive shows as "1 TB" on a Mac and "931 GB" on a PC.
Who uses which
- Decimal (1000-based): drive manufacturers, network speeds, macOS, Ubuntu
- Binary (1024-based): Windows Explorer, RAM capacity, most programming APIs
RAM is genuinely binary — an 8 GB stick really is 8 GiB — which is why memory never shows the discrepancy that storage does.
The right names
The IEC standardised unambiguous prefixes in 1998: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB). Linux tooling adopted them; consumer software largely did not. Convert either way with the gigabytes to gibibytes converter.
The other missing space
Formatting also costs you. A filesystem reserves space for its own metadata — journals, allocation tables, inodes — typically 0.5–3%. Combine that with the decimal/binary gap and a 1 TB drive realistically offers about 915–930 GiB of usable space.
Related conversions
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy