| Bits (bit) | Gibibits (Gibit) |
|---|---|
| 1 Bit | 9.31322574615 × 10-10 Gibit |
| 2 Bits | 0.00000000186264514923 Gibit |
| 3 Bits | 0.00000000279396772385 Gibit |
| 4 Bits | 0.00000000372529029846 Gibit |
| 5 Bits | 0.00000000465661287308 Gibit |
| 10 Bits | 0.00000000931322574615 Gibit |
| 20 Bits | 0.0000000186264514923 Gibit |
| 25 Bits | 0.0000000232830643654 Gibit |
| 50 Bits | 0.0000000465661287308 Gibit |
| 100 Bits | 0.0000000931322574615 Gibit |
| Reference | Bits (bit) | Gibibits (Gibit) |
|---|---|---|
| A plain text message (160 characters) | 1280 bit | 0.00000119209 Gibit |
| A three-minute MP3 | 24000000 bit | 0.0223517 Gibit |
| A smartphone photo | 32000000 bit | 0.0298023 Gibit |
| A high-definition film | 3.2 × 1010 bit | 29.8023 Gibit |
| A dual-layer Blu-ray disc | 4 × 1011 bit | 372.529 Gibit |
The bit is the fundamental unit of information. Its symbol is bit, and its name is a contraction of binary digit, coined by the statistician John Tukey and put into print by Claude Shannon in his 1948 paper A Mathematical Theory of Communication, the work that founded information theory.
A bit is the amount of information carried by a single choice between two equally likely possibilities. A coin landing heads or tails, a switch open or closed, a voltage high or low: each of those settles one bit. That definition is what makes the bit a unit rather than a mere convention of notation. It measures how much uncertainty an answer removes, and it does so in a way that is independent of what the question was about.
Shannon's insight was that this could be counted. A message drawn from an alphabet of thirty-two equally likely symbols carries five bits per symbol, because thirty-two is two to the fifth. If the symbols are not equally likely — as letters in English are not — the average drops, and that gap between the naive count and the true average is exactly what compression exploits. A well-compressed file is one from which the redundant bits have been removed.
In hardware the bit is a physical state: a charge trapped on a floating gate in flash memory, the direction of magnetisation of a domain on a hard disc platter, a pit or land on an optical disc, a pulse of light present or absent in a fibre. All of these encode the same abstract quantity, which is why data can move between them without loss.
Bits are almost never counted singly in storage. They are grouped into octets of eight, and storage capacity is quoted in octets or their multiples. Transmission is different: network and interface speeds are quoted in bits per second, so a connection described as 100 megabits per second delivers about 12.5 megaoctets per second. Confusing the two is the commonest arithmetic error in the whole field.
Where single bits do get counted is in specifications of precision and range. A colour channel with 8 bits holds 256 levels; one with 10 bits holds 1,024. Audio at 16 bits per sample has about 96 decibels of dynamic range, and at 24 bits about 144. A 64-bit address can name about 18 quintillion locations. In every case, each added bit doubles what can be distinguished.
One bit equals 0.125 octets, 0.001 kilobits, or about 0.0009766 kibibits.
The gibibit is a unit of digital information equal to 1,073,741,824 bits, which is 1,024 mebibits or two to the thirtieth power. Its symbol is Gibit. It is the binary counterpart of the gigabit, and the two now differ by 7.4 per cent — a gap wide enough that mistaking one for the other in a purchase order is a genuine error.
Its practical home is the memory industry. Dynamic memory chips are made in binary capacities because their internal organisation is a rectangular grid of rows and columns, both addressed in binary. A part described as 8 gigabits in marketing material is an 8-gibibit die, holding 8,589,934,592 bits, and eight of those make the 8 gibioctets a memory module advertises as 8 GB.
Flash memory follows the same rule for the same reason, though with a twist: flash dies include spare blocks to replace cells that wear out, so the usable capacity of a drive is deliberately less than the raw capacity of its chips. Manufacturers then quote the usable figure in decimal units, which is how a drive built from binary parts ends up labelled with a decimal number.
The 7.4 per cent gap explains a great deal of everyday confusion. A memory module of 8 gibioctets and a solid-state drive of 8 gigaoctets are not the same size, though both are written 8 GB. The module holds 8.59 gigaoctets; the drive holds 8.00. Nothing is wrong with either figure, but they are counted in different bases and cannot be compared without conversion.
A gibibit is 134,217,728 octets, or 128 mebioctets. In everyday terms that is roughly a hundred and thirty megaoctets — a couple of dozen photographs, or two minutes of high-definition video. It is not a large amount of data by modern standards, which is why the unit appears in component specifications rather than in descriptions of files.
The correct symbol, Gibit, is used in datasheets and in standards but almost never in advertising. When a document writes Gb without further explanation, the safe assumption is that memory means the binary quantity and networking means the decimal one, and that a careful writer would have written Gibit or Gbit to say which.
One gibibit equals 1,073,741,824 bits, 1,024 mebibits, 134,217,728 octets, or about 1.074 gigabits.