Data Storage Converter

There are two competing definitions of a gigabyte, and the gap between them is exactly why a '1 TB' drive shows up as 931 GB.

Two systems, one name

Computers address memory in powers of two, so 1,024 (2¹⁰) was historically called a “kilo” even though the SI prefix means 1,000. For small quantities the 2.4 % discrepancy hardly mattered. It compounds at every level:

The IEC introduced unambiguous binary prefixes in 1998 — kibibyte (KiB), mebibyte (MiB), gibibyte (GiB) — but adoption has been patchy. Linux and macOS now report decimal gigabytes; Windows still labels binary quantities “GB”.

Why your drive looks smaller

A drive marketed as 1 TB contains 1,000,000,000,000 bytes, using the decimal definition. Windows divides that by 1,024³ and reports 931 GB. Nothing is missing; the same bytes are being described in a different unit that happens to share a name.

A genuine reduction comes on top of that: filesystem metadata, reserved blocks and recovery partitions typically consume a few percent more. Between the two effects, expect around 91–93 % of the advertised capacity to be usable.

Class action lawsuits over this were filed against several manufacturers in the 2000s and largely failed, on the grounds that the SI definition is the correct one and the operating systems were the ones using the word loosely.

Bits versus bytes

A byte is eight bits, and the difference matters most with internet connections, which are quoted in bits per second:

A 100 Mbps connection therefore delivers at most 12.5 MB/s, and in practice rather less after protocol overhead. The capital B means bytes and the lowercase b means bits — a distinction that looks pedantic until it produces an eight-fold error.

Rough reference points: a plain text page is a few kilobytes, a photo from a phone 2–5 MB, a minute of 4K video around 400 MB, and a modern game 50–150 GB.