| Zettabits (Zbit) | Bits (bit) |
|---|---|
| 1 Zettabit | 1 × 1021 bit |
| 2 Zettabits | 2 × 1021 bit |
| 3 Zettabits | 3 × 1021 bit |
| 4 Zettabits | 4 × 1021 bit |
| 5 Zettabits | 5 × 1021 bit |
| 10 Zettabits | 1 × 1022 bit |
| 20 Zettabits | 2 × 1022 bit |
| 25 Zettabits | 2.5 × 1022 bit |
| 50 Zettabits | 5 × 1022 bit |
| 100 Zettabits | 1 × 1023 bit |
| Reference | Zettabits (Zbit) | Bits (bit) |
|---|---|---|
| A plain text message (160 characters) | 1.28 × 10-18 Zbit | 1280 bit |
| A three-minute MP3 | 2.4 × 10-14 Zbit | 24000000 bit |
| A smartphone photo | 3.2 × 10-14 Zbit | 32000000 bit |
| A high-definition film | 3.2 × 10-11 Zbit | 3.2 × 1010 bit |
| A dual-layer Blu-ray disc | 4 × 10-10 Zbit | 4 × 1011 bit |
The zettabit is a unit of digital information equal to a thousand exabits, or a sextillion bits — a one followed by twenty-one zeros. Its symbol is Zbit. It is the largest unit in which real, measured quantities of data are still routinely reported, and it entered common use only in the last fifteen years.
A zettabit is 125 exaoctets. The phrase that made the prefix familiar was the zettabyte era, coined for the moment around 2016 when annual global internet traffic first exceeded one zettaoctet. That threshold arrived a little over a decade after annual traffic first passed one exaoctet, which gives a fair sense of the growth rate involved.
Estimates of the total quantity of data in existence are given in zettaoctets. The global datasphere — everything stored on every server, disc, phone and memory card, plus everything created and immediately discarded — is put at somewhere above a hundred zettaoctets, which is more than eight hundred zettabits. Nobody counts this directly; the figures come from manufacturing statistics and modelling.
Almost all of that is machine-generated and never read by anyone. Surveillance video that is recorded and overwritten, sensor telemetry, logs, backups and the many copies that redundancy demands account for the overwhelming majority. The portion that a human being has ever looked at is a tiny fraction of the total, and shrinking as a share every year.
The prefix zetta was adopted in 1991, along with yotta, at a conference that also had to consider what letters remained available. Z and Y were chosen partly because they were unused, and the pattern of naming from the Greek and Latin numerals had by then become loose. The 2022 additions of ronna and quetta continued the ladder upward for the same reason.
For perspective, a zettabit of information written as text in a single line of characters would stretch far beyond the solar system. The unit does not describe anything a person can hold or handle; it describes a civilisation's accumulated output, and it is useful precisely because that quantity now needs a name.
One zettabit equals 1,000 exabits, 125 exaoctets, or about 0.8470 zebibits.
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.