| Reference | Octets (octet) | Megaoctets (Mo) |
|---|---|---|
| A plain text message (160 characters) | 160 octet | 0.00016 Mo |
| A three-minute MP3 | 3000000 octet | 3 Mo |
| A smartphone photo | 4000000 octet | 4 Mo |
| A high-definition film | 4 × 109 octet | 4000 Mo |
| A dual-layer Blu-ray disc | 5 × 1010 octet | 50000 Mo |
The octet is a unit of digital information equal to exactly eight bits. Its symbol is octet, and its name comes from the Latin for a group of eight. It is the unit in which nearly all stored data is counted, and the reason it is called an octet rather than a byte is worth knowing.
A byte was originally the number of bits a machine used to hold one character, and that number was not fixed. Early computers used six-bit bytes, some used seven, and a few used nine. The IBM System/360 of 1964 settled on eight and its commercial success made eight the norm, but the word byte kept its older, machine-dependent meaning in specifications where ambiguity was dangerous.
The networking world therefore adopted octet. When a protocol standard says a header field is four octets long, no reader anywhere can interpret that as anything but thirty-two bits. The internet standards published by the IETF use octet throughout for exactly that reason, and French, Romanian and several other languages adopted the word for ordinary use as well, so that a hard disc sold in France is measured in gigaoctets.
Eight bits hold 256 distinct values, which is why so many things come in units of 256. A colour channel runs from 0 to 255. An IPv4 address is four octets, written as four numbers each between 0 and 255. The original ASCII character set used seven bits, leaving one spare, and the various eight-bit extensions that followed each filled that spare bit with a different set of accented letters — the mess that Unicode eventually replaced.
Modern text encoding still counts in octets. UTF-8 uses one octet for the unaccented Latin alphabet, two for most European accented letters and Greek and Cyrillic, three for Chinese, Japanese and Korean, and four for the rest. That is why the same sentence occupies different amounts of storage in different languages, and why a text-length limit measured in octets is not a limit on characters.
Storage capacity, file sizes, memory and network payloads are all quoted in octets or their multiples. Transmission rates are the exception, being quoted in bits per second, and the factor of eight between the two conventions is the single most common source of confusion in the field.
One octet equals 8 bits, 0.008 kilobits, or 0.001 kilooctets.
The megaoctet is a unit of digital information equal to one million octets, or eight million bits. Its symbol is Mo. It is the unit of the individual file: a photograph, a song, a document, an application download are all sized in megaoctets, which makes it the most frequently read data unit in daily life.
The typical figures are worth carrying in the head. A photograph from a phone is 2 to 5 megaoctets, a raw photograph from a system camera 25 to 50. A compressed song is 3 to 10. A minute of high-definition video is around 100. A long text document is under one. An operating system update is several thousand, which is why it is quoted in gigaoctets instead.
The megaoctet also carries the most notorious unit error in computing history. The 1.44 MB floppy disc holds neither 1.44 million octets nor 1.44 times 1,048,576. Its capacity is 1,440 kibioctets, which is 1,474,560 octets — the manufacturers multiplied a binary kilo by a decimal thousand and produced a figure that is correct in no system at all. It remains the standard illustration of why the IEC prefixes were needed.
Compact discs are cleaner: a standard disc holds about 700 megaoctets of data, or 74 to 80 minutes of audio, and that capacity was the practical limit for distributing software for a decade. Before them, distributing a program meant a box of floppies, and after them a DVD held about 4.7 gigaoctets, nearly seven times as much.
Memory sizes reached megaoctets in the early 1990s and stayed there for a decade. Four megaoctets was a comfortable amount in 1993 and inadequate by 1998, which tracks the arrival of graphical interfaces and the web. Today a single browser tab routinely holds more memory than an entire computer of that era.
The decimal-binary gap matters at this scale. A megaoctet is a million octets, but a mebioctet is 1,048,576, and a file reported as 100 MB by one program may appear as 95.4 MB in another. The difference is 4.9 per cent, small enough to overlook and large enough to cause arguments about whether a download completed correctly.
One megaoctet equals 1,000,000 octets, 1,000 kilooctets, 8 megabits, or about 0.9537 mebioctets.