Why does my 1 TB drive show 931 GB?

The manufacturer counts a terabyte as 1,000,000,000,000 bytes. Windows displays that same number of bytes in binary gibibytes but labels them "GB", giving 931. No space is actually missing.

What is the difference between MB and MiB?

A megabyte (MB) is 1,000,000 bytes. A mebibyte (MiB) is 1,048,576 bytes — 2 to the twentieth. MiB is about 4.9% larger.

Data Storage Converter

Convert between bytes, kilobytes, megabytes, gigabytes and their binary equivalents.

Draft — figures checked, awaiting an editorial read. Not indexed.

Result—

Formula: Result = amount × (factor of source unit ÷ factor of target unit). Decimal units step by 1000; binary units step by 1024.

What this calculates

This converts between data units, and it deliberately keeps decimal and binary units separate rather than pretending they are the same thing. A kilobyte is 1,000 bytes. A kibibyte is 1,024. Both are legitimate; they are simply different quantities. This distinction is the entire explanation for the most common complaint in consumer computing — that a "1 TB" drive shows up as roughly 931 GB in the operating system. Nothing has been lost. The drive maker counted in decimal terabytes and the operating system displayed binary ones while labelling them "GB".

How it works

Every unit has a factor in bytes. The decimal units follow SI: kilo is 1,000, mega is 1,000,000, and so on, stepping by a thousand each time. The binary units follow the IEC standard adopted in 1998: kibi is 1,024 — that is 2 to the tenth — and mebi, gibi and tebi step by 1,024 each. The gap between the two systems widens as the units get larger, from 2.4% at the kilo level to about 10% at tera.

Worked example

One decimal terabyte in gibibytes: 1 TB is 1,000,000,000,000 bytes, divided by 1,073,741,824 bytes per GiB = 931.3 GiB. That is the missing 69 GB, and it is purely a labelling difference. Another everyday case: a 100 Mbps connection is 100 megabits, not megabytes, per second — divide by 8 to get 12.5 MB/s, which is why a large download never seems as fast as the advertised number.

Common mistakes

Confusing bits with bytes. Network speeds are quoted in bits per second and file sizes in bytes, a factor of eight, which accounts for most "my internet is slower than advertised" confusion. Second, assuming decimal and binary units are interchangeable — at the terabyte scale that is a 10% error. Third, expecting a drive's formatted capacity to match its label even after accounting for the above; the file system itself also reserves some space.

Common questions

Why does my 1 TB drive show 931 GB?: The manufacturer counts a terabyte as 1,000,000,000,000 bytes. Windows displays that same number of bytes in binary gibibytes but labels them "GB", giving 931. No space is actually missing.
What is the difference between MB and MiB?: A megabyte (MB) is 1,000,000 bytes. A mebibyte (MiB) is 1,048,576 bytes — 2 to the twentieth. MiB is about 4.9% larger.