Why Your 1 TB Drive Shows 931 GB: Bits, Bytes and Storage Units Explained
You buy a 1 terabyte drive, plug it in, and your computer reports only about 931 gigabytes. Nothing is broken and nothing is missing. The difference comes from two ways of counting data, and understanding them also explains why your internet speed and your download speed never quite match.
Bits and bytes
A bit is the smallest unit of digital information, a single 0 or 1. A byte is a group of eight bits, enough to store one letter of basic text. File sizes and storage capacity are measured in bytes (capital B), while network speeds are measured in bits per second (lowercase b). That is why a connection advertised as 100 Mbps (megabits per second) downloads at most 12.5 megabytes per second: 100 divided by 8.
Two ways of counting
The SI system, used by storage manufacturers, counts in powers of 1,000: a kilobyte is 1,000 bytes, a megabyte is 1,000 kilobytes, and a gigabyte is 1,000 megabytes. Computers, however, work naturally in powers of two, and for decades 1,024 was used as the multiplier instead of 1,000. To resolve the confusion, standards bodies introduced separate names for the binary versions: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB) and tebibyte (TiB).
| Decimal (SI) | Bytes | Binary (IEC) | Bytes |
|---|---|---|---|
| 1 kilobyte (KB) | 1,000 | 1 kibibyte (KiB) | 1,024 |
| 1 megabyte (MB) | 1,000,000 | 1 mebibyte (MiB) | 1,048,576 |
| 1 gigabyte (GB) | 1,000,000,000 | 1 gibibyte (GiB) | 1,073,741,824 |
| 1 terabyte (TB) | 1012 | 1 tebibyte (TiB) | 1,099,511,627,776 |
Where the “missing” space goes
A 1 TB drive holds 1,000,000,000,000 bytes. Windows divides that by 1,073,741,824 and shows the result, 931, with the label “GB” even though it is really gibibytes. macOS, by contrast, uses the decimal definition and would show 1 TB. The gap grows with size: a 256 GB phone shows about 238 GiB, a 512 GB drive about 477 GiB, and a 2 TB drive about 1.82 TiB. Some space is also used by the file system and the operating system, so the usable amount is slightly less still.
Practical tips
- When comparing storage products, treat the advertised size as decimal.
- When you check whether a file fits, compare it in the same units as the limit you are working to.
- For internet speeds, divide megabits by 8 to estimate the download in megabytes per second.
Use the kilobytes to megabytes, megabytes to gigabytes and gigabytes to terabytes converters to do the sums. Each lets you pick binary units from the drop-down if you want the 1,024-based answer.