| Reference | Bits (bit) | Gigaoctets (Go) |
|---|---|---|
| A plain text message (160 characters) | 1280 bit | 0.00000016 Go |
| A three-minute MP3 | 24000000 bit | 0.003 Go |
| A smartphone photo | 32000000 bit | 0.004 Go |
| A high-definition film | 3.2 × 1010 bit | 4 Go |
| A dual-layer Blu-ray disc | 4 × 1011 bit | 50 Go |
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 gigaoctet is a unit of digital information equal to one thousand million octets, or eight gigabits. Its symbol is Go. It is the unit in which most things people actually buy are sized: the memory in a computer, the storage in a phone, the monthly allowance on a mobile contract.
The reference points are familiar. A DVD holds 4.7 gigaoctets and a dual-layer disc 8.5. A Blu-ray holds 25 or 50. A feature film in high definition is 4 to 15 gigaoctets depending on compression, and in ultra-high definition 40 or more. A phone with 128 gigaoctets of storage holds roughly thirty thousand photographs, or a few hundred hours of music.
Memory sizes now sit in this range and are stated in gibioctets even when written as gigaoctets. A module labelled 8 GB holds 8 gibioctets, which is 8.59 gigaoctets, because memory addressing is binary and always has been. The same is true of processor caches and of the page tables that map memory, so anything on the memory side of a computer is binary while anything on the storage side is decimal.
That split is the source of the most familiar consumer complaint about units. A hard drive sold as one teraoctet holds a million million octets, which the operating system divides by 1,024 three times and reports as 931 gigaoctets. Nothing has been lost; the drive holds exactly what the box says, but the two are counting in different bases. Lawsuits over this were settled in the manufacturers' favour, since the decimal usage matches the metric system.
Mobile data allowances made the gigaoctet a household figure. A few gigaoctets a month was generous around 2012 and is now minimal, because video dominates: an hour of standard-definition streaming is about 0.7 gigaoctets, an hour in high definition around 3, and an hour in ultra-high definition 7 or more.
For scale in text, a gigaoctet holds around a thousand full-length books as plain text, or the complete works of most authors many times over. Digital storage stopped being a constraint on text decades ago, and every capacity discussion since has really been about images, sound and video.
One gigaoctet equals 1,000,000,000 octets, 1,000 megaoctets, 8 gigabits, or about 0.9313 gibioctets.